BIOLOGICAL CONTROL OF ENGLISH BROOM: BROOM GALL MITE Background
English broom, Cystisus scoparius (also known as broom and Scotch broom) is a leguminous shrub that is native to central and western Europe and has now spread to many temperate areas of the world. English broom has now become naturalised over large areas of south-eastern Australia.
In Tasmania, it occurs throughout areas of human settlement but is particularly common in higher rainfall areas. Plants are locally abundant on roadsides, waste areas, poor quality pastures and in disturbed bushland. In pastures, English broom causes production losses by forming thickets that prevent grazing. Dense thickets provide cover for pest animals such as rabbits and can also inhibit access to bushland and recreational areas.
English broom can be controlled to varying extents by herbicides, burning, mechanical clearing and grazing using sheep and goats. However, the limited success, expense and difficulty in controlling English broom in many areas has resulted in biological control being investigated.
Many insect species have been recorded on English broom in its native range and several of these have been under investigation to determine whether they have a sufficiently narrow host range to enable them to be released safely in Australia as biological control agents. These investigations resulted in the release of the broom twig-mining moth, Leucoptera spartifoliella, which has now established in Tasmania from releases conducted since 2004. Releases of this agent are continuing in order to accelerate its slow dispersal rate. A second biological control agent, the broom gall mite was first released in Tasmania in 2009. A mass rearing program is being conducted by TIA to establish the mite at sites throughout the State. If this establishment program is successful, the best sites will be used to collect and redistribute the mite to new sites to accelerate dispersal. Introduction
The broom gall mite, Aceria genistae (Figure 1), belongs to a large family of microscopic plant eating mites known as the Eriophyidae. Mites in this family feed by inserting their mouthparts into plant cells to extract sap. Although some Eriophyids are agricultural pests due to the damage they cause to crops, other species can cause substantial damage to weeds and are a useful tool in suppressing them. The broom gall mite originates from western Europe and feeds and develops exclusively on English broom. Description
Adults are light-pink in color and have a worm-like appearance. The anterior part of the body comprises
Figure 1. Broom gall mite adults: female (top) and male (bottom); white arrows indicate genital region (Photo courtesy of Jean-Louis Sagliocco, DPI Victoria). a complex head (the gnathosoma) followed the prodorsum, while the longer posterior part (the opisthosoma) looks like an annulated abdomen. Eriophyids have only two pairs of developed legs located ventrally near the head. Adult females measure 160-225 microns in length (0.16-0.225 mm) and about 50 microns (0.05 mm) wide, while males measure only 165 microns in length and 44 microns in width. Along with size, identification of the sex in adults is based on the differences of the genital region (Figure 1). Eggs are white, spherical and about 50 microns in diameter. Immature mites have two different stages (larva and nymph) and are pinkish-white in color and measure up to about 100-120 microns in length (0.10-0.12 mm). There is no sexual dimorphism among immatures. Life cycle
Colonies establish in stem buds, transforming them into rounded pubescent galls (Figure 2). Like the majority of Eriophyids, Aceria genistae has four life stages: egg, larva, nymph and adult. At maturity, the male deposits a spermatophore which is collected by a female via her genital flap. Fertilised eggs are produced by the female, one at a time. The female lays eggs either inside the gall or leaves the protection of the gall to colonise new buds on the same plant or be carried by wind to a new host plant. If females land on another broom plant they will locate a shoot bud in which to feed and shelter during winter. Several overlapping generations develop in galls during spring and summer. The non-woody galls senesce in late summer and autumn, stimulating mites to crawl into dormant stem buds where mites overwinter.
Weed Biological Control Pamphlet October 2009
Figure 2. Comparison of uninfested stem (on left) with infested stem (on right) showing distinct galls of the broom gall mite (Photo: W. Chatterton, TIA). Feeding damage
In spring, feeding induces abnormal growth of plant tissues which develop into a tiny gall after 8-12 weeks. Over the course of a year, the gall may grow 8-10 mm in diameter and contain hundreds of mites (Figure 3). As galls grow they become more hairy until they senesce. The continued formation of galls on successive years of growth can cause stunting, reduced flowering and ultimately plant death.
Although the broom gall mite will not be capable of controlling English broom by itself, it has the potential of causing considerable damage to a plant, weakening it and making a useful contribution to its control. Introductions of more biological control agents are planned and these will place additional stress on plants, reducing vigour and may ultimately decrease the density of infestations. Biological control should be considered as a long-term program that is best used on large, chronic infestations with a low priority for control by other methods.
Figure 3. Dissected gall containing egg, immature and adult stages of the broom gall mite (Photo: W. Chatterton, TIA). Releases
Releases are made by cutting gall infested branches and tying them amongst the branches of broom plants at new sites (Figure 4). The mites will then
transfer across onto the buds of the live plant as the galls on the severed branch senesce. A minimum of 50 galls per new site should result in establishment.
Figure 4. Releasing broom gall mite by using a ‘twist tie’ to secure the infested stem within a cluster of stems on the release plant (Photo: W. Chatterton, TIA). Acknowledgements
Information on the biology of the broom gall mite used in this pamphlet was provided by Jean-Louis Sagliocco, Department of Primary Industry, Victoria. The initial importation and release of the broom gall mite was federally funded by the Defeating the Weeds Menace Program administered by Land & Water Australia with financial contribution from Parks Victoria, TIA and the Adelaide and Mt. Lofty Natural Resources Management Board, South Australia. A mass rearing and field establishment program from 2009-11 was federally funded by the ‘Caring for our Country’ program through the Department of the Environment, Water, Heritage and the Arts/Department of Agriculture Fisheries and Forestry. Further information For further information on this project contact: John Ireson Tasmanian Institute of Agriculture, 13 St. John’s Avenue, New Town, 7008. Phone: (03) 6233 6821; Fax: (03) 62782716 Email: [email protected]
