a preliminary model for slug control in vegetable crops · 2017-05-05 · ha project no: vg00030...

A preliminary model for slug control in

vegetable crops

Sally Bound University of Tasmania

Project Number: VG00030

danikah

Stamp

VG00030 This report is published by Horticulture Australia Ltd to pass on information concerning horticultural research and development undertaken for the vegetable industry. The research contained in this report was funded by Horticulture Australia Ltd with the financial support of the Vegetable Industry. All expressions of opinion are not to be regarded as expressing the opinion of Horticulture Australia Ltd or any authority of the Australian Government. The Company and the Australian Government accept no responsibility for any of the opinions or the accuracy of the information contained in this report and readers should rely upon their own enquiries in making decisions concerning their own interests. ISBN 0 7341 0900 8 Published and distributed by: Horticultural Australia Ltd Level 1 50 Carrington Street Sydney NSW 2000 Telephone: (02) 8295 2300 Fax: (02) 8295 2399 E-Mail: [email protected] © Copyright 2004

FINAL REPORT

Project VG00030

(Completed 30 April 2004)

A preliminary model for slug control in vegetable crops

Sally Bound

Tasmanian Institute

of Agricultural Research

HA Project No: VG00030

Project title: A preliminary model for slug control in vegetable crops Project Chief Investigator: Sally A Bound Address: Tasmanian Institute of Agricultural Research New Town Research Laboratories 13 St Johns Avenue New Town TAS 7008 Email: [email protected] The purpose of this report is to document the findings of three years of research examining control methods for slugs, a major horticultural pest.

Tasmanian Institute of Agricultural Research

4 May 2004 Any recommendations contained in this publication do not necessarily represent current HA policy. No person should act on the basis of the contents of this publication, whether as to matters of fact or opinion or other content, without first obtaining specific, independent professional advice in respect of the matters set out in this publication.

Final Report VG00030 Tasmanian Institute of Agricultural Research page 3

Contents

Acknowledgments................................................................................................................................4

Media Summary ..................................................................................................................................5

Technical Summary ............................................................................................................................6

Introduction.........................................................................................................................................7

Literature review / Background..........................................................................................................7 Introduction................................................................................................................................................. 7

Ecology & biology ....................................................................................................................................... 7

Chemical control ....................................................................................................................................... 11

Cultural and biological control ................................................................................................................ 15

Sampling .................................................................................................................................................... 17

Project objectives...............................................................................................................................18

Achievement of objectives.................................................................................................................18

Materials and Methods .....................................................................................................................18 (i) Grower survey ..................................................................................................................................... 18

(ii) Replicated small plot trials ................................................................................................................ 18 Data analysis ........................................................................................................................................................... 25

Results................................................................................................................................................31 (i) Grower survey ..................................................................................................................................... 31

(ii) Replicated small plot trials ................................................................................................................ 38 A. Food Type / Cultural / Slug Species.................................................................................................................. 38 B. Barrier Trials ..................................................................................................................................................... 42 C. Baiting Efficacy................................................................................................................................................. 44 D. Caffeine / Carvone Studies................................................................................................................................ 53

Discussion..........................................................................................................................................55 Grower survey / cultural control ............................................................................................................................. 55 Barriers.................................................................................................................................................................... 56 Baiting efficacy ....................................................................................................................................................... 57 Caffeine/carvone studies ......................................................................................................................................... 58

Recommendations / Conclusions......................................................................................................59

Technology Transfer.........................................................................................................................59

Industry implications and recommendations...................................................................................60 Recommendations for continuing work:................................................................................................. 60

References .........................................................................................................................................61 Appendix 1 - Survey on slugs ................................................................................................................. 66


Acknowledgments

Funding for this project was provided by Horticulture Australia through the Vegetable Levy.

The technical support of Justin Direen is gratefully appreciated.

Valuable advice was provided by Dr Brian Smith, Queen Victoria Museum, Mr Bob Hardy, Specialist Agricultural Services Pty Ltd, Dr Lionel Hill, Entomologist with the Tasmanian Department of Primary Industry, Water & Environment and Dr Geoff Dean, Agronomist with the Tasmanian Institute of Agricultural Research.

Thanks are also due to Dr Colin Young and Mr Gavin Armstrong for technical advice and provision of Multiguard and the iron based products assessed.

Seedlings used in this work were kindly donated by Colin Houston of Houstons Farm. Hills Transpants Pty Ltd, Devonport supplied the iceberg lettuce speedlings.


Media Summary

Slugs are a major economic pest in most horticultural crops. In some crops the main issue is at planting, while in others problems occur during the growing season and/or at harvest time. Slug management is difficult - often single agent control methods are ineffective and a concerted approach must be taken to adequately protect crops from slug attack.

Results from a survey of growers across Australia demonstrate that cultural management can play an important role in controlling slug populations. In regions where slug activity is rampant, resulting in economic crop losses, a combination of cultural and chemical control methods should be utilised to minimise damage. Effective cultural practices include: reducing soil moisture and removal of materials that provide favourable habitats, soil cultivation to expose eggs and slugs, production of fine seed beds to reduce movement between slug habitats and the crop, a cultivated weed free strip between the headland and the crop to reduce damage by immigrating large slugs.

Survey results reinforced the importance of timing when using chemical baits – baits need to be applied before populations build up to damaging levels. Establishment of refuge traps and regular monitoring will provide the grower with information on fluctuations in slug populations, allowing baits to be laid before the slugs become a major problem. Chemical baits were most effective when they were the only food source, or where the food source was not highly desirable. Methiocarb baits were consistently more effective in this work than metaldehyde based baits. Although slower acting, Multiguard baits proved to be more effective than either methiocarb or metaldehyde baits. Formulation of baits influenced efficacy.

Caffeine and carvone showed promise as potential control agents. Caffeine achieved 100% mortality of adult slugs when applied as a soil drench at rates as low as 0.5%. Both carvone (+) and carvone (-) gave similar results at concentrations of 0.25% or greater. Further studies need to be conducted with both caffeine and carvone.

Aluminium oxide sandpaper proved to be an effective barrier and may have potential in organic systems as well as in small plot research trials. Oxidised copper bands, limil, sawdust and iron sulphate were also effective barriers.

While slug control can be considered a management issue there is little information available to growers on how to integrate cultural, biological and chemical control methods to achieve effective control. This project has provided a starting point for Australian vegetable growers on integration of slug control measures. Many growers are already successfully integrating cultural and chemical control methods, but the number of growers still experiencing problems suggests that there is a major need for extension of existing information.


Technical Summary

Slugs are a major economic pest in most horticultural crops. In some crops the main issue is at planting, while in others problems occur during the growing season and/or at harvest time. Slug management is difficult - often single agent control methods are ineffective and a concerted approach must be taken to adequately protect crops from slug attack.

Results from a survey of growers across Australia demonstrate that cultural management can play an important role in controlling slug populations. Practices commonly implemented by growers include: reducing soil moisture; removal of materials that provide favourable habitats; soil cultivation; production of fine seed beds; retention of a cultivated weed free strip between the headland and the crop; keeping gully lines clean; and use of refuge traps to monitor changes in slug populations.

Chemical baits were most effective when they were the only food source, or where the food source was not highly desirable. Methiocarb baits were consistently more effective in this work than metaldehyde based baits. Although slower acting, Multiguard baits proved to be more effective than either methiocarb or metaldehyde baits. Formulation of baits influenced efficacy. Weathering improved the efficacy of some baits, particularly those with hard pellets. Survey results reinforced the importance of timing when using chemical baits – baits need to be applied before populations build up to damaging levels.

Aluminium oxide sandpaper proved to be an effective barrier and may have potential in organic systems as well as in small plot research trials. Oxidised copper bands, limil, sawdust and iron sulphate were also effective barriers.

Caffeine and carvone showed promise as potential control agents. Preliminary studies suggest that both chemicals are effective ovicides. Caffeine achieved 100% mortality of adult slugs when applied as a soil drench at rates as low as 0.5%. Both carvone (+), derived from caraway, and carvone (-), derived from spearmint, gave similar results at concentrations of 0.25% or greater, but at lower concentrations (0.1%), carvone (+) was more effective than carvone (-). Further studies need to be conducted with both caffeine and carvone.

This project has provided a starting point for Australian vegetable growers on integration of slug control measures. Many growers are already successfully integrating cultural and chemical control methods, but the number of growers still experiencing problems suggests that there is a major need for extension of existing information.


Introduction

Slugs are a major economic pest in most horticultural crops. In some crops the main issue is at planting, while in others problems occur during the growing season and/or at harvest time. Slug management is difficult - often single agent control methods are ineffective and a concerted approach must be taken to adequately protect crops from slug attack. For the effective management of any pest a sound knowledge of its ecology and biology is required. While slug control can be considered a management issue there is little information available to growers on how to integrate cultural, biological and chemical control methods to achieve effective control.

During the course of this project a number of strategies were undertaken to provide information for the development of an integrated package for slug control. Although lettuce growing in Tasmania is being used as the model system, it is anticipated that management tools and control principles developed will be readily transferred to other vegetable and horticultural crops in temperate Australia.

Literature review / Background

Introduction Slugs have been classed as major economic pests, and although Australia has native slugs, all the pest species are introduced, mainly from the Mediterranean region (Davis 1994c).

Slugs belong to the Phylum Mullusca, in the subgroup Gastropoda. The Australian terrestrial mollusc fauna contains eight families of slugs (van Regteren Altena and Smith 1975). Three of these, the Athoracophoridae, Cystopeltidae and Rathouisiidae, consist of endemic species. The other five are represented only by species introduced into Australia, the Veronicellidae probably from the Southern Pacific islands, and the Testacellidae, Arionidae, Limacidae and Milacidae originated from Britain or Europe.

Slugs are primarily pests of ground crops such as vegetable and cereals. They can damage tuber crops such as potato and cause losses of seeds, seedlings and fruit. Damage to seedlings usually leads to major losses following the death of plants.

Ecology & biology For the effective management of any pest a sound knowledge of its ecology and biology is required.

The life of the field slug is closely connected to its environment, with temperature and humidity directly affecting biological processes (Dmitrieva 1978). Young and Port (1989) report that air temperature, soil surface temperature, windspeed, humidity and soil moisture content are all


correlated with slug activity. The effect of these may be direct or indirect through their effect on temperature and humidity. Slugs have a preference for areas of high humidity and the choice of daytime resting site can be crucial to their survival. Slugs frequent moist, shady places and retire into seclusion to avoid excessive loss of water during periods of drought. As well as environmental factors slug activity is also governed by endogenous rhythms.

Slugs are active throughout the year and will cause trouble whenever the temperature and moisture conditions are favourable, being particularly active when humidity reaches 100% (Hunter 1968b). In studies of Deroceras caucasicum, Uvalieva (1978) reported that these slugs are active at air temperatures of 11-25°C, soil temperatures of 8-20°C and a relative humidity (RH) of 47-69%. He found that relative humidity of <45% in air and <18% in soil inhibited slug activity, with slugs retreating deep into cracks in the soil or hiding themselves at the base of plants and under clods on the ground. Carrick (1942) found that the effective temperature range for Agriolimax agrestis extends from just above freezing point to 25°C, however he also reported a very high degree of resistance to freezing following the complete recovery of two slugs after 1 hour at -5°C. His conclusion was that frost must be severe and continuous to have a pronounced lethal effect on adults, although less so on eggs, the development of which is much retarded by exposure to temperatures below 5°C. According to Anon (1999) slugs are very sensitive to ambient temperature and can detect gradual temperature changes. Hunter (1968b) reported maximal feeding at 20°C, although some feeding occurred at very low temperatures of just above 0°C, particularly in Agriolimax reticulatus. While slugs prefer temperatures of 17-18°C, they lay eggs and develop normally (but slower) at lower temperatures, with development ceasing below 5°C.

Slugs are extremely susceptible to dehydration due to evaporative water loss across their integument and lung surface, and through deposition of their slime trail (Prior et al 1983; Prior 1985). Consequently active slugs can lose up to 40% of their initial body weight in less than 2 hours (Prior 1989). They can however rapidly rehydrate through integumental absorption of water (Prior 1984, 1989). While slugs are active during periods of rain they do not like heavy rain or wind (Davis 1994c). Ideal conditions are damp, mild (15-25°C) and calm periods. Slugs can withstand brief periods of immersion under water, although they drown after several hours (Anon 1999).

Slugs favour heavier soils, being able to survive over summer in cracks in the soil and under clods. According to Davis (1994c) they do not survive well in fine, light or compacted soils.

Duval and Banville (1989) suggest that there is a relationship between the seasonal surface activity pattern of Deroceras reticulatum and the sexual maturation pattern of the population. Temperature directly affects the growth rate of both slugs and eggs, and it is the most vital factor in determining the time the slug takes to reach maturity and commence reproduction (Carrick 1942).


Slugs are hermaphrodites and both members of a mating couple can lay eggs (Davis 1994c). As slugs mature, they become functional males and then true hermaphrodites; older slugs are females (Anon 1999). Mating usually takes place from mid-autumn to mid-winter when conditions are favourable. Deroceras reticulatum lays eggs in the soil at a depth of 2-3 cm (Dmitrieva 1978). Eggs, which contain 85% water (Carrick 1942), become dry and are destroyed unless contact is maintained with a moist surface. Carrick (1942) found that eggs laid in soil which was too dry (25% moisture content) or too wet (100% satuarated) did not complete their development and so failed to hatch. In the drier soil there was insufficient moisture to keep the eggs turgid, resulting in embryo death; in saturated soils the eggs were immersed and many died at an early stage of development. Carrick also reported that the depth at which egg masses were laid varied according to the amount of moisture present – eggs being placed more deeply as the soil dried on the surface. Hunter (1968a) reported that while Agriolimax reticulatus had two distinct generations per year (spring and autumn), eggs were found throughout the year. This agrees with D Glen (pers. comm.) who states that Deroceras reticulatum (syn. Agriolimax reticulatus) is active all year providing conditions are suitable. According to Carrick (1942), low temperatures inhibit hatching, while a change from cold to warm conditions appears to stimulate hatching. He found that in the grey field slug, the minimum hatching temperature was 5°C while the maximum hatching temperature was 21°C. Time of development of Agriolimax agrestis eggs varies from 105 days at 5°C to 18 days at 20°C, and egg mortality increases from zero at 5°C to 37% at 20°C (Carrick 1942). Hunter (1978) reported that Milax budapestensis eggs hatch in about 3 weeks at 20°C but take over 4 months at 7.5°C. Once hatched, growth rates also vary depending on temperature: when kept at a constant temperature of 20°C, Arion hortensis will grow to a weight of 500 mg in 12 weeks, but at 5°C will only reach a weight of 50 mg in this time.

The number of eggs varies with species, in confined cultures Arion hortensis lays about 50 eggs per individual, Milax budapestensis about 30 eggs, and Deroceras reticulatum about 200 eggs (Hunter 1978).

Homing behaviour is exhibited by slugs (South, 1965) with slugs leaving their resting site soon after sunset and returning shortly before dawn. A distinct nocturnal rhythm has been reported by Hunter (1968b) with slugs feeding most in the early part of the night. Hunter also reported that day length did not influence slug activity.

Observations by Hunter (1968b) show that slugs do not move far to feed, species that live underground tend to feed underground and those that live on the surface are surface feeders. He illustrates this with the following examples: Agriolimax reticulatus which eats mostly green material is mainly surface dwelling; Arion hortensis is usually found under soil level and eats less green food; and Milax budapestensis, which eats still less green food, is found deeper under ground. Hunter and Symonds (1970) determined that Agriolimax reticulatus covered distances up to 1.5 m in a night. Personal observations suggest that this distance can be considerably greater. Slugs do


immigrate from headlands, however Pinder (1974) suggested that this is unlikely to be sufficiently penetrative to be of importance on a field scale.

Glen and Wilson (1995) have suggested that changes in agronomic practices and wet weather have contributed to an increased severity of slug problems in cereals and oilseed rape. According to these authors, agronomic changes that have favoured slugs include a ban on straw burning and the change from ‘single-low’ to ‘double-low’ rape cultivars which contain low concentrations of glucosinolates, hence are more acceptable to slugs. Moens (1989) also discusses the importance of glucosinolate level in rape. As well as providing suitable conditions for slug population growth, wet weather also prevents preparation of seed beds which can deter slug attack.

Slug problems are greatly influenced by crop rotations and other aspects of cropping systems. Movement of plant material facilitates the dispersal of pest slugs (Anon 1999). Carrick (1942) reports that practices which tend to increase the moisture holding capacity of the soil also render it capable of sustaining an increased population of slugs. Glen et al. (1996) report that winter wheat following oilseed rape is at risk because of the build-up of slug populations in the rape crop, and similarly oilseed rape is at risk where incorporation of straw or stubble from previous cereal crops (especially shallow incorporation by non-plough tillage) has resulted in high slug populations. Moens (1989) concluded that wet summers and a crop rotation scheme containing many crops favouring slug reproduction (rape, vetch and other legumes, lodged cereals, permanent crops) are key factors for harmful slug populations. Growers are reluctant to use conservation tillage practices, especially no-tillage, if slugs cannot be managed (Hammond et al. 1996).

According to Davis (1994a), effective control of pest species involves a combination of measures, including cultural, biological and chemical methods.

Plant growth stage is an important factor in susceptibility to slug damage. In monocotyledons the most vulnerable stages occur shortly after drilling as soon as the slugs are able to hollow the weakened germ (Moens 1989), but vulnerability diminishes sharply at the development of the coleoptiles which provide a mechanical barrier protecting shoots before they emerge. Dicotyledons become highly susceptible to slug damage when the cotyledons rise to the surface and spread out above the soil level. At these stages the stems, cotyledons and terminal buds are very exposed to slug attacks and the vulnerability of the plant is extremely high compared with monocotyledons at the same stages. Once normal leaves develop, vulnerability decreases quickly and hole grazing in the leaves can be compensated by growth. Damage only occurs when the consumption level is greater than leaf production.

The grey field slug (Deroceras reticulatum) is one of the most widely distributed agricultural and horticultural pests. It is very difficult to provide a useful threshold of population density above which a treatment will be economically justified, because slug damage is largley influenced by environmental and climatic factors acting on the feeding activity of slugs, food accessibility,


duration of vulnerable stages and growth (Moens 1989). Varietal influence is also of importance in crops such as rape.

Chemical control Early chemical control was based on burnt lime, hydrated lime and dehydrated copper sulphate. The chemical control of pest slugs is usually reliant on the application of baits containing a molluscicide. Slugs can be difficult to kill by contact poisons because they are covered by a layer of slime which prevents chemicals from coming in contact with the skin (Hunter and Symonds 1970). Four chemicals are registered in Australia for slug control – methiocarb (Baysol, Mesurol), metaldehyde (Defender, Budget, PestMaster, Blitzem), iron chelate (Multiguard) and copper silicate (Four-S, Escar-go).

Most commercial baits comprise an extruded cereal-based bait containing either metaldehyde and methiocarb. Bait composition, particle size, application rate, persistence, and the concentration and formulation of the active ingredient, all interact to affect the efficacy of baits against specific gastropod pests (Barker et al. 1991).

There is considerable contradiction in the results of trials comparing molluscicides. Proude (1970) and Rayner (1975) found metaldehyde baits of equal value to methiocarb. Symonds (1975) reported metaldehyde baits were superior to methiocarb baits, while Getzin (1965) and Rings et al. (1975, cited in Prystupa et al. 1987) reported that methiocarb killed more slugs than did metaldehyde. However, metaldehyde baits do not give good control under humid conditions as the slugs are able to rehydrate (Anon 1999; M. Williams, pers. comm.). Proude (1970) also reports that under conditions of high humidity methiocarb is a superior molluscicide to metaldehyde. Anon (1999) reports that slugs become more susceptible to carbamate pesticides as they mature.

Barker et al. (1984) reported that broadcasting of methiocarb and metaldehyde baits at sowing gave similar levels of control. Metaldehyde baits deteriorate rapidly in moist soil (Barker et al. 1984). Drilling of methiocarb baits with seed is a common grower practice in direct drilling, however Barker et al. (1984) questioned the efficacy of this approach to slug control. They found that drilled methiocarb baits were rapidly invaded by soil fungi, making them less effective than broadcast applications. These authors also showed that coating seeds with methiocarb is phytotoxic. Barratt et al. (1993) states that while methiocarb is currently the most effective form of chemical control, this method of slug control is expensive, environmentally unacceptable to some growers, and only partially effective. Davis (1994a) suggests that since some slug species may be naturally tolerant to methiocarb, metaldehyde baits should be used for slug control, especially in crop situations.

These variable results may be explained by the lack of uniformity among the numerous published comparisons of the field efficacy of metaldehyde and methiocarb bait formulations. This may be in active ingredient content of the baits, rates of application, timing of bait application in relation to


crop sowing or growth stage, method of bait application – whether banded, broadcast or drilled, and assessment of slug populations before and after baiting. Frain and Newell (1983) suggested that, as slugs poisoned by methiocarb are more conspicuous than those killed by metaldehyde, the practice of counting poisoned slugs to assess the number killed by the treatments may be one source of the variation of results in efficacy trials.

Thiodicarb has been shown to be as effective as methiocarb and in some cases more effective than metaldehyde in causing slug mortality in small plot trials (Firth et al. 1991). In a further comparison between methiocarb and thiodicarb, Ferguson et al. (1995) found no differences in slug mortality associated with the two types of bait

It has long been known that metal salts are toxic to terrestrial molluscs. As early as 1899 Tyron (cited in Henderson et al. 1990) suggested poison baits containing Paris Green (copper aceto-arsenate), while in 1939 Shropshire and Compton (cited in Henderson et al. 1990) mooted various metallic compounds such as calcium, barium and sodium fluorosilicate as stomach poisons. However, metal salts tend to be caustic or otherwise repellent and baits containing them are not readily eaten, hence they have not been successful as bait poisons. Henderson et al. (1989) and Henderson and Martin (1990) overcame this problem by using metals chelated with organic ligands to produce compounds with a range of chemical properties and different biological effects. These authors demonstrated in field trials that wheat baits containing aluminium and iron could kill slugs as effectively as baits containing metaldehyde or methiocarb, were more efficient under very wet conditions, and showed no insecticidal activity. They suggest that with these metal complexes ingestion is the most effective delivery method as impractically high rates were required when used as contact poisons. Differences in recovery rates between bait poisons were reported by Henderson et al. (1990), with 81% of metaldehyde and 59% of methiocarb poisoned slugs recovering compared with <2% of slugs ingesting aluminium acetylacetonate. These authors hypothesised that this reflects the fact that amounts of metal chelates well in excess of the lethal dose are usually ingested.

Laboratory studies of the effect of commonly used fungicides, insecticides and herbicides by van der Gulik and Springett (1980) suggest that many chemicals have the potential to kill adults, juveniles and/or eggs. Para-quat dichloride was an effective ovicide while combinations of carbaryl + methiocarb, carbaryl + methomyl, methiocarb + methomyl were extremely effective in killing adults, juveniles and eggs. Many of the fungicides were very effective ovicides, including benomyl, thiram and thiophanate. These results however were not trialed in a field situation.

In bioassay screenings, Barratt et al. (1993) found that extracts of the leaves of the native New Zealand angiosperm Coriara arborea exhibited strong feeding deterrents. In screening test of candidate molluscicides, Judge and Kuhr (1972) reported that the 6 best materials all contained sulphur in their chemical structure.


Caffeine has been shown to act as both a repellent and toxicant against slugs and snails (Hollingsworth et al. 2003). These authors also showed that drench treatments caused slugs to exit treated soil. Rates as low as 0.1% sprayed on plant leaves reduced feeding activity. Carvone applied directly to plants or incorporated into mulch reduced slug feeding on lettuce (Frank et al. 2002).

According to Maurin et al. (1989) the effect of molluscicidal treatment against slugs damaging crops varies according to: (1) efficiency of active ingredient and formulation, (2) the persistence of activity as affected by climatic conditions (particularly rainfall) and the population level, and (3) the method and timing of application. They suggest that molluscicides are insufficiently effective and that integrated control measures against slugs need to be improved, including cultural practices, biological control, chemical control, application techniques and the use of treatment thresholds.

Chemical use can cause damage to natural predators and awareness needs to be raised of these issues. The application of methiocarb bait in the field for slug control has been found to be toxic to carabid populations, natural predators of slugs in the UK (Purvis 1996). Caffeine has been shown to be toxic to frogs (Anon 1991). The recently released iron complex bait, Multiguard, is claimed to have low toxicity to domestic pets and wildlife and to be non-toxic to beneficial insects (Dyer 2000).

Baiting

For successful control when using baits, timing of application is critical. Davis (1994b) suggests that trying to control slugs with chemical baits when they are a problem, usually in spring, is the least effective method for the following reasons:

1. the population is at its greatest at this time

2. most of the population is juvenile and not very mobile, hence has a reduced chance of encountering baits

3. there is ample alternative feed available which competes with the baits

4. rainfall is relatively high, reducing the life of baits in the field.

Baiting in autumn (late March to April) is preferable as adult slugs are killed before they get the chance to lay their eggs – eggs are laid in soil which is damp enough to germinate grasses (Davis 1994b). Rain is infrequent during this period which means that the field life of baits is extended. The ground is comparatively bare in autumn, so the chance of a slug contacting a bait is increased, and after spending the summer period inactive slugs are hungry and there is little alternative feed to compete with the baits.

In USA studies, Hammond et al. (1996) found that molluscicides applied in early May (spring) did not prevent juvenile slugs from becoming numerous in mid-June, reporting that only late May and early June molluscicide applications significantly reduced the number of juvenile slugs and


prevented defoliation injury. They concluded that this was probably because the molluscicides were applied near, or following, egg hatch.

The efficiency of poison baits depends on the amount eaten by the slugs, the toxicity of the active ingredients and the probability that slugs will eat the bait. The amount eaten is determined by the feeding activity of the slugs and the palatability of the baits (Hunter and Symonds 1970). However, Barker et al. (1991) suggest that the effectiveness of the bait is dependent more on the quantity of molluscicide ingested by each animal than on slug activity or bait attractiveness. Davis (1994b) also suggests that the size of the bait is important, especially for broad scale applications. The smaller the bait pellet, the more baits there are per unit weight and hence the better the coverage.

Recovery from paralysis induced by methiocarb is dose-dependent and is not greatly influenced by environment, however recovery from metaldehyde poisoning is both dose and humidity dependent (Getzin 1965). This explains why normal doses of metaldehyde often prove ineffective in moist conditions. Metaldehyde shortens the period slugs feed at baits by interfering with the neural control of feeding and inducing paralysis in the gut musculature (Wedgwood and Bailey 1988; Bailey et al. 1989). To increase the likelihood of a lethal dose being ingested, Wedgwood and Bailey (1988) concluded that a bait formulation was needed that reduced the rate of absorption of the molluscicide from the alimentary tract.

Methiocarb affects the larger mature individuals (Glen et al. 1987). Prystupa et al. (1987) found that molluscicide efficacy was greatest when the proportion of adult slugs was at a maximum.

To prevent re-invasion of an area, Davis (1994b) suggests application of baits around the perimeter. This can be done by applying baits in a 2-3 metre wide strip to clean-cultivated borders or fire-breaks, or alternatively applying in a continuous line along the bottom of a furrow. He also suggests that the performance of baits can be improved by mowing or cultivating and spraying weeds along tree-lines and fences prior to baiting.

Prystupa et al. (1987) suggest that if long term depression of slug populations, rather than short term control of adults, is the aim of a molluscicide program, then efficacy trials should use population sampling methods which will detect immature stages effectively.

Sprays

Two Mesurol sprays are registered for slug control. However they are restricted in their registration and have a long with-holding period when used on fruit producing trees.

Copper silicate (Four-S) acts primarily as a repellent and can be sprayed on tree butts and plants (Davis 1994b). Copper containing sprays such as Bordeaux mixture, copper sulphate or copper oxychloride, although not registered for slug control, also have some effect in killing slugs (usually juveniles) and in protecting plants by making them repellent (Davis 1994b).


Cultural and biological control Good hygiene, weed control and removal of harbourages can reduce slug problems over time (Davis 1994a). However Davis cautions that pest problems may increase in the short term following this process as the slugs have no alternative feed source. In 1923, the use of hedgehogs was recommended in New Zealand where there were no frogs in the vicinity (Coleman 1970).

Cultivation

Glen and Wilson (1995) suggest that cultivation, seed-bed preparation and adjustment of drilling depth are the foundation of successful strategies for slug control in field crops. It has also been reported that ploughing of soil to depths of 5, 15 or 25 cm after harvest of cereal crops reduced slug numbers substantially compared to zero tillage (Glen et al. 1996). Cultivation kills slugs directly, hence control can be effected by disturbing shelters. This can be achieved by cultivation and seed bed preparation. Seed-bed conditions and drilling depth have a critical influence in determining whether slugs are able to find and kill seeds and seedlings below ground.

Work by Hunter (1967) has suggested that slug populations can be significantly reduced when the number of macrospaces and cracks in the soil are reduced by extensive compaction. Laboratory studies by Glen et al. (1989) have demonstrated that larger soil aggregates, uncompacted soil and shallow sowing enable slugs to gain access to wheat seeds and thus cause damage. In field trials these authors saw least damage (3-5%) in fine or medium tilth unconsolidated seedbeds, and most damage (31-33%) on fine or medium tilth consolidated seedbeds – this was partly related to differences in seed depth. Seed was drilled deeper (5cm) into unconsolidated than consolidated (2 cm) seedbeds; and the shallower the depth the greater the damage.

According to Glen and Wilson (1995), ploughing and subsequent operations to produce a seed-bed have greater effect in reducing slug numbers than methods of non-inversion tillage. Fine, firm seed-beds make it difficult for slugs to move through soil to find seed. Where it is not possible to prepare a fine seed-bed, deeper drilling of seeds can effectively reduce slug damage. Glen et al. (1990b) reported that drilling at 4 cm rather than 2 cm provided as much protection to seeds as an application of methiocarb pellets. Coleman (1970) reports that the use of organic fertilisers in vegetable cropping contributes to the build-up of slug populations. Increasing the organic matter content of the soil helps to increase its moisture content, making a more favourable environment for slugs and increasing the food supply (Davis 1994a). Newly hatched slugs largely subsist on decomposing plant matter (Carrick 1942).

Glen et al. (1996) reported high slug numbers in plots with straw residues, with few slugs on burnt plots. They suggest that burning itself does not kill slugs, but rather the lack of food and shelter following a burn may be more important. Hammond and Stinner (1987) reported that slug populations were highest in no-tillage systems where corn crop residue cover was greatest and lowest where no residue was present. They also found a trend for numbers of slugs to be greater when the previous crop was soybeans; this was thought to be related to either the increased amount


of dead, weed biomass present or a preference for decomposing soybean tissue in the no-tillage system.

According to Ferguson and Barratt (1983) direct drilling does not provide sufficient mechanical disturbance of the soil to reduce slug density significantly, but in fact enhances slug survival by providing a moist dark refuge in the form of the drill slit. Davis (1994a) also advises that minimum tillage and straw retention techniques can help slugs survive and make seedlings more susceptible to damage.

Good hygiene will improve the effectiveness of other methods, particularly baiting (Davis 1994a). Hunter (1978) suggests that the amount of trash left on the surface after harvesting crops has an influence on the density of populations in the following year. The reason given for this effect is that the intensity of the controlling effect exerted by extremes of weather or by predators depends to a considerable extent on the amount of shelter available to the slug population.

Barriers

Physical barriers such as continuous lines of sawdust or ash provide a dry surface which slugs avoid (Davis 1994a), however the effectiveness of these barriers is reduced once they become wet. Lines of lime and copper sulphate are repellent and Davis (1994a) suggests that these can be used to prevent migration into an area. Lush (2002) reports that bands of copper sheeting successfully prevent snails from entering tree canopies.

Biological control

Pest slugs in Australia are an introduced species, hence there are limited biological control agents. Some predatory beetles and lizards feed on slugs (Davis 1994a), but birds and rats are the most effective at controlling slugs. Ducks, chickens and Guinea fowl can provide effective long-term control in orchards and vineyards (Davis 1994a).

In field experiments in organic farming carried out with the nematode biocontrol agent Phasmarhabditis hermaphrodita, Speiser and Andermatt (1996) concluded that P. hermaphrodita is a promising alternative to chemical molluscicides.

Reidenbach et al. (1989) explored the possibilities of using Sciomyzidae (Diptera) as biological control agents for crop pest molluscs with some success. Symondson (1989) reported that carabid beetles reduced slug populations by up to 80%. Carabid beetles have also been shown to be at least as effective as methiocarb at controlling slugs in grass/clover swards (Asteraki 1993)

After investigating proteinase activities present in crop, digestive gland and salivary gland extracts of the pest slug species Deroceras reticulatum, Walker et al. (1998) suggest that the expression of phytocystatins in transgenic plants may be an alternative method for controlling slug populations in the field.


Sampling Several methods of estimating slug populations have been developed, but most are labour intensive or have other undesirable characteristics that make them unsuitable in some situations. The first accurate means of assessing absolute slug populations was developed by South (1964). This method depended on washing and flooding of soils. Although it was time consuming, this method enables accurate determination of the slug population, including adults, juveniles and eggs. Barker and Pottinger (1982) refined the technique by reducing the size of sample units.

Refuge traps

Refuge traps have been constructed from a wide range of materials. Byers et al. (1989) used a 30 cm x 30 cm fibre glass roof shingle wrapped in aluminium foil, while Clements and Murray (1991) tried an inverted saucer of 15 cm diameter and a 50 cm x 50 cm piece of wadding. Young et al. (1994) investigated the use of a plate technique and out of many alternatives they found the best was simply a 40 cm x 40 cm hardboard square, shiny side up. They also investigated the addition of non-toxic baits and found layer mash to be the best. The best results were achieved with the combination of hardboard square and layer mash.

Defined area traps (DATs)

Ferguson et al. (1989) developed the defined-area trap (DAT) and demonstrated it was equally as accurate as the methods of both South (1964) and Barker and Pottinger (1982). The DAT apparatus consists of a galvanised cylinder 357 mm diameter and 150 mm tall hammered into the ground to a depth of 40-50 mm. Each ring encloses an area of 0.1 m2. After cutting and removal of vegetation, wet sacking is placed inside each cylinder to maintain a humid atmosphere. A hardboard cover painted white is placed on top of the cylinder to keep the environment cool and dark and to keep the slugs from escaping. A weight is placed on top of the hardboard to ensure a tight fit. Slugs in the soil accumulate on the underside of the sacking and can be simply counted on lifting the sacking. DATs do not operate well when soil is frozen (Byers et al. 1989)

Clements and Murray (1991) examined the efficiency of DAT and two different kinds of refuge traps and found the refuge traps gave a good, but over-estimated, assessment of the density of slugs by a factor of about two. As well the refuge traps tended to be biased towards larger slugs. In comparing DAT and refuge traps, Byers et al (1989) also concluded that refuge traps, which measure surface activity, gave a good general assessment of the slug population but tended to underestimate density of grey field slug in spring, whereas in autumn they overestimated slug density. The reasons given for these differences are that in spring slugs haven’t yet become active on the surface and are mostly juveniles which are less mobile than adults. In autumn slugs may seek refuge from the cold, and hence the traps provide an overestimate of population numbers. These authors suggest that refuge traps are not sufficient for estimating density and probably should be restricted to relative comparisons between pesticide treatments.


Project objectives

This project aimed to examine a number of strategies to provide information for the development of an integrated package for slug control. Although lettuce growing in Tasmania is being used as the model system, it is anticipated that management tools and control principles developed will be readily transferred to other vegetable and horticultural crops in temperate Australia.

Achievement of objectives

Due to unseasonal weather conditions in Tasmania during the period of this project, not all field work could be conducted. However to compensate, additional small plot trials were undertaken and laboratory studies on the effect of caffeine and carvone on slug mortality were also included.

Materials and Methods

(i) Grower survey A grower survey (appendix 1) was developed with the assistance of DPIWE Entomologist Lionel Hill. The survey specifically targeted growers of leafy vegetables such as lettuce, brassica and Asian greens, and aimed to clarify the extent of the problem in different growing regions, drawing together the practices currently used for slug control by growers in the different regions. Copies of the survey were distributed to growers in each state through the Industry Development Officers.

(ii) Replicated small plot trials A number of trials were conducted to determine the impact of various factors on slugs. Experimental design varied slightly between trials, but treatments were replicated in all trials, with the number of replicates varying from 3 to 10. The trials conducted were: 1. slug preference for food type 2. comparison of commercial slug baits for efficacy 3. effect of time and weathering on bait efficacy 4. impact of weathered baits combined with a food source on slug mortality 5. food source palatability/toxicity 6. preliminary barrier study 7. development of mechanical barriers 8. the effect of humidity levels on bait efficacy 9. slug population study in cultivated vs non-cultivated soils 10. assessment of damage caused by different slug species 11. laboratory examination of caffeine and carvone as slug ovicides 12. effect of biofumigation on slug populations


13. efficacy of slug baits following exposure to UV light 14. preliminary examination of caffeine and carvone as slug control agents 15. further studies on the impact of caffeine and carvone on slug mortality

Trials are not described in order conducted, but rather have been grouped according to study type.

A. food type / cultural / slug species 1. slug preference for food type 5. food source palatability/toxicity 9. slug population study in cultivated vs non-cultivated soils 10. assessment of damage caused by different slug species 12. effect of biofumigation on slug populations B. barrier trials 6. preliminary barrier study 7. development of mechanical barriers C. baiting efficacy 2. comparison of commercial slug baits for efficacy 3. effect of time and weathering on bait efficacy 4. impact of weathered baits combined with a food source on slug mortality 8. the effect of humidity levels on bait efficacy 13. efficacy of slug baits following exposure to UV light D. caffeine/carvone studies 11. laboratory examination of caffeine and carvone as slug ovicides 14. preliminary examination of caffeine and carvone as slug control agents 15. further studies on the impact of caffeine and carvone on slug mortality

A. FOOD TYPE / CULTURAL / SLUG SPECIES

Trial 1: susceptibility of a range of leafy green vegetables

This trial assessed the preference of slugs for a particular crop when given a choice. Black plastic pots of 20 cm diameter were filled with a standard potting mix and planted with six plants, one each of: iceberg lettuce, red oakleaf lettuce, green oakleaf lettuce, rocket, Muzuna, and Tatsoi. Half the pots were left with a clean surface while the other half had the surface covered with plant debris (cut up lettuce). In total there were 12 treatments with 10 replicates per treatment. Four slugs (Deroceras reticulatum) were placed in each pot. To prevent the slugs escaping, each pot was covered with a clear plastic lid with a gauze insert to allow air movement (see Figure 1). Plant damage was recorded using the scale in Table 1. Assessments were undertaken from 7 days after introduction of the slugs to the pots through to 20 days.


Table 1: Plant damage rating scale Rating Degree of damage 1 No damage 2 1 – 20% eaten 3 21 – 40% eaten 4 41 – 60% eaten 5 61 – 80% eaten 6 81 – 100% eaten 7 Dead

Trial 5: Food source palatability/toxicity trial

Following on from the results of Trial 1 where Tatsoi appeared to be toxic to the slugs, a smaller trial was undertaken to confirm this finding. Pots were planted with either 3 Tatsoi plants or 3 lettuce plants, a control treatment with no plants was also included. Trial design was a randomised complete block with 5 replicates per treatment. Four slugs (Deroceras reticulatum) were placed in each pot at day 0. Assessments were undertaken at day 1, day 3, day 5, day 7 and day 14.

Trial 9: slug population study in cultivated vs non-cultivated soil

A study was made of slug populations in cultivated and non-cultivated soils. The cultivated area was also herbicided to prevent weed growth. The non- cultivated area was ley pasture that was regularly mown. Six refuge traps were placed in each area and assessments undertaken weekly, counting all slugs present under traps. Slug species located and counted were: 1. Deroceras reticulatum 4. Milax gagetes 2. Deroceras panormitanum 5. Arion hortensis 3. Lehmannia nycetlia

Trial 10: assessment of damage caused by different slug species

An assessment was made of the damage caused by four different slug species (Figure 2): 1. Deroceras reticulatum 2. Deroceras panormitanum 3. Lehmannia nycetlia 4. Milax gagetes

Each pot was planted with 1 plant each of iceberg lettuce, cascade lettuce and tatsoi. Trial design was a randomised complete block with 4 replicates per treatment. Four slugs were placed in each pot at day 0. Assessments were undertaken at day 1, day 3, day 5, day 7 and day 14.

Trial 12: Effect of biofumigation on slugs

Due to an extremely wet winter and spring, this trial was unable to be planted.


B. BARRIER TRIALS

Trial 6: Preliminary barrier study

There are many materials that can be used for barriers and repellents against slugs, some of these materials are quite common. In this experiment several of these materials were observed for their efficacy in forming a barrier against slug invasion and escape. Choice of materials for these observations was dependent on cost, application suitability, weather restraints and availability. The materials chosen were: 1. Copper fungicide (copper oxychloride) 2. SOCUSIL (copper complex spray) 3. Vaseline (petroleum gel) 4. Aluminium Foil (household grade) 5. Copper sheet (0.9 guage, new) 6. Galvanised tin sheet (0.9 guage, new) 7. Sandpaper 80 grade (aluminium oxide) 8. Sandpaper 80 grade (glass grit) 9. Sandpaper 80 grade (sand grit)

All treatments were applied to black plastic laid on a wooden laboratory bench. Where appropriate a stencil was used to create a 50 mm wide barrier enclosing an area of 900 cm2. Three slugs (Deroceras reticulatum) were released into the enclosed area and observed for 20 minutes.

Copper fungicide (copper oxychloride) was mixed with water at the label rate, sprayed onto the black plastic background and allowed to dry. SOCUSIL (copper complex spray) was sprayed onto the black plastic background and allowed to dry. Vaseline (petroleum gel) was smeared onto the black plastic background to create a barrier as described above.

The following materials were all was cut into four strips 50 mm wide and at least 30 cm long, and arranged on the black plastic to enclose a 900 cm2 area:- household grade aluminium foil; copper sheet (0.9 gauge, new); galvanised tin sheet (0.9 gauge, new); aluminium oxide impregnated 80 grade sandpaper; glass grit impregnated 80 grade sandpaper; and sand grit impregnated 80 grade sandpaper.

Following on from these observations, different grades of aluminium oxide sandpaper were examined using the same methodology.

Trial 7: Development of mechanical barriers

Based on the results in the preliminary observations, twelve different barrier types were examined in this trial (detailed below). Each treatment was replicated three times.


Each treatment was prepared on a black plastic background placed on the wooden laboratory bench (Figure 3). For treatments 1-8 the barriers were created using a cardboard stencil to form a 50 mm wide barrier enclosing an area of 900 cm2. Compounds for treatments 1-4 were prepared as indicated below. Liquid solutions were sprayed onto the background with the stencil in place to form the barrier. The barrier was allowed to completely dry. Barriers 9-12 were created by using four 50 mm wide strips of the appropriate materials and placing them together to enclose a square area of 900 cm2. Detailed methods of preparation for each treatment are given below.

Iron Sulphate: 2 g of iron sulphate was mixed with 500 ml water and poured into a spray bottle. Bordeaux Mix: 10 g Copper Sulphate and 10 g slaked lime were dissolved in 1 L water and poured into a spray bottle. Socusil (buffered copper complex) Garlic Spray: Garlic cloves were peeled and crushed, 50 g of the crushed garlic was mixed with 500 ml warm water. The mixture was left for 2 hours before separating the liquid from the garlic pulp. The liquid was poured into a spray bottle. Limil (slaked lime): sprinkled lightly. Dolomite (calcium magnesium carbonate): sprinkled lightly. Sawdust - green eucalypt: applied to an approximate depth of 1 cm (patted down). Sawdust - dried eucalypt: applied to an approximate depth of 1 cm. Copper Strip – unoxidised Copper Strip - oxidised: strips of copper placed into a tray of 5% vinegar and left for 24 hours. Sandpaper - aluminium oxide (Al2O3) Sandpaper - glass fragments

After each of the treatments were set up, four grey field slugs (Deroceras recticulatum) were introduced into the central 900 cm2 area. The slugs were placed separately into the four quarters of the inside area (Figure 3). Slugs were observed for 30 minutes and observations taken of their reactions to the barrier such as baulk, retreat, traverse, crossing and traverse/cross (Figure 3). Where a slug escaped from the enclosed area, it was picked up and returned to the enclosure. Toxicity effects were recorded 15 hours after treatment.

C. BAITING EFFICACY

Trial 2: bait efficacy

A comparison of the efficacy of a range of baits, including the traditional metaldehyde and methiocarb baits and the new iron chelate based bait Multiguard. Two crops were used in this trial – iceberg lettuce and Tatsoi (brassica), with 4 plants per pot. Four slugs (Deroceras reticulatum)


were placed in each pot. Experimental design was a split plot with 10 replicates per treatment, 5 replicates planted with each crop.

Treatment Label rates *Application rate for 270 cm2 1. BAYSOL Methiocarb 20g/kg (pellet) 100 pellets per m2 3 pellets 2. PESTMASTER Metaldehyde 15g/kg (pellet) 5 grams per m2 3 pellets 3. DEFENDER Metaldehyde 15g/kg (pellet) 5 grams per m2 3 pellets 4. BLITZEM Metaldehyde 18g/kg (granule) 6 grams per m2 13 granules 5. MULTIGUARD Iron EDTA Complex 60g/kg (pellet) 25 kilos per ha 1 pellet 6. Fe lll EDTA GO1 (pellet) ** 7.5 kilos per ha 3 pellets 7. Fe ll EDTA GO1 (pellet) ** 7.5 kilos per ha 3 pellets 8. Untreated Control nil nil * surface area of each pot ** trial product supplied by Multicrop

Trial 3: Effect of time and weathering on bait efficacy

Examined the efficacy of a number of the baits used in Trial 2, with the additional factors of time of exposure (weathering) and water (wet or dry). Baits were weathered for either 4, 2, 1 or 0 weeks with or without simulated rain, before placing slugs into pots. Rain was simulated by overhead watering for 10 minutes per day. There was no alternate food source other than baits in this trial. The same potting mix as used in trials 1 and 2 was used in each pot. Four slugs (Deroceras reticulatum) were placed in each pot. Each treatment was replicated four times.

Trial 4: Bait weathering trial with additional food source

Following on from Trial 3, this trial compared the efficacy of weathered and fresh baits with a food source present. Weathered baits were exposed to light and simulated rain for 2 weeks prior to slug introduction. Baits used in this trial were: - Baysol Methiocarb 20 g/kg pellets - Defender Metaldehyde 15 g/kg pellets - Blitzem Metaldehyde 18 g/kg granules - Multiguard Iron EDTA Complex 60 g/kg pellets - Fe III EDTA GO1 pellets

All pots were planted with 3 Tatsoi plants. Trial design was a randomised complete block with 4 replicates per treatment. An unbaited control was also included. Four slugs (Deroceras reticulatum) were placed in each pot at day 0. Assessments were undertaken at day 1, day 3, day 5, day 7 and day 14.

Trial 8: Bait x humidity


Following on from the earlier baiting trials, a comparison was made with closed pots (as used in the previous trials) and open pots with a slug proof barrier (Figure 4). Closed pots represent high humidity while open pots are at ambient humidity. A split plot design was used with 8 replicates per treatment, 4 replicates planted with tatsoi and 4 left without. Four slugs (Deroceras reticulatum) were placed in each pot at day 0. Assessments were undertaken on days 1, 3, 5, 7 and 14.

1 Open, tatsoi, baysol 9 Closed, tatsoi, baysol 2 Open, tatsoi, defender 10 Closed, tatsoi, defender 3 Open, tatsoi, multiguard 11 Closed, tatsoi, multiguard 4 Open, tatsoi, control 12 Closed, tatsoi, control 5 Open, baysol 13 Closed, baysol 6 Open, defender 14 Closed, defender 7 Open, multiguard 15 Closed, multiguard 8 Open, control 16 Closed, control Trial 13: Efficacy of slug baits after exposure to UV light

Following on from previous trials, this trial compared the efficacy of baits exposed to UV light with fresh baits. Baits were exposed to UV light for 1, 2 or 4 weeks prior to slug introduction. The baits used were: BAYSOL Methiocarb, 20 g/kg pellets (100 pellets per square metre) DEFENDER Metaldehyde, 15 g/kg pellets (5 grams per square metre) MULTIGUARD Iron EDTA Complex, 60 g/kg pellets (25 kilos per hectare)

Trial design was a randomised complete block with 4 replicates per treatment. Four slugs (Deroceras reticulatum) were placed in each pot at day 0. Assessments were undertaken 1, 3, 5 and 7 days after introduction of the slugs.

D. CAFFEINE/CARVONE STUDIES

Trial 11: Laboratory examination of caffeine and carvone as slug ovicides

Caffeine and carvone (both positive and negative forms) were examined in this trial for their effects on slug eggs and compared with an untreated control. Caffeine was applied at either 0.5 or 2.0 g per 100 ml distilled water. Carvone concentrations were either 0.05 or 0.5 ml/100 ml distilled water.

Each treatment was replicated 4 times. Containers were lined with moist cardboard and held at 18°C and 100% humidity. Twenty eggs of Deroceras reticulatum were placed in each container and examined at weekly intervals.


Trial 14: Preliminary examination of caffeine and carvone as slug control agents

Caffeine and carvone (both positive and negative forms) were examined in this trial for their effects as a soil drench. Caffeine was applied at either 0.5 or 2.0 g per 100 ml distilled water. Carvone concentrations were either 0.05 or 0.5 ml/100 ml distilled water.

As for previous trials, 200 mm pots filled with a standard potting mix were wetted until they had reached field capacity. Four slugs (Deroceras reticulatum) were placed in each pot and given no shelter so they would bury themselves in the potting soil. Solutions of caffeine and carvone were formulated as above and 250ml of solution applied to each pot using a small watering can with a shower rose.

Trial design was a randomised complete block with 4 replicates per treatment. Four slugs were placed in each pot at day 0. Assessments were undertaken 1, 3 and 5 days after drenching.

Trial 15 : Efficacy of caffeine and carvone

Using the results of trial 14, a further examination was conducted with caffeine and carvone (both positive and negative forms) to determine the most effective concentration range. Caffeine was applied at either 0.25, 0.5, 1.0 or 2.0 g per 100 ml distilled water. Carvone concentrations were 0.1, 0.25 or 0.5 ml/100 ml distilled water.

As for previous trials, 200 mm pots filled with a standard potting mix were wetted until they had reached field capacity. Four slugs (Deroceras reticulatum) were placed in each pot and given no shelter so as they would bury themselves in the potting soil. Solutions of caffeine and carvone were formulated as above and 250ml of solution applied evenly to the surface of each pot using a small watering can with a shower rose.

Trial design was a randomised complete block with 4 replicates per treatment. Four slugs were placed in each pot at day 0. Assessments were undertaken 1, 3 and 5 days after drenching.

Data analysis

Data was subjected to analysis of variance using Genstat 5 (Rothamsted Experimental Station, Harpenden, Herfordshire, UK). Significance was calculated at p = 0.05 and least significant difference (LSD) was used for comparison of mean values in the tables and figures. Data are presented as mean values for each treatment combination. Means in the tables followed by the same letter within each column are not significantly different at the 0.05 probability level using Fisher’s LSD test.


Figure 1: showing design of slug-proof containers used in pot trials.

(a) View of covered pots

(b) top view of slug proof lid showing gauze placement

(c) Modified lids with larger gauze insert as used in later trials.


Figure 2: Slug species used in trial 10.

(a) Deroceras reticulatum

(b) Deroceras panormitanum

(c) Lehmannia nycetlia

(d) Milax gagetes


Figure 3: Barriers.

(a) Limil (b) Aluminium oxide sand paper

(c) Slugs placed inside barrier

(d) Reaction to oxidised copper strip


Figure 4: Baiting trials.

(a) Aluminium oxide sandpaper barrier

(b) humidity trial – open vs closed pots

(c) Deroceras reticulatum feeding on baysol bait


Figure 5: Slug reactions to caffeine and carvone soil drenches.

(a) Reaction to 0.5% caffeine

(b) Reaction to 0.05% carvone (-)

(c) Reaction to 0.5% carvone (+)


Results

(i) Grower survey A total of 1,841 surveys were distributed across all states (Table 2). However only 9.2 were returned. Queensland and Tasmania had the highest return rates, 14.6 and 11.9% respectively. Victoria (4.6%), South Australia (5%) and New South Wales (5.6%) had the lowest returns.

Table 2: Surveys distributed and returned: State IDO No. distributed No. returned % returned Tasmania Roger Tyshing 700 83 11.9 Queensland Samantha Heritage 191 28 14.6 New South Wales Alison Anderson 300 17 5.6 South Australia Craig Feutrill 100 5 5.0 Western Australia David Ellement 400 29 7.2 Victoria Patrick Ulloa 150 7 4.6 1,841 169 9.2

Of the 169 surveys returned, 27% of growers had frequent problems with slugs, 50% had problems sometimes, and 23% had no problems. Figures for each state are given in Table 3. Although the data suggests that slug problems are worse in South Australia than in any other state, with only 4 surveys returned, this data may not be representative of the state as a whole.

Table 3: percentage growers with slug problems in each state (from returned surveys)

Slug problems NSW Vic Qld SA WA Tas

Frequent 25 33 0 75 41 31 Occasional 56 67 45 25 36 53 Never 19 0 55 0 23 16

Crops that were listed as having the most problems with slugs were: asparagus, brussel sprouts, cabbage, cauliflower, lettuce, pasture and poppies (Table 4). From question 1, those crops that were regarded as having the least problems were: beans, capsicum, celery, grain/cereals, onion, garlic, spring onions, potato, melons, pumpkin, squash, zucchini, sweet-corn and tomato. This data was confirmed by the response to question 6.

There was a range of slug species in all states. From the descriptions given, Deroceras reticulatum appeared to be prevalent in most states, however it was too difficult to successfully identify other species from the descriptions given.

Growers with fewer slug problems tended to be from drier regions or those who used cultivation and/or burning between crops. Damage was related to rainfall distribution in Victoria, Western Australia and Tasmania. Several growers noted that slug problems were increasing each year.


In general, where minimum tillage was used or organic matter added to the soil through composting, green crops etc there were problems. Most damage occurred during winter and spring. Growers from Victoria, Queensland and South Australia also listed autumn as being problematic. Several growers noted that long damp seasons favoured build-up of slug populations, and if farms are continually cropped and never allowed to dry out the problem increases.

Baiting was a common control measure, although there were varying degrees of success. Use of bait stations to monitor slug populations was successful in assisting with timing of baiting. A fallow period before planting and rolling of seedbeds also reduced slug populations. Weed control was a common control measure across all states.

The majority of growers did not use boundary control measures. However those growers who did reported that it assisted in reducing slug populations. Measures ranged from cultivation of headlands, slashing, baiting, spraying of fencelines, or keeping free of weeds and grass. It was also noted that gully lines appeared to be more important than headlands.

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-

x

1

Her

bs

-

x

1

Lettu

ce

7

x

5

Luce

rne

-

x

1

Mel

ons

1

x

2

Oni

ons

-

x

1

Peas

-

x 2

Po

tato

-

x 5

Pu

mpk

in

-

x

1

Spin

ach

-

x

2

Squa

sh

-

x

1

Leek

s 2

x -

Le

ttuce

1

x -

O

nion

1

x -

Pa

rsle

y -

x 1

Po

tato

-

x 1

R

adis

h 1

x -

Si

lver

beet

1

x -

Sp

inac

h 1

x -

Sp

ring

onio

n -

x 1

Her

bs

-

x

1

Leek

s -

x 1

Le

ttuce

3

x -

Lu

cern

e -

x 1

M

elon

s -

x 1

O

nion

-

x 5

Pe

as

-

x

2

Pota

to

-

x

8

Silv

erbe

et

-

x

1

Gra

in/c

erea

ls

5

x

24

Gra

ss se

ed

3

x

2

Oni

ons

3

x

13

Pac

choy

1

x -

Pa

rsle

y 2

x 1

Pa

stur

e 3

x -

Pe

as

9

x

14

Popp

ies

48

x

8

Pota

to

3

x

36

Pum

pkin

-

x 1

Py

reth

rum

8

x 6

Sq

uash

-

x 1

Fina

l Rep

ort V

G00

030

Tasm

ania

n In

stitu

te o

f Agr

icul

tura

l Res

earc

h pa

ge 3

5

Que

stio

n N

SW (2

000)

V

ic (3

000)

Q

ld (4

000)

SA

(500

0)

WA

(600

0)

Tas

(700

0)

Stra

wbe

rry

1

x

-

Swee

tcor

n 2

x 2

Swee

tcor

n -

x 3

Swee

t pot

ato

-

x

1

Swee

tcor

n -

x 3

To

mat

o -

x 9

V

eget

able

s -

x 2

Zu

cchi

ni

1

x

1

Zucc

hini

-

x 1

Swed

e/tu

rnip

-

x 1

Sw

eetc

orn

-

x

6

Tom

ato

-

x

3

Zucc

hini

-

x 1

Swed

e/tu

rnip

-

x 3

W

asab

i 1

x -

Slug

pro

blem

s Fr

eque

ntly

Som

etim

es

N

ever

4 9 3

2 4 0

0 10

12

3 1 0

9 8 5

23

39

12

Tim

e of

yea

r dam

age

suff

ered

Pr

edom

inan

tly w

inte

r /

sprin

g A

utum

n –

sprin

g W

et p

erio

ds

Aut

umn

– sp

ring

Wet

per

iods

A

utum

n –

win

ter

Win

ter –

sprin

g W

inte

r – sp

ring

Is d

amag

e re

late

d to

ye

s ra

infa

ll di

strib

utio

n?

no

3 5 6 1

4 4 1 0

10

4 22

11

A

re so

me

crop

s mor

e

yes

susc

eptib

le th

an o

ther

s?

no

5 4 5 2

8 1 1 0

16

1 39

2

I

f so,

whi

ch c

rops

Cab

bage

C

aulif

low

er

Leaf

y cr

ops

Lettu

ce

Arti

chok

es

Asp

arag

us

Cau

liflo

wer

C

eler

y Le

ttuce

Cab

bage

C

eler

y C

hine

se c

abba

ge

Lettu

ce

Bru

ssel

spro

uts

Cab

bage

C

arro

t C

hine

se c

abba

ge

Lettu

ce

Asp

arag

us

Cau

liflo

wer

Le

ttuce

Can

ola

Cau

liflo

wer

Pa

stur

e Po

ppie

s Py

reth

rum

Is

dam

age

wor

se a

fter

yes

certa

in c

rops

in a

rota

tion?

no

0 7

1 2 0 9

0 1 2 11

19

15

If y

es, t

hen

whi

ch c

rops

/ ro

tatio

ns

Lettu

ce to

cau

liflo

wer

•

Dou

ble

crop

ped

caul

iflow

er

• ca

nola

• A

fter p

astu

re,

popp

ies o

r gra

in

• D

irect

dril

ling

Wha

t typ

e of

dam

age

occu

rs

See

ds /

un-e

mer

ged

plan

ts

Cot

yled

ons /

1st le

aves

S

eedl

ings

L

ater

gro

wth

stag

es

0 2 5 11

1 2 2 3

2 3 7

0 0 1 2

0 3 4 15

6 34

23

13

Fina

l Rep

ort V

G00

030

Tasm

ania

n In

stitu

te o

f Agr

icul

tura

l Res

earc

h pa

ge 3

6

Que

stio

n N

SW (2

000)

V

ic (3

000)

Q

ld (4

000)

SA

(500

0)

WA

(600

0)

Tas

(700

0)

Do

you

burn

ye

s be

twee

n cr

ops?

so

met

imes

1 1

- - - 1

- - - -

7 11

Do

you

culti

vate

ye

s be

twee

n cr

ops?

so

met

imes

11

-

3 - 16

-

1 - 14

1

39

8 D

o yo

u us

e ye

s m

inim

um ti

llage

?

som

etim

es

6 - 1 -

4 1 - -

3 - 16

10

D

o yo

u gr

ow c

over

ye

s / g

reen

man

ure

crop

s?

so

met

imes

4 1

1 - 9 -

1 - 3 1

22

10

Do

you

inco

rpor

ate

yes

prev

ious

cro

p/ st

ubbl

e?

som

etim

es

8 1 2 -

9 - - 1

15

1 33

2

Do

you

appl

y ye

s co

mpo

sts?

so

met

imes

3 -

1 1 2 1

- - 3 -

5 - C

ontro

l mea

sure

s use

d

• B

ayer

pel

lets

•

Mes

urol

pel

lets

•

Uns

peci

fied

baits

•

No

cont

rol u

sed

by

sign

ifica

nt n

umbe

r of

grow

ers w

ith

occa

sion

al p

robl

ems

• W

et su

lphu

r kel

ps

& c

oppe

r

• B

aits

•

Wee

d co

ntro

l •

No

cont

rol u

sed

by

sign

ifica

nt n

umbe

r of

grow

ers w

ith

occa

sion

al p

robl

ems

• W

eed

cont

rol &

ba

iting

• B

aitin

g lit

tle &

of

ten

with

Mes

urol

, m

ethi

ocar

b •

Cop

per s

pray

s •

Mow

ing

and

baiti

ng

• C

rop

rota

tion

&

mai

ntai

n so

il ba

lanc

e •

Pelle

ts &

spra

y •

pelle

ts e

very

3

wee

ks fr

om A

ug to

Se

ptem

ber

• pe

llets

, wee

d co

ntro

l, cl

ear h

eadl

ands

•

plan

t cro

ps a

way

fr

om d

rain

s &

vege

tate

d ar

eas

• fla

me

wee

d •

culti

vatio

n •

Mes

urol

and

m

etal

dehy

de p

elle

ts

mos

t com

mon

•

Rai

n di

ssol

ves

man

y pe

llets

•

Cop

per s

pray

s, m

esur

ol n

ot v

ery

effe

ctiv

e

• B

ait s

tatio

ns to

m

onito

r •

Pelle

ts

• R

oll s

eed

beds

•

Bur

n st

ubbl

e •

Cul

tivat

e pr

e-pl

antin

g •

Bai

t bef

ore

emer

genc

e •

Spra

y fe

ncel

ines

to

redu

ce h

abita

t •

Wee

d co

ntro

l •

Met

alde

hyde

bai

ts

mos

t com

mon

and

su

cces

sful

Bou

ndar

y co

ntro

l mea

sure

s

• B

aitin

g •

Rot

ary

hoe

head

land

s •

Kee

p fr

ee o

f wee

ds

& g

rass

•

Slas

h he

adla

nds

• M

ow –

but

can

’t co

ntro

l wha

t com

es

• B

aitin

g •

Wee

d co

ntro

l •

No

cont

rol u

sed

by

sign

ifica

nt n

umbe

r of

grow

ers

• N

o co

ntro

l use

d by

si

gnifi

cant

num

ber o

f gr

ower

s •

Slas

h &

her

bici

de

• M

ow

• W

eed

cont

rol

• B

aitin

g

• B

aitin

g •

Mow

ing

• N

o co

ntro

l use

d by

si

gnifi

cant

num

ber o

f gr

ower

s •

Bai

ting

• C

ultiv

ate

perim

eter

•

Spra

y fe

ncel

ines

• N

o co

ntro

l use

d by

si

gnifi

cant

num

ber o

f gr

ower

s •

Leav

e 1-

2m b

are

earth

•

Rem

ove

any

cove

r •

Slas

h •

Bai

ting

Fina

l Rep

ort V

G00

030

Tasm

ania

n In

stitu

te o

f Agr

icul

tura

l Res

earc

h pa

ge 3

7

Que

stio

n N

SW (2

000)

V

ic (3

000)

Q

ld (4

000)

SA

(500

0)

WA

(600

0)

Tas

(700

0)

over

pro

perty

bou

ndar

y •

Stub

ble

rem

oval

•

No

cont

rol u

sed

by

sign

ifica

nt n

o. o

f gr

ower

s Sl

ug sp

ecie

s on

prop

erty

•

rang

e of

spec

ies

•

rang

e of

spec

ies

• ra

nge

of sp

ecie

s •

rang

e of

spec

ies

• ra

nge

of sp

ecie

s •

rang

e of

spec

ies

Furth

er c

omm

ents

• le

ss c

ultiv

atio

n gr

eate

r the

pro

blem

•

serio

us p

robl

em

• sl

ugs w

orse

if

cove

r cro

p gr

own

&

plou

ghed

in

• lo

ng d

amp

seas

on

favo

urs b

uild

up

• sl

ugs m

ostly

pr

esen

t on

head

land

s

• ne

ed to

bai

t bef

ore

prob

lem

occ

urs

• C

u m

oves

slug

s off

th

e cr

op

• M

ust i

nspe

ct w

hen

wea

ther

suits

slug

s •

Bai

ts w

ork

if ap

plie

d of

ten

enou

gh

but e

xpen

sive

• B

est c

ontro

l ap

pear

s to

be a

fallo

w

perio

d be

fore

pla

ntin

g •

Prob

lem

whe

n pl

ant i

nto

grou

nd th

at

has n

ot b

een

left

clea

r •

Impo

rtant

to

com

men

ce b

aitin

g ea

rly –

eve

n w

hen

no

sign

of s

lugs

•

Prob

lem

s with

coo

l w

eath

er &

irrig

atio

n •

Cau

se si

g.

Econ

omic

dam

age

– un

able

to g

row

cro

ps in

so

me

year

s •

Slug

s wor

se w

here

w

eeds

get

aw

ay

• Sn

ails

– u

se

mur

iate

of p

otas

h (K

Cl)

trails

– sn

ails

die

whe

n cr

ossi

ng

• gu

lly li

nes a

ppea

r m

ore

impo

rtant

than

he

adla

nds

• fe

ncel

ines

goo

d ha

rbou

r •

stub

ble

rete

ntio

n le

ads t

o pr

oble

ms

• w

et su

mm

ers &

au

tum

ns p

robl

em

• sl

ug p

robl

em

incr

easi

ng

• ro

lling

to fi

rm

seed

beds

impr

oved

slug

pr

oble

m

• ne

ed a

n ea

sier

&

chea

per s

olut

ion

to

baits

•

if fa

rm c

ontin

ually

cr

oppe

d &

nev

er

allo

wed

to d

ry o

ut

prob

lem

incr

ease

s •

ferti

liser

spre

ader

m

ount

ed o

n 4W

D b

ike

mak

es b

ait s

prea

ding

ea

sy


(ii) Replicated small plot trials

A. FOOD TYPE / CULTURAL / SLUG SPECIES

Trial 1: Susceptibility of a range of leafy green vegetables

The presence of organic matter (OM) on the soil surface reduced the degree of damage to the growing plants (Table 5(i)), presumably by providing an alternate food source. There were significant differences between the six leafy green vegetables assessed (Table 5(ii)). The red and green oakleaf lettuces were the least preferred as a food source, followed by rocket and iceberg lettuce. Tatsoi received the greatest degree of damage, being eaten completely by day 9 where there was no surface OM present. Muzuna also received a high degree of damage.

Table 5: Susceptibility of a range of leafy greens to slug damage. (Rating: 1=no damage; 7=total destruction resulting in plant death). OM, organic matter.

Degree of damage to plants Day 7 Day 9 Day 13 Day 15 Day 20 (i) organic matter absent 2.16 b 3.47 b 3.87 b 3.94 b 4.36 b present 1.16 a 1.56 a 2.53 a 2.76 a 3.04 a lsd (p=0.05) 0.165 0.26 0.25 0.262 0.26 F prob. <0.001 <0.001 <0.001 <0.001 <0.001

(ii) plant Iceberg lettuce 1.25 a 2.01 b 3.15 b 3.46 b 4.19 b Rocket 1.56 b 2.42 b 3.27 b 3.49 b 3.96 b Red oakleaf lettuce 1.07 a 1.03 a 1.13 a 1.23 a 1.33 a Muzuna 1.93 c 3.62 c 4.88 c 4.91 c 5.18 c Green oakleaf lettuce 1.09 a 1.01 a 0.99 a 1.06 a 1.42 a Tatsoi 3.07 d 4.98 d 5.76 d 5.96 d 6.12 d lsd (p=0.05) 0.28 0.46 0.44 0.45 0.46 F prob. <0.001 <0.001 <0.001 <0.001 <0.001

(iii) organic matter * plant Iceberg lettuce 1.52 bc 3.04 c 3.87 c 4.03 c 4.94 e Iceberg lettuce + OM 0.98 a 0.98 a 2.44 b 2.90 b 3.44 c Rocket 2.19 d 3.41 c 4.19 cd 4.35 cd 4.87 de Rocket + OM 0.93 a 1.43 ab 2.35 b 2.64 b 3.06 c Red oakleaf lettuce 1.15 ab 1.05 a 1.04 a 1.22 a 1.43 ab Red oakleaf lettuce + OM 1.00 a 1.00 a 1.21 a 1.24 a 1.24 ab Muzuna 2.72 e 5.25 d 6.09 e 5.93 e 6.08 f Muzuna + OM 1.14 ab 1.98 b 3.66 c 3.89 c 4.28 d Green oakleaf lettuce 1.19 ab 1.04 a 1.00 a 1.14 a 1.87 b Green oakleaf lettuce + OM 0.99 a 0.98 a 0.98 a 0.98 a 0.98 a Tatsoi 4.21 f 7.00 e 7.00 f 7.00 f 7.00 g Tatsoi + OM 1.92 cd 2.97 c 4.52 d 4.92 d 5.24 e lsd (p=0.05) 0.40 0.65 0.62 0.64 0.65 F prob. <0.001 <0.001 <0.001 <0.001 0.002


Trial 5: Food source palatability/toxicity trial

There was no significant difference between the control, lettuce or tatsoi treatments on slug mortality or injury (Table 6).

Table 6: Toxicity of lettuce and tatsoi plants following 14 days feeding.

% unaffected slugs Lettuce 85 Tatsoi 85 Control 100 lsd (p=0.05) ns F prob. 0.262

Trial 9: Slug population study in cultivated vs non-cultivated soil

Cultivated soil was not conducive as a slug habitat (Table 7). The number of slugs increased in the non-cultivated areas from August (late winter) through to mid November, with slug numbers being significantly higher in the non-cultivated areas compared with the cultivated areas at all assessment dates, except for the final assessment. Prolonged hot weather commenced mid November and this combined with the lack of rain resulted in a sharp reduction in slug numbers towards the end of November. Species present were: Arion hortensis, Deroceras panormitanum, Deroceras reticulatum, Lehmannia nycetelia, and Milax gagetes.

Table 7: The effect of cultivation on slug populations.

Mean number of slugs per trap Aug 19 Aug 26 Sep 2 Sep 9 Sep 16 Sep 23 Sep 30 Oct 7 Cultivated 0 a 0 a 0 a 0 a 0 a 0 a 0 a 0 a Non-cultivated 3 b 9 b 11 b 16 b 14 b 16 b 16 b 12 b lsd (p=0.05) 1 2 4 4 3 5 3 3 F prob. 0.005 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Mean number of slugs per trap Oct 14 Oct 21 Oct 28 Nov 4 Nov 11 Nov 18 Nov 25 Dec 2 Cultivated 0 a 1 a 1 a 0 a 0 a 0 a 0 a 0 Non-cultivated 13 b 19 b 16 b 17 b 6 b 22 b 2.3 b 3.5 lsd (p=0.05) 3 4 3 4 1 20 2 ns F prob. <0.001 <0.001 <0.001 <0.001 <0.001 0.041 0.040 0.241


Trial 10: Assessment of damage caused by different slug species

Deroceras reticulatum and Milax gagetes caused significantly more damage to plants than the other two species (Table 8(i)). Tatsoi again proved to be the food source of choice in preference to either Iceberg or Cascade lettuce (Table 8(ii)). At days 1 and 3, all slug species showed a low level of feeding on both Iceberg and Cascade lettuce compared with Tatsoi (Table 8(iii)). However at days 5 and 7 there were no differences between individual treatments. The only species not exhibiting major damage on the Tatsoi plants was Lehmannia nycetlia.

Table 8: Feeding preferences of different slug species. (Rating: 1=no damage; 7=total destruction resulting in plant death).

Degree of damage to plants Day 1 Day 3 Day 5 Day 7 (i) species Deroceras reticulatum 2.3 b 3.0 b 3.8 b 4.0 b Deroceras panormitanum 1.3 a 1.6 a 2.5 a 2.7 a Lehmannia nycetlia 1.1 a 1.3 a 1.6 a 2.1 a Milax gagetes 2.3 b 2.9 b 3.7 b 4.1 b lsd (p=0.05) 0.3 0.7 0.9 1.1 F prob. <0.001 <0.001 <0.001 0.001

(ii) plant type Iceberg lettuce 1.2 a 1.4 a 2.1 a 2.3 a Tatsoi 2.9 b 3.9 b 5.0 b 5.5 b Cascade lettuce 1.1 a 1.3 a 1.6 a 1.9 a lsd (p=0.05) 0.3 0.6 0.8 0.9 F prob. <0.001 <0.001 <0.001 <0.001

(iii) species * plant type Iceberg lettuce + D. reticulatum 1.5 ab 1.8 a 2.5 2.8 Iceberg lettuce + D. panormitanum 1.0 a 1.0 a 1.8 1.5 Iceberg lettuce + L. nycetlia 1.0 a 1.0 a 1.3 1.5 Iceberg lettuce + M.gagetes 1.3 ab 1.8 a 2.8 3.3 Tatsoi + D. reticulatum 4.3 c 5.5 b 6.3 6.3 Tatsoi + D. panormitanum 1.8 b 5.8 b 4.8 5.5 Tatsoi + L. nycetlia 1.3 ab 1.8 a 2.5 3.5 Tatsoi + M. gagetes 4.3 c 5.5 b 6.5 6.8 Cascade lettuce + D. reticulatum 1.3 ab 1.8 a 2.8 3.0 Cascade lettuce + D. panormitanum 1.0 a 1.0 a 1.0 1.0 Cascade lettuce + L. nycetlia 1.0 a 1.0 a 1.0 1.3 Cascade lettuce + M. gagetes 1.3 ab 1.5 a 1.8 2.3 lsd (p=0.05) 0.6 1.0 ns ns F prob. <0.001 0.001 0.114 0.539


No eggs were laid by Milax gagetes during the seven day assessment period (Table 9). Both Deroceras reticulatum and D. panormitanum laid eggs in all replicates, while Lehmannia nycetlia only laid eggs in 1 replicate.

Table 9: Egg laying of different slug species. The effect of baiting and humidity on egg laying (0 = no eggs present, 1 = eggs present).

Eggs present (Mean of each treatment) Day 1 Day 3 Day 5 Day 7 Deroceras reticulatum 0.50 b 0.50 bc 1.00 c 1.00 c Deroceras panormitanum 0.50 b 0.75 c 1.00 c 1.00 c Lehmannia nycetlia 0 a 0.25 ab 0.25 b 0.25 b Milax gagetes 0 a 0 a 0 a 0 a lsd (p=0.05) 0.31 0.33 0.18 0.18 F prob. <0.001 <0.001 <0.001 <0.001

Trial 12: Effect of bio-fumigation on slugs

No results due to unseasonal weather conditions


B. BARRIER TRIALS

Trial 6: Preliminary barrier study (observations only)

Copper fungicide, vaseline, aluminium foil, galvanised tin sheet, and sandpaper sheet impregnated with either glass or sand grit showed no repellent qualities and slugs escaped, the slugs exhibited no sensory or toxic reaction on contact with these barriers.

Socusil displayed good efficacy as a repellent with the slugs exhibiting a severe sensory and toxic reaction on contact with the barrier. Copper sheet showed slight repellent qualities but did not prevent slugs from escaping. The slugs exhibited slight sensory but no toxic reaction on contact with the copper barrier.

Sandpaper sheet impregnated with aluminium oxide was an excellent repellent and barrier, the slugs exhibited a severe sensory but no toxic reaction on contact with the barrier. The slug’s reaction was to limit the area of contact between it’s sole (foot) and the aluminium oxide coating. In the follow up observations with different grades of aluminium oxide sandpaper, there was a drop in efficacy as the grades of paper became finer.

Trial 7: Development of mechanical barriers

There wre no significant differences between treatments in the number of baulks (Table 10). However, the aluminium oxide sandpaper had the greatest number of touch and retreats. The oxidised copper strip, Limil, Socusil and the iron sulphate treatments also had high rates of touch and retreats.

Table 10: Reaction of slugs to different barrier materials.

Baulk Touch & retreat Traverse/cross 1. Iron Sulphate 4.67 10.67 abcd 1.00 f 2. Bordeaux Mix 1.33 9.67 bcd 11.67 abcde 3. Socusil 3.33 10.33 abcd 0.00 f 4. Garlic Spray 0.33 4.33 de 13.67 abcd 5. Limil 6.00 17.33 ab 0.00 f 6. Dolomite 2.00 4.00 de 17.67 ab 7. Sawdust - Green 3.33 6.00 de 4.00 cdef 8. Sawdust - Dried 5.00 9.00 cde 2.33 ef 9. Copper Strip - Unoxidised 3.00 7.67 cde 14.00 abc 10. Copper Strip - Oxidised 3.67 15.00 abc 0.00 f 11. Sandpaper - Aluminium Oxide 5.33 18.33 a 3.00 ef 12. Sandpaper - Glass Fragments 0.67 1.33 e 18.33 a Lsd (p=0.05) ns 7.72 10.06 F probability 0.116 0.002 0.001


The glass sandpaper, copper strip, dolomite, garlic spray, and Bordeaux mix treatments had the greatest number of traverse/cross measurements, while iron sulphate, Socusil, limil, oxidised copper strip, aluminium oxide sandpaper, and both green and dry sawdust were the most successful at preventing escape.

Although the toxic effects were not significant (Table 11), there were three cases of more than one slug being dehydrated and in some cases dead. Those treatments were iron sulphate, dried sawdust and the most reactive of all, the oxidised copper strip. All of the other treatments had none or no other toxic reactions.

Table 11: Toxic effect of barrier treatments on slugs 15 hours after exposure.

Trt Unaffected Slight Moderate Severe

1. Iron Sulphate 3.0 0.3 0.3 0.3 2. Bordeaux Mix 3.3 0.6 0.0 0.0 3. Socusil 3.6 0.3 0.0 0.0 4. Garlic Spray 4.0 0.0 0.0 0.0 5. Limil 4.0 0.0 0.0 0.0 6. Dolomite 4.0 0.0 0.0 0.0 7. Sawdust - Green 3.6 0.3 0.0 0.0 8. Sawdust - Dried 3.3 0.0 0.3 0.3 9. Copper Strip - Unoxidised 3.0 0.6 0.3 0.0 10. Copper Strip - Oxidised 2.0 0.6 0.6 0.6 11. Sandpaper - Aluminium Oxide 3.6 0.3 0.0 0.0 12. Sandpaper - Glass Fragments 4.0 0.0 0.0 0.0 Lsd (p=0.05) ns ns ns ns F probability 0.2 0.4 0.3 0.1


C. BAITING EFFICACY

Trial 2: Comparison of commercial slug baits for efficacy

Slugs in lettuce plots were more severely affected by the baits than those in the tatsoi plots (Table 12 & 13). As the previous trial demonstrated that slugs prefer tatsoi to lettuce as a food source, it is likely that slugs in the lettuce plots ingested more bait than those in the tatsoi plots, thus accounting for the greater effect in the lettuce plots.

Table 12: The effect of plant species and bait type on slugs

% slugs severely affected % dead slugs % affected slugs Day 1 Day 3 Day 5 Day 5 (i) plant Lettuce 70 b 68 b 48 b 86 b Tatsoi 45 a 39 a 34 a 64 a lsd (p=0.05) 10 10 9 8 F prob. <0.001 <0.001 <0.001 <0.001 (ii) Bait Baysol 93 e 85 d 58 de 90 cd Pestmaster 90 de 60 c 33 c 73 b Defender 83 de 60 c 13 ab 70 b Blitzem 73 d 70 cd 30 bc 80 bc Multiguard 23 b 65 cd 88 f 100 d Fe III EDTA 50 c 63 c 68 e 98 d Fe II EDTA 50 c 28 b 43 cd 90 cd Control 0 a 0 a 0 a 0 a lsd (p=0.05) 19 20 18 16 F prob. <0.001 <0.001 0.003 <0.001 (iii) plant * bait Lettuce + Baysol 95 c 75 ef 60 100 d Lettuce + Pestmaster 100 c 70 def 35 90 d Lettuce + Defender 85 c 85 f 15 100 d Lettuce + Blitzem 90 c 90 f 40 100 d Lettuce + Multiguard 20 a 95 f 95 100 d Lettuce + Fe III EDTA 85 c 85 f 85 100 d Lettuce + Fe II EDTA 85 c 45 cd 55 100 d Lettuce + Control 0 a 0 a 0 0 a Tatsoi + Baysol 90 c 95 f 55 80 c Tatsoi + Pestmaster 80 bc 50 cde 30 55 b Tatsoi + Defender 80 bc 35 bc 10 40 b Tatsoi + Blitzem 55 b 50 cde 20 60 bc Tatsoi + Multiguard 25 a 35 bc 80 100 d Tatsoi + Fe III EDTA 15 a 40 c 50 95 d Tatsoi + Fe II EDTA 15 a 10 ab 30 80 cd Tatsoi + Control 0 a 0 a 0 0 a lsd (p=0.05) 27 29 ns 23 F prob. <0.001 0.003 0.494 0.004


Baysol, Pestmaster and Defender were the fastest acting baits, with >83% of slugs severely affected in these treatments at day 1. However, by day 3 some slugs had recovered, no longer being in the severely affected category. By day 5 Multiguard was the most effective treatment, with 88% of slugs dead. The next most effective treatments were Fe III EDTA and Baysol.

Persistence assessments at day 14 showed that Multiguard was the most effective bait with 95% of slugs dead. Fe III EDTA was the next most effective bait with an 80% kill rate. Baysol resulted in a 73% kill rate, followed by, in order of effectiveness, Fe II EDTA, Blitzem, Pestmaster and Defender. The 20% kill rate seen in the control, ie no baits (Table 13(ii)), was the result of dead slugs in the tatsoi plots, not the lettuce plots (Table `3(iii)). This suggests that tatsoi itself may be toxic to slugs if it is the only available food source.


Table 13: The effect of plant species and bait type on slug persistence and presence of eggs. Eggs present: 0 = no, 1 = yes

Day 14 Day 14 Eggs present % slugs dead % slugs affected at day 14 (i) plant Lettuce 61 83 b 0.40 a Tatsoi 54 65 a 0.93 b lsd (p=0.05) ns 8 0.16 F prob. 0.208 <0.001 <0.001

(ii) Bait Baysol 73 d 78 bc 0.4 a Pestmaster 45 bc 80 bcd 0.7 abc Defender 33 ab 68 b 0.8 bc Blitzem 53 bcd 85 cd 0.6 ab Multiguard 95 f 95 d 0.6 ab Fe III EDTA 80 ef 88 cd 0.5 ab Fe II EDTA 65 cde 75 bc 0.7 abc Control 20 a 23 a 1.0 c lsd (p=0.05) 21 16 0.3 F prob. <0.001 <0.001 0.015

(iii) plant * bait Lettuce + Baysol 75 85 ef 0.0 a Lettuce + Pestmaster 50 100 f 0.4 abc Lettuce + Defender 40 95 f 0.6 bcd Lettuce + Blitzem 65 100 f 0.4 abc Lettuce + Multiguard 95 95 f 0.2 ab Lettuce + Fe III EDTA 90 95 f 0.0 a Lettuce + Fe II EDTA 75 85 ef 0.6 bc Lettuce + Control 0 0 a 1.0 d Tatsoi + Baysol 70 70 de 0.8 cd Tatsoi + Pestmaster 40 60 bcd 1.0 d Tatsoi + Defender 25 40 b 1.0 d Tatsoi + Blitzem 40 65 cde 0.8 cd Tatsoi + Multiguard 95 95 f 1.0 d Tatsoi + Fe III EDTA 70 80 def 1.0 d Tatsoi + Fe II EDTA 55 65 cde 0.8 cd Tatsoi + Control 40 45 bc 1.0 d lsd (p=0.05) ns 23 0.5 F prob. 0.097 <0.001 0.039

Trial 3: Effect of time and weathering on bait efficacy

Wet conditions reduced both the percentage slugs killed and the percentage slugs affected (Table 14). Fresh baits were more effective than older baits. Although a 93% kill rate was achieved with


baits that had been weathered for 4 weeks, there was a decline in the effectiveness of the baits with time.

At day 1, Blitzem and Defender (both metaldehyde baits) were the most effective, with Baysol (methiocarb) being the next most effective. However at day 3 and day 5, Multiguard showed the highest kill rate with > 80% of slugs dead. By day 14 all baits showed similar results, except for the Fe3 EDTA.

Table 14: The effect of weathering and time of exposure on bait efficacy

------------------% slugs dead----------------- -------------% slugs affected------------- Day 1 Day3 Day 5 Day 14 Day 1 Day 3 Day 5 Day 14 (i) rain exposure dry 14 69 b 81 b 99 b 98 b 100 b 100 b 100 b wet 13 46 a 58 a 92 a 90 a 87 a 88 a 98 a lsd (p=0.05) ns 9 7 3 5 4 4 1 F prob. 0.658 <0.01 <0.001 <0.001 0.002 <0.001 <0.001 0.002

(ii) Time 0 weeks 19 69 c 83 b 99 c 98 b 100 b 100 b 99 1 week 11 63 bc 75 b 97 bc 97 b 97 b 98 b 99 2 weeks 12 51 ab 64 a 94 ab 97 b 90 a 90 a 99 4 weeks 13 46 a 57 a 93 a 84 a 86 a 87 a 98 lsd (p=0.05) ns 12 10 4 7 6 5 ns F prob. 0.133 <0.001 <0.001 0.017 <0.001 <0.001 <0.001 0.306

(iii) bait Baysol 14 ab 66 c 83 c 97 b 100 c 100 c 100 b 100 b Blitzem 23 b 50 b 62 b 98 b 96 bc 92 b 96 b 99 b Defender 18 b 29 a 45 a 95 b 99 c 98 bc 97 b 99 b Fe3 EDTA 6 a 58 bc 70 b 89 a 87 a 80 a 80 a 96 a Multiguard 8 a 83 d 89 c 99 b 88 ab 96 bc 95 b 99 b lsd (p=0.05) 9 14 11 5 8 6 6 2 F prob. <0.001 0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.025

Trial 4: Impact of weathered baits combined with a food source on slug mortality

Plants in pots with weathered baits received less damage that those with fresh baits (Table 15), suggesting that weathered baits are more palatable to slugs than fresh baits. The level of damage in the fresh iron based baits (Multiguard and Fe EDTA) was similar to the untreated control.


Table 15: Susceptibility of tatsoi speedlings to slug damage with a range of fresh and weathered baits. (Rating: 1=no damage; 7=total destruction resulting in plant death).

Degree of damage to plants Day 1 Day 3 Day 5 Day 7 Day 14 Fresh Baysol 2.3 2.3 bc 2.5 abcde 3.0 bcd 3.5 bcd Fresh Defender 2.0 2.3 bc 3.0 cde 4.0 de 4.8 def Fresh Blitzem 2.0 2.3 bc 2.8 bcde 2.8 abc 3.8 bcde Fresh Multiguard 2.0 2.5 bc 3.3 de 3.3 cd 4.3 cdef Fresh Fe EDTA 2.3 2.8 c 3.5 e 4.0 de 5.3 ef Weathered Baysol 1.8 1.5 a 1.8 ab 1.8 a 1.8 a Weathered Defender 1.5 2.0 ab 2.0 abc 2.3 abc 2.8 abc Weathered Blitzem 1.5 1.3 a 1.5 a 1.8 a 3.0 abc Weathered Multiguard 1.8 2.0 ab 2.0 abc 2.0 ab 2.3 ab Weathered Fe EDTA 2.0 2.0 ab 2.3 abcd 2.0 ab 2.3 ab Control – no bait 2.0 2.8 c 3.5 e 4.8 e 5.8 f lsd (p=0.05) ns 0.7 1.0 1. 1 1. 6 F prob. 0.080 0.001 0.001 <0.001 <0.001

The fresh baits had a higher percentage of unaffected slugs than weathered baits (Table 16). Baysol, and weathered Multiguard and Fe EDTA treatments showed the least number of unaffected slugs. The rise and fall in the number of unaffected slugs from day 1 to day 14 suggests that slugs are able to recover depending on the degree to which they have been affected by the baits.

Table 16: Effect of fresh and weathered baits on slug survival.

Percentage of unaffected slugs Day 1 Day 3 Day 5 Day 7 Day 14 Fresh Baysol 13 ab 0 a 13 ab 31 bcd 19 ab Fresh Defender 63 de 31 ab 63 c 69 e 75 e Fresh Blitzem 25 abc 25 ab 25 b 38 cd 44 cd Fresh Multiguard 63 de 44 b 69 c 56 de 50 d Fresh Fe EDTA 81 ef 100 c 94 d 81 ef 75 e Weathered Baysol 0 a 0 a 0 a 0 ab 0 a Weathered Defender 0 a 13 ab 31 b 13 abc 25 bc Weathered Blitzem 0 a 0 a 19 ab 31 bcd 44 cd Weathered Multiguard 38 bcd 19 ab 19 ab 13 abc 13 ab Weathered Fe EDTA 50 cde 19 ab 13 ab 6 ab 6 ab Control – no bait 100 f 100 c 100 d 100 f 94 e lsd (p=0.05) 31 32 24 29 24 F prob. <0.001 <0.001 <0.001 <0.001 <0.001

Slug mortality increased with time for all treatments (Table 17). However, fresh Defender baits had no effect compared with the unbaited control. The weathered Fe EDTA and Multiguard treatments had the greatest effect on slug mortality (Table 17), closely followed by both fresh and weathered Baysol.


Table 17: Effect of fresh and weathered baits on slug death over 14 days.

Percentage of dead slugs Day 1 Day 3 Day 5 Day 7 Day 14 Fresh Baysol 6 13 50 cd 50 de 69 de Fresh Defender 0 0 0 a 0 a 6 a Fresh Blitzem 6 13 31 abcd 31 bcd 44 bcd Fresh Multiguard 0 0 13 ab 19 abc 31 abc Fresh Fe EDTA 0 0 0 a 6 ab 19 ab Weathered Baysol 0 6 19 abc 25 abcd 69 de Weathered Defender 0 13 13 ab 38 cd 50 cd Weathered Blitzem 6 13 19 abc 25 abcd 38 bc Weathered Multiguard 0 31 56 d 69 e 88 e Weathered Fe EDTA 0 31 44 bcd 69 e 94 e Control – no bait 0 0 0 a 0 a 6 a lsd (p=0.05) ns ns 33 28 30 F prob. 0.614 0.154 0.007 <0.001 <0.001

Trial 8: The effect of humidity levels on bait efficacy

Humidity levels had no effect on the level of slug mortality (Table 18(i). By day 3, 53% of slugs treated with Multiguard and 35% treated with Baysol were dead (Table 18(ii)). Multiguard proved to be the most effective bait, with 99% mortality by day 14. Baysol was the next most effective bait, followed by Defender with a 52% mortality rate.

Table 18: Impact of humidity and bait on the efficacy of slug baits.

Percentage of dead slugs Day 3 Day 5 Day 7 Day 14 (i) humidity Open (low) - - 47 61 Closed (high) - - 42 57 lsd (p=0.05) ns ns F prob. 0.375 0.556 (ii) bait Control 0 a 0 a 6 a 13 a Baysol 35 b 50 b 60 c 74 c Defender 8 a 5 a 24 b 52 b Multiguard 53 c 80 c 88 d 99 d lsd (p=0.05) 17 15 17 18 F prob. <0.001 <0.001 <0.001 <0.001


Seven and fourteen days after exposure to baits, treatments with higher humidity levels showed the greatest number of unaffected slugs (Table 19(i)). This is probably due to the slugs being able to recover from mild cases of poisoning under the higher moisture levels.

Although at day 1, Baysol and Defender had fewer unaffected slugs than Multiguard, by day 3 all baits showed similar levels of unaffected slugs (Table 19(ii)). However, from day 5 onwards, there was a greater number of unaffected slugs in the Defender treatment, suggesting that this bait was not as toxic as Baysol or Multiguard and the slugs were able to recover.

Table 19: Slug survival following exposure to baits under high and low humidity conditions.

Percentage of unaffected slugs Day 1 Day 3 Day 5 Day 7 Day 14 (i) humidity Open (low) - - 30 23 a 23 a Closed (high) - - 37 35 b 32 b lsd (p=0.05) ns 6 7 F prob. 0.090 ,0.001 0.008 (ii) bait Control 100 c 100 b 100 c 94 c 88 c Baysol 3 a 0 a 5 a 2 a 5 a Defender 13 a 5 a 23 b 17 b 16 b Multiguard 81 b 5 a 9 a 5 a 2 a lsd (p=0.05) 12 6 11 9 10 F prob. <0.001 <0.001 <0.001 <0.001 <0.001 (iii) humidity * bait Open + Control - - 94 91 c 84 c Open + Baysol - - 0 0 a 6 a Open + Defender - - 19 3 a 0 a Open + Multiguard - - 6 0 a 0 a Closed + Control - - 97 97 c 91 c Closed + Baysol - - 9 3 a 3 a Closed + Defender - - 28 31 b 31 b Closed + Multiguard - - 13 9 a 3 a lsd (p=0.05) ns 13 14 F prob. 0.938 0.037 0.008

Humidity level had no effect on whether the slugs laid eggs (Table 20(i)). Fewest eggs were laid in the Multiguard treatment (Table 20(iii)).


Table 20: The effect of baiting and humidity on egg laying (0 = no eggs present, 1 = eggs present).

Eggs present Day 7 Day 14 (i) humidity Open (low) 0.50 0.66 Closed (high) 0.63 0.78 lsd (p=0.05) ns ns F prob. 0.223 0.209 (ii) bait Control 0.9 b 1.00 b Baysol 0.5 a 0.69 a Defender 0.5 a 0.69 a Multiguard 0.3 a 0.50 a lsd (p=0.05) 0.28 0.28 F prob. <0.001 0.008

(iii) humidity * bait Open + Control 0.88 d 1.00 b Open + Baysol 0.38 abc 0.63 ab Open + Defender 0.25 ab 0.38 a Open + Multiguard 0.50 abc 0.63 ab Closed + Control 1.00 d 1.00 b Closed + Baysol 0.63 bcd 0.75 ab Closed + Defender 0.75 cd 1.00 b Closed + Multiguard 0.13 a 0.38 a lsd (p=0.05) 0.41 0.39 F prob. 0.029 0.023

Trial 13: Efficacy of slug baits following exposure to UV light

Methiocarb was not affected by UV exposure (Table 21), with 100% mortality by day 3 with both fresh and exposed baits. Multiguard showed similar results, although UV exposed baits did not achieve 100% mortality until days 5 or 7. Metaldehyde was the least effective bait when applied fresh. Exposure to UV for 1 week actually improved efficacy, however even after 7 days, slug mortality did not reach 100%. While the mortality levels in the control treatment cannot be fully explained, this may be due to natural aging of the slugs as slugs were collected from the field and age and health status were unknown.


Table 21: Effect of fresh and UV exposed baits on slug survival.

Percentage of dead slugs Day 1 Day 3 Day 5 Day 7 Control – no bait 0 6 a 13 a 13 a Metaldehyde – fresh 0 13 a 31 ab 31 ab Methiocarb – fresh 13 100 d 100 e 100 e Multiguard – fresh 6 100 d 100 e 100 e Metaldehyde – 1 week exposure 0 50 b 50 bc 63 c Methiocarb – 1 week exposure 6 100 d 100 e 100 e Multiguard – 1 week exposure 0 81 cd 100 e 100 e Metaldehyde – 2 week exposure 0 56 bc 69 c 69 cd Methiocarb – 2 week exposure 13 100 d 100 e 100 e Multiguard – 2 week exposure 0 88 d 88 de 88 de Metaldehyde – 4 week exposure 0 31 ab 50 bc 50 bc Methiocarb – 4 week exposure 6 100 d 100 e 100 e Multiguarb – 4 week exposure 0 88 d 94 e 100 e lsd (p=0.05) ns 26 24 27 F prob. 0.578 <0.001 <0.001 <0.001 <0.001

By day 3 there were no unaffected slugs in any of the Methiocarb or Multiguard treatments (Table 22). Metaldehyde baits, both fresh and exposed, were the least effective, with up to 38% unaffected slugs at day 7.

Table 22: Effect of fresh and weathered baits on slug survival.

Percentage of unaffected slugs Day 1 Day 3 Day 5 Day 7 Control – no bait 100 c 94 b 88 c 88 d Metaldehyde – fresh 0 a 6 a 19 b 38 c Methiocarb – fresh 0 a 0 a 0 a 0 a Multiguard – fresh 0 a 0 a 0 a 0 a Metaldehyde – 1 week exposure 0 a 6 a 6 ab 13 ab Methiocarb – 1 week exposure 0 a 0 a 0 a 0 a Multiguard – 1 week exposure 31 b 0 a 0 a 0 a Metaldehyde – 2 week exposure 0 a 6 a 13 ab 25 bc Methiocarb – 2 week exposure 0 a 0 a 0 a 0 a Multiguard – 2 week exposure 13 ab 0 a 6 ab 6 ab Metaldehyde – 4 week exposure 0 a 6 a 0 a 19 ab Methiocarb – 4 week exposure 0 a 0 a 0 a 0 a Multiguard – 4 week exposure 13 ab 0 a 0 a 0 a lsd (p=0.05) 19 11 14 24 F prob. <0.001 <0.001 <0.001 <0.001


D. CAFFEINE / CARVONE STUDIES

Trial 11: Laboratory examination of caffeine and carvone as slug ovicides

Eggs in the control treatment began hatching at 8 weeks with no indication of hatching in caffeine or carvone treated eggs (results not presented). However an equipment failure resulting in an increase in temperature to 35°C and a decrease in humidity to 41% resulted in death of all eggs and the trial was aborted. The trial was unable to be repeated due to lack of availability of sufficient slug eggs.

Trial 14: Preliminary examination of caffeine and carvone as slug control agents

The mortality levels in the control treatment cannot be fully explained (Table 23). Caffeine resulted in high slug mortality, with 100% death by day 3 at 2% concentration. At the lower concentration of 0.5%, mortality reached 80% by day 5. Carvone was not effective at the lower concentration of 0.05%, however at 0.5% mortality was high after 24 hours exposure. Both forms of carvone (positive and negative) showed the same level of efficacy in relation to slug mortality.

Table 23: Effect of caffeine and carvone drenches on slug survival.

Percentage of dead slugs Percentage of unaffected slugs Day 1 Day 3 Day 5 Day 1 Day 3 Day 5 Control 0 a 12 a 12 b 100 d 88 c 88 bc 0.5% Caffeine 19 a 31 b 81 c 6 ab 0 a 6 a 2% Caffeine 69 b 100 c 100 d 0 a 0 a 0 a 0.05% Carvone (+) 0 a 0 a 0 a 31 bc 75 bc 100 c 0.5% Carvone (+) 94 c 100 c 100 d 0 a 0 a 0 a 0.05% Carvone (-) 0 a 0 a 0 a 38 c 63 b 81 b 0.5% Carvone (-) 100 c 100 c 100 d 0 a 0 a 0 a lsd (p=0.05) 20 17 10 26 23 17 F prob. <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Trial 15: Further studies on the impact of caffeine and carvone on slug mortality

Caffeine at 2% achieved 100% mortality by day 1 (Table 24). The 1% rate resulted in 100% mortality by day 3 while 0.5% caffeine caused 100% mortality by day 5. At 0.25%, the mortality rate increased over time, reaching 56% by day 5. Both the positive and negative forms of carvone showed similar results, with high mortality from day 1. However, the 0.1% carvone rate was not as effective as the higher rates. The negative form of carvone was also less effective than the positive form by day 5.


Table 24: Effect of caffeine and carvone drenches on slug survival.

Percentage of dead slugs Percentage of unaffected slugs Day 1 Day 3 Day 5 Day 1 Day 3 Day 5 Control 0 a 0 a 0 a 100 c 100 c 100 c 0.25% Caffeine 6 a 25 b 56 b 44 b 31 b 25 b 0.5% Caffeine 13 a 88 d 100 c 0 a 0 a 0 a 1% Caffeine 75 c 100 d 100 c 0 a 0 a 0 a 2% Caffeine 100 d 100 d 100 c 0 a 0 a 0 a 0.1% Carvone (+) 50 b 88 d 88 c 0 a 0 a 6 ab 0.25% Carvone (+) 100 d 100 d 100 c 0 a 0 a 0 a 0.5% Carvone (+) 94 cd 100 d 100 c 0 a 0 a 0 a 0.1% Carvone (-) 38 b 56 c 63 b 0 a 6 a 0 a 0.25% Carvone (-) 100 d 100 d 100 c 0 a 0 a 0 a 0.5% Carvone (-) 100 d 100 d 100 c 0 a 0 a 0 a lsd (p=0.05) 23 23 24 16 22 23 F prob. <0.001 <0.001 <0.001 <0.001 <0.001 <0.001


Discussion

Grower survey / cultural control

The survey results illustrated the important role played by cultural practices in slug control. Disturbance of the soil through cultivation between crops is one tool that has been successfully used by growers in all states to disrupt the slug cycle. Control of weeds and removal of refuges also assists in reducing slug populations.

Addition of organic matter to the soil aids in greater retention of soil moisture, any practice that increases soil moisture holding capacity also renders the soil capable of sustaining an increased population of slugs (Carrick 1942). The presence of organic matter in the soil also tends to reduce soil compaction. Increasing soil aggregate size and number of macrospaces in soils contribute to a favourable environment for slugs (Hunter 1967; Glen et al. 1989). Growers following what are now recognised as sustainable farming practices by addition of organic matter to the soil are unfortunately providing a more favourable habitat for slugs, and are finding that slug populations are increasing. However, even though slugs may be present, it is only the surface feeders that are likely to cause damage to crops. These species are also more likely to be affected by soil cultivation. Identification of species on the farm will enable growers to determine whether they are likely to have crop losses due to slug damage. One of the species most likely to be damaging crops is Deroceras reticulatum. Species such as Milax budapestensis are unlikely to become a problem as they are found deeper underground and do not feed on green material (Hunter 1968b).

While some growers considered that headlands were a source of infestation, gully lines are probably more important as they tend to remain damp and ideal habitats. Removal of habitats in headland areas may assist in reducing slug populations, however Pinder (1974) has suggested that slug immigration from headlands is unlikely to be sufficiently penetrative to be of importance on a field scale. Observations by Hunter (1968b) show that slugs do not move far to feed. Hunter and Symonds (1970) also reported that the surface feeder Agriolimax reticulatus (syn. Deroceras reticulatum) only covered distances up to 1.5 m in a night.

Slugs are capable of a high degree of selection in their feeding behaviour (South 1992). While the survey highlighted slug feeding preferences between major crops, the small plot trials demonstrated that, even among the leafy ‘salad’ vegetables there are definite feeding preferences. Oakleaf lettuce were less attractive to slugs than iceberg lettuce, while the Asian green crops Tatsoi and Muzuna were highly attractive as food sources. According to South (1962), slugs actively select certain foods in preference to others. He suggests that the basis of food selection can be broadly defined by two concepts: the presence of inhibiting substances and the presence of feeding stimulants, stating the two are not necessarily mutually exclusive. Plants generally avoided by land molluscs are those containing condensed tannins and polyphenolics, such as caffeic acid (Mølgaard 1986). Gelperin (1975) demonstrated rapid food-aversion learning in Limax maximus. Avoidance was most commonly associated with complete rejection of the unsafe food, based on olfactory cues alone.,


with some slugs learning food avoidance in a single trial and remembering without error for three weeks. Sahley et al. (1981a, cited in South 1962) demonstrated associative learning first-order conditioning in experiments in which L. maximus was capable of associating the odour and taste properties of food. This evidence suggests that one potential method of preventing crop damage is to spray the crop with a substance that would normally result in rejection as a food source.

While growers who added organic matter to their soils found that slug problems were increasing, the small plot trial studying feeding preferences demonstrated that the presence of green organic matter on the soil surface reduced crop damage. Most organic matter added to soils is dead and/or decomposed, so surface feeders who tend to feed on green material (Hunter 1968b) would ignore this as a food source and concentrate on green crop plants. Perhaps in badly affected areas or high risk crops, consideration could be given to interplanting of a decoy crop to reduce the damage to the main crop. This however, is unlikely to be viable in broad acre cropping.

In the examination of population studies in cultivated vs non-cultivated soils, the cultivated soils were not conducive to slugs, even under mild spring conditions with good rainfall. In the non-cultivated soils, slug numbers built up over a three month period from late winter through to mid spring. Populations remained high until the onset of prolonged hot dry weather conditions. Where growers use practices such as minimum tillage, careful note should be taken of weather conditions and trends, and where possible, delaying planting so that the susceptible phase of the crop is shifted to dry periods when slug activity is low.

Barriers

Small plot trials are often performed under enclosed conditions to prevent slugs escaping and wandering into other ‘treatments’. This increases humidity, often enabling slugs to re-hydrate and recover following bait ingestion. Consequently this can have an effect on final results of baiting trials. To this end, a non-toxic barrier that is able to prevent slugs escaping and moving between plots is highly desirable to allow trials to be conducted under natural humidity conditions.

Clement (2000) suggests that the best commercial barriers against slugs are strips of copper that can be fastened around beds, greenhouse benches etc. He also suggests that the green oxidation of the copper doesn’t seem to decrease its effectiveness. The popular belief is that snails and slugs experience a slight electric current on contact with the copper. Lush (2002) also reported that copper bands successfully prevented snails from entering tree canopies. However, the results of this work suggest that new copper strips are ineffective. It may be that, had the copper strips been vertically oriented rather than horizontal, they may have been more effective. Although copper strips were not successful as a barrier in this work, oxidised copper strips were effective. The copper oxide on the surface may have been acting as a repellent and/or a toxin. The commercial copper silicate formulation (Socusil) was also effective as a barrier, with slugs displaying a toxic


reaction on contact. The fact that Bordeaux mix, which also contains copper, was ineffective demonstrates that the formulation of copper is important.

Sandpaper sheet impregnated with aluminium oxide proved to be an excellent repellent and barrier, with the slugs exhibiting a severe sensory but no toxic reaction on contact. The slug’s reaction was to limit the area of contact between it’s sole (foot) and the surface of the sandpaper. In a vertical application the slug would not be able to maintain it’s cohesive attachment. As glass impregnated sandpaper of the same grade was ineffective as a barrier, the slugs must be reacting to the aluminium oxide rather than to the surface texture. In the follow up observations with different grades of aluminium oxide sandpaper, the drop in efficacy as the grades of paper became finer suggest that the slug is able to maintain its grip better on a smoother surface, despite its obvious discomfort.

Other materials that were effective barriers in this work were Limil, sawdust and iron sulphate. The effectiveness of both Limil and sawdust can be explained by their dehydrating properties. However while these materials are effective when dry, they lose effectiveness when they become wet. In regard to the iron sulphate, it is well known that metal salts are toxic to slugs (Henderson et al. 1990).

Baiting efficacy

Baits were more effective when they were the only source of food, or where the food source was not highly desirable.

In all baiting trials undertaken in this project, Multiguard proved to be the most effective bait. This most likely reflects the fact that amounts of metal chelates well in excess of the lethal dose are usually ingested (Henderson et al. (1990). Henderson et al. (1989) and Henderson and Martin (1990) demonstrated that chelated iron baits could kill slugs as effectively as baits containing metaldehyde or methiocarb.

Methiocarb baits were consistently more effective in this work than metaldehyde baits. This agrees with the findings of Getzin (1965), Proude (1970) and Barratt et al. (1993). Recovery following ingestion of metaldehyde baits can be explained by the fact that metaldehyde induces paralysis of the gut musculature (Wedgwood and Bailey 1988; Bailey et al. 1989), shortening the period slugs feed at baits and often resulting in ingestion of a non lethal dose. In addition to this, Getzin (1965) found that recovery from metaldehyde poisoning is both dose and humidity dependent, explaining why normal doses often prove ineffective under moist conditions. Getzin also states that the effects of methiocarb are not influenced by the environment, but are purely dose dependent. However it has been suggested by Davis (1994a) that some slug species may be naturally tolerant to methiocarb.


Formulation of baits will influence efficacy. In this work, up to three metaldehyde formulations were used (Defender, Blitzem and Pestmaster). In all cases, Blitzem was more effective than either Defender or Pestmaster, while Defender was more effective than Pestmaster. The active ingredient was highest in Blitzem, with 18 g/kg compared with Defender and Pestmaster at 15 g/kg. However the other difference between the products was size. Blitzem is formulated as granules and 13 granules were required to cover the plot area. Both Defender and Pestmaster are produced in larger pellets, requiring only 3 pellets of each to cover the treatment area. The smaller the bait unit the more baits are required per unit weight, and hence better coverage is achieved. Davis (1994b) also suggested that the size of the bait was an important consideration.

This work produced some evidence that weathering and exposure to UV light is not necessarily detrimental to bait efficacy. According to Barker et al. (1984), metaldehyde baits deteriorate rapidly in moist soil, however in this work weathering for 2 weeks actually improved the efficacy of Defender baits, but had no effect on Blitzem baits.

Caffeine/carvone studies

This work confirms the report Hollingsworth et al. (2003) that caffeine has potential as a slug control agent. These authors reported that soil drenches of 2% caffeine resulted in 92% mortality 48 hours after treatment, while a rate of 1% achieved 92% mortality after 7 days. In this work, 100% mortality was achieved within 24 hours following a 2% application, and 3 days following application of 1%. A 0.5% drench achieved 100% mortality within 5 days, while at 0.25% a 56% mortality rate was achieved by 5 days. Hence this work extends that of Hollingsworth et al. (2003) and shows that, although 2% caffeine drenches are fast acting, good control can be achieved at much lower concentrations.

This work is the first report of the use of carvone as a soil drench. Like caffeine, carvone has proved to be an efficient slug control agent. Frank et al. (2002) reported a clear molluscicidal effect (50% mortality) when 0.75% carvone (+) was incorporated into mulch, but this work found increased efficacy when used as a soil drench, with 0.25% achieving 100% mortality within 1 day. This work also compared carvone sourced from caraway (+) and spearmint (-) and found that both forms gave similar results at concentrations of 0.25% or greater, but at very low concentrations (0.1%) carvone (+) was more effective than carvone (-).


Recommendations / Conclusions

In regions where slug activity is rampant, resulting in economic crop losses, a combination of cultural and chemical control methods should be utilised to minimise damage. Reducing soil moisture and removal of materials that provide favourable habitats are relatively simple practices that can assist in reducing slug populations. Soil cultivation will expose eggs and slugs, usually resulting in desiccation. The production of fine seed beds reduces movement between slug habitats and the crop. Leaving a cultivated weed free strip between the headland and the crop will reduce damage by immigrating large slugs.

Chemical baits can be effective but timing is critical. Establishment of refuge traps and regular monitoring will provide the grower with information on fluctuations in slug populations, allowing baits to be laid before the slugs become a major problem. Once crop damage is evident it is too late to lay baits. Choice of bait is also important. Look for baits with small, even sized pellets/granules and ensure spread is even. Metaldehyde baits were the least effective in this project, while Multiguard was the most effective.

Technology Transfer

A brief summary of project results was presented annually at the Tasmanian Vegetable Research days. The following publications are currently being prepared:

1. Scientific papers Aluminium oxide as an effective barrier against slugs. A comparative study of metaldehyde, methiocarb and iron baits for slug control. Caffeine and carvone as molluscicides.

2. Popular press articles Reducing crop damage by slugs. Using slug baits effectively.

3. Grower Information Bulletin Slug control measures


Industry implications and recommendations

This project has provided a starting point for Australian vegetable growers on integration of slug control measures. Many growers are already successfully integrating cultural and chemical control methods, but the number of growers still experiencing problems suggests that there is a need for extension of information.

Recommendations for continuing work: Further work is needed to study the effects of caffeine and carvone as molluscicides. This includes confirmation of application rates when used as a soil drench. The effect of these chemicals when applied to mulches and directly to plants also needs to be determined. It is also important to undertake a study of the effect of both caffeine and carvone on beneficial insects and other soil fauna.

Carabid beetles are important predators of slugs in some countries. Studies on local carabid beetle populations may find a local beetle that is an effective predator.


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Glen, D.M.; Jones, H. and Fieldsend, J.K. (1990a) Damage to oilseed rape seedlings by the field slug Deroceras reticulatum in relation to glucosinolate concentration. Annals of Applied Biology, 117, 197-207.

Glen, D.M.; Milsom, N.F. and Wiltshire, C.W. (1987) Pest forecasting: preventing slug damage. Report Long Ashton Research Station for 1986, 31-32.

Glen, D.M.; Milsom, N.F. and Wiltshire, C.W. (1989) Slug damage forecasting. Report Long Ashton Research Station for 1987, 26-28.

Glen, D.M.; Milsom, N.F. and Wiltshire, C.W. (1990b) Effect of seed depth on slug damage to winter wheat. Annals of Applied Biology, 117, 693-701.

Glen, D.M.; Wiltshire, C.W.; Walker, A.J.; Wilson, M.J. and Shewry, P.R. (1996) Slug problems and control strategies in relation to crop rotations. Apects of Applied Biology, 47, 153-160.

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Hammond, R.B.; Smith, J.A. and Beck, T. (1996) Timing of molluscicide applications for reliable control in no-tillage fields crops. Journal of Economic Entomology, 89(4), 1028-1032.

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Henderson, I.F.; Martin, A.P. and Parker, K.A. (1990) Laboratory and field assessment of a new aluminium chelate slug poison. Crop Protection, 9, 131-134.

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Appendix 1 - Survey on slugs

Dear Vegetable Grower,

This survey is being undertaken as a component of the Horticulture Australia project VG00030 – A preliminary model for slug control in vegetable crops.

Your responses will provide valuable information to the project team. All returned surveys will be treated as confidential and results will be combined to provide baseline data on the extent of slug damage / problems and current management strategies in different growing regions. Results will be reported in industry newsletters.

Please use the reply paid envelope enclosed to return your response by 15 December 2001.

Farm location: ............................................................... Postcode:........................................

Soil type:......................................................................... Soil pH (if known) .........................

Annual rainfall ..............................................................................................................................

Month/s most rainfall received ....................................................................................................

Crops produced (please also outline when each crop is normally planted):

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Do you have problems with slug damage: Frequently / Sometimes / Never

Which crops are affected: ............................................................................................................

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What time of year do you suffer the most damage: ...................................................................

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Do you consider this damage is related to rainfall distribution?..............................................

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Do you consider some crops are more susceptible than others? Yes / No

If so, which crops ............................................................................................................................

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Do you find slug damage is worse after particular crops in a rotation? Yes / No

If yes, then which crops ..................................................................................................................

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What type of slug damage occurs on your property? please circle and list crops affected:

1. seeds / un-emerged plants .....................................................................................................

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2. damage to newly emerged cotyledons/first leaves...............................................................

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3. damage to seedlings ............................................................................................................

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4. damage at later stages of growth..........................................................................................

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What control measures do you use? ............................................................................................

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How successful do you consider these control measures? .........................................................

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If you use chemicals or baits, what do you use and when do you apply (please include Brand name, active ingredient and concentration, and rate of application)

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Do you: burn between crops Yes / No / Sometimes

cultivate between crops Yes / No / Sometimes

use minimum tillage Yes / No / Sometimes

grow cover/green manure crops Yes / No / Sometimes

incorporate previous crop or stubble Yes / No / Sometimes

apply composts Yes / No / Sometimes

If you apply composts, how often are they applied and at what rate

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If you grow cover or green manure crops, what do you grow? .........................................................................................................................................................

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Do you vary cultural practices from year to year? Yes / No

If yes, please describe .....................................................................................................................

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What are your most common crop rotations .............................................................................

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Interval between crops .................................................................................................................

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Do you irrigate? Yes / No Irrigation frequency ......................................................

Bed tilth: Fine Medium Coarse

Planting material: Seed Speedling

What fertilisers do you apply and when .....................................................................................

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What herbicides are used and when are they applied (ie pre-emergent/post-emergent/between crops etc)

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What slug control measures do you adopt on the boundaries of cultivated areas?

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If you know the slug species on your property please name .....................................................

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If not, describe colour and whether any distinguishing marks such as stripes, mottling, spots etc

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Further comments.........................................................................................................................

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Name (optional):............................................................................................................................

Contact number (optional):..........................................................................................................

Thank you for your cooperation. If you have any queries relating to this survey or to the Horticulture Australia slug project in general, please contact project leader Sally Bound.

Please fax responses through to 03 6233 6145 or use the reply paid envelope enclosed to return your response by 15 December 2001.

Sally Bound Tasmanian Institute of Agricultural Research 13 St Johns Ave Ph: 03 6233 6857 New Town 7008 Fax: 03 6233 6145 TAS Email: [email protected]

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