植物保護學會會刊 42：213 - 222, 2000 213

Occurrence of the Legume Pod Borer, Maruca testulalis Geyer (Lepidoptera: Pyralidae) on

Cowpea (Vigna nguiculata Walp) and its Insecticides Application Trial

Liao C. T.＊, and Lin C. S.

Taichung District Agricultural Improvement Station, Tatsuan, Changhua, Taiwan, ROC

(Accepted for publication: Aug. 5, 2000)

ABSTRACT

Liao C. T.＊ and Lin C. S. 2000. Occurrence of the Legume Pod Borer, Maruca testulalis Geyer (Lepidoptera: Pyralidae) on Cowpea (Vigna nguiculata Walp) and its Insecticides Application Trial. Plant Prot. Bull. 42: 213-222

The legume pod borer (LPB), Maruca testulalis Geyer (Lepidoptera : Pyralidae), is a serious pest of cowpea. LPB larvae prefer to attack floral parts and pods. The distribution of LPB larvae on flowers and pods and their population fluctuation were investigated in Changhua, Taiwan. These results suggested that larval vertical distribution of LPB did not significantly different among the strata of the flowers and pods of cowpea plants. Most of the 1st and 2nd instar larvae were observed on flowers, while the majority of the 4th to 5th instars were found on pods. The infestation level of LPB was greater for spring crops than for those in summer and autumn. Meanwhile, the maximum larval density occurred in late-May for the spring crop. Eight insecticides were tested to evaluate their efficiency in controlling LPB. Deltamethrin and carbaryl appeared more effective than others. Furthermore, the highest pod-yield (an average of 4950.9 kg/ha) was recorded in the thiodicarb-treated plots. Pod yield losses ranging from 17-53 % resulted from LPB damage. Thus, the application of insecticides significantly increased the pod yield and reduced the heavy losses caused by LPB.

(Key words: Maruca testulalis, occurrence, insecticides, control, cowpea)

*Corresponding author. E-mail: liaoct@tdais.gov.tw

214 植物保護學會會刊 第 42 卷 第 4 期 2000

INTRODUCTION

The legume pod borer (LPB), Maruca testulalis Geyer, is an important pest affecting grain legumes in the tropics and subtropics(16). LPB has been observed to feed on 40 host plants(9, 14), and has been found attacking cowpea, common bean, and vegetable soybean in Taiwan (2, 3). Sesbania, a green manure for soil improvement, was also the ultimate host for LPB and elevated the population density in the field(9). Although the biology of LPB on cowpea has been studied extensively (5, 6, 11, 12, 13, 17, 18), agroclimatic conditions differ completely among countries and these differences could influence the population fluctuation of LPB. Thus, researching local information is valuable.

LPB larvae feed on flowers, buds, and pods by webbing them. This feeding habit protects the larvae from natural enemies and from the application of insecticides(17). LPB control worldwide has traditionally relied upon chemical insecticides(14). However, effective insecticides such as endosulfan and DTT are prohibited in Taiwan, and thus screening of insecticides useful for controlling LPB is necessary.

In this study, we evaluated the distribution and occurrence of M. testulalis on cowpea in the field. Furthermore, the efficiency of eight insecticides in controlling LPB was examined, and the pod yield was also estimated.

MATERIALS AND METHODS

Distribution and insecticide application studies on M. testulalis were conducted at

the experimental farm at the Taichung District Agricultural Improvement Station (DAIS), Changhua, Taiwan. Meanwhile, LPB larval population was examined in five area of Changhua. Each plot had two rows, 5 m long and spaced 0.9 m apart, with 0.6 m between plant stands. The plots were planted with cowpea variety, Tung kua bean, from the Vegetative laboratory of Taichung DAIS. The experiment was replicated four times. The plants were maintained under irrigation during the dry periods of the year. For vertical distribution of LPB at different heights on cowpea, samples were stratified with respect to distance from the base and categories were designated as upper, central and lower parts of the plant. Twenty flowers and pods were sampled at each stratum at 60, 80 and 100 days after planting (DAP). Each pod was horizontally divided onto three equal parts, i.e., distal, middle and basal from the stipe of pod. The various stages of LPB were also counted on flowers and pods. Cowpea are cultivated in three cropping seasons, with spring, summer and autumn crops. Twenty pods were randomly sampled and examined for LPB infestation at every 10-day interval after planting.

Insecticides were sprayed three times weekly with randomized complete block design starting from 50 % of flowering in 1998. The experiment comprised nine treatments (see Table 1) and four replicates. The incidence of LPB larvae on the pods was determined by counting the total number of larvae on 20 pods selected randomly from each plot four times before the first spraying and 7 days after each application. Pod yield was determined by weighting all pods collected from each tested plot of cowpea

Occurrence of the Legume Pod Borer, Maruca testulalis Geyer on Cowpea and its Insecticides Application Trial 215

plants. Finally, pod yield loss from LPB infestation was also calculated, using current

Taiwan cowpea prices.

Table 1. Treatments and rate of application of insecticides used in the experiments Code Treatment Formulation Rate of application in g (AI) / 1000 liters water / ha

1 Chlorpyrifos + cypermethrin

25 EC 1000

2 Deltamethrin 2.8 EC 1500 3 Bifenthrin 2.8 EC 1500 4 Naled 58 EC 1000 5 Thiodicarb 75 WP 4000 6 Carbaryl 85 WP 1700 7 Permethrin 10 EC 3000 8 Bacillus thuringiensis 3 WP 1500 9 Control -- No insecticide

Fig. 1. Percentage of different stages of M. testulalis between flowers (■) and pods (□) on cowpea plants.

0

20

40

60

80

100

1st 2nd 3rd 4th 5th prepupa

Larval instars

%

216 植物保護學會會刊 第 42 卷 第 4 期 2000

RESULTS

Larval distribution of LPB on pods and flowers

The early larvae feed on young tender shoots, peduncles and stems in the absence of flower buds and flowers. However, the preferred feeding sites are floral parts and pods. The 1st instar larvae were exclusively observed on flowers, and most of the 2nd instar (93.2 %) were also found on flowers (Fig. 1). The 3rd instars displayed no significant difference in their preferences for flowers and pods, while the 4th and 5th instar larvae preferred to feed on the pod. Additionally, the prepupa stage larvae were observed exclusively on pods. The incidence of LPB larvae on cowpea flowers at different cultivation dates did not differ significantly among the three strata (Table 2). However, LPB mainly attacked pods in the central stratum of the cowpea plant at 60 DAP, just as larvae were first observed on pods. Meanwhile, at 80 and 100 DAP, the mean number of LPB larvae on pods were equal at different strata. These results revealed that LPB larvae developed randomly on cowpea flowers and pods of at different plant heights.

Nevertheless, larvae were more concentrated on flowers than on pods (Table 2). The LPB larvae prefer to attack pods at the junction between two pods or between a pod and a leaf or stem. The distribution of larvae among pods was 56.4 % on the distal portion, 30.8 % on the middle and 12.8 % on the basal portion.

Infestation level of LPB Damage severity is more important than

the number of insect pests on the crop. A high positive correlation (r = 0.96) was observed between the number of LPB larvae and the percentage of pods damaged. Therefore, the percentage of pods infested was used instead of the number of LPB l a r va e to e va l ua t e LP B p op u la t i on fluctuation. LPB infestation levels for the LPB for the different cropping seasons are given in Fig. 2. One small peak in LPB population, followed by a large peak was observed for each cultivation season. The LPB populations on cowpea remained low before 60 DAP, but displayed a small peak at 80 DAP. Meanwhile, a large peak occurred at 110 DAP for the spring crop, and at 100 DAP for the summer and autumn crops.

Table 2. Occurrence of M. testulalis larvae on flowers and pods at different strata of cowpea

plants in 19991)

Mean No. M testulalis/20 flowers or pods 60 DAP 80 DAP 100 DAP

Stratum

Flower Pod Flower Pod Flower Pod

Upper 7.25 a2) 0 13.75 a 1.50 a 14.00 a 4.50 a Central 6.25 a 5.25 7.00 a 1.25 a 19.00 a 2.50 a Lower 8.25 a 0 7.00 a 1.50 a 12.00 a 6.00 a 1) Values are means of four replicates. 2) Different letter denotes mean separation within each column by Duncan's multiple range test

at 5 % level.

Occurrence of the Legume Pod Borer, Maruca testulalis Geyer on Cowpea and its Insecticides Application Trial 217

0

2

4

6

8

10

12

14

2 3 4 5 6 7 8 9 10 11

Month

% P

od

s d

am

ag

ed

Fig. 2. Infestation level of M. testulalis larvae on cowpea during three cropping seasons. ■, spring crop; ▲, summer crop; ●, autumn crop. The first symbol represented the date just planting for each crop.

Among the three seasons, the LPB population of the spring crop was more remarkable than that of the other two crops. The peak LPB population for the spring crop occurred between April and June. At its maximum, the damage to the spring crop was 2.2 fold greater than that to summer crop, and 3.0 fold greater than that to the autumn crop.

Insecticides application for control the LPB and pod yield loss

The number of larvae collected was

significantly lower for plots treated with insecticides than for control plots. After three applications, pod infestation in the eight insecticide treated plots was significantly lower than in the control plots (Table 3). The experimental results also indicated that insecticides varied in their efficiency in controlling LPB larvae, with deltamethrin and carbaryl appearing more effective than the other six treatments (Table 3).

However, all insecticide treatments produced significantly higher pod yields (Table 4). Plots treated with thiodicarb

218 植物保護學會會刊 第 42 卷 第 4 期 2000

produced an average yield of 4950.9 kg of pods per ha. Meanwhile, plot receiving applications of permethrin and chloropyrifos + cypermethrin also produced high yields. Pod-yield losses of 17-53 % were recorded due to damage by LPB larvae (Table 4). Application of insecticide also resulted in a net monetary gain ranging between NT$ 1,149 for treatment with naled and NT$ 62,747 for spraying with thiodicarb (Table 4).

DISSCUSSION

When LPB attack on cowpea, infestation is highest on flowers>flower buds>terminal shoots>pods(5). Karel also observed more larvae (52.3 %) on flowers than on pods (37.8 %), and leaves (9.9 %)(7). Third- to fifth-instar larvae are capable of boring into the pods (17). According to the results in the study, the results in Fig. 1 and

Table 2 closely corresponded to previous literature.

Among the three cropping seasons, the maximum larval density occurred in late-May for the spring crop (Fig. 1). Okeyo-Owuor et al. reported that in Kenya, LPB populations are lower during the short rainy season (13). Alghali observed three peaks of LPB infestation appearing on cowpea in Nigeria(1). Significant relationships were also observed between LPB incidence and cumulative rainfall, as well as the number of rainy days between crop emergence to flowering. Above results indicated that the agrometerological factors may participate in population fluctuation of the LPB(1). In this paper, the larval density increased from 80 DAP to 110 DAP for the three crops (Fig. 2), implying that the population dynamics were also associated with the growth stage of cowpea plants.

Table 3. Efficiency of various insecticides in controlling M. testulalis on cowpea plants

7 days after 1st application

7 days after 2nd application

7 days after 3rd application

Treatment Mean No. damaged

pod before test

Damaged pod

%Control

Damaged pod

%Control

Damaged pod

%Control

Chlorpyrifos+ cypermethrin

0.3 6.0 a1) -- 3.3 ab 43.1 0.8 ab 53.3

Deltamethrin 1.0 2.3 a 67.7 0.8 a 91.3 0.5 a 92.0

Bifenthrin 2.5 5.0 ab 71.4 5.0 abcd 77.2 2.0 ab 87.2

Naled 0.8 5.0 ab 6.0 8.0 cd 20.2 2.5 ab 47.5

Thiodicarb 1.0 3.0 a 57.1 5.0 abcd 57.1 0.8 ab 87.9

Carbaryl 0.5 2.3 a 35.4 3.5 abc 79.9 0.3 a 91.7

Permethrin 1.3 3.3 ab 63.0 5.8 bcd 47.8 1.3 ab 84.0

Bacillus thuringiensis

1.3 4.3 ab 51.7 1.5 ab 86.4 3.0 b 62.0

Control 1.0 7.0 b -- 8.8 d -- 6.3 c -- 1) The same as Table 2.

Occurrence of the Legume Pod Borer, Maruca testulalis Geyer on Cowpea and its Insecticides Application Trial 219

Table 4. Percentage of pod yield loss caused by M. testulalis infestation and its net monetary gain under various insecticides application

Cost of insecticide application (NT $/ha)

Treatment

Mean pod yield

(kg/ha) ± S. E.

% Yield loss

without M. testulalis control

Total monetary

gain due to insecticide application (NT $/ha)2)

Insecticide3) Labor4) Total

Net monetary

gain due to insecticides application

pre ha Chlorpyrifos +cypermethrin

4,512.9± 433.3 a1) 47.9 62,040 1,800 10,500 12,300 49,740

Deltamethrin 3,549.5± 757.4 ab 33.7 34,330 1,900 10,500 12,400 21,930

Bifenthrin 3,122.7± 623.9 bc 24.6 22,046 2,800 10,500 13,300 8,746

Naled 2,823.0± 1,105.5 bc 16.6 13,449 1,800 10,500 12,300 1,149

Thiodicarb 4,950.9± 1,053.3 a 52.5 74,597 1,350 10,500 11,850 62,747

Carbaryl 3,895.4± 758.0 ab 39.6 44,272 1,765 10,500 12,265 32,007

Permethrin 4,550.6± 496.1 a 48.3 63,081 450 10,500 10,950 52,131

Bacillus thuringiensis

3,935.0± 1,312.9 ab 40.2 45,400 1,200 10,500 11,700 33,700

Control 2,353.2± 750.2 c -- -- -- -- -- --

1) The same as table 2. 2) Cowpea pod value calculated at 1998 Taiwan agricultural prices and costs monthly farm

price of NT. 28.2 pre kg (1 NT $ = US $ 0.03). 3) Based on current prices for eight insecticides. 4) The labor cost of insecticide application based on an application pre ha for NT. 3500.

In addition, several insecticides were

evaluated for the control of LPB. Endosulfan, cypermethrin, biphenthrin, cyhalothrin, and in combination with dimethoate effectively control the pod borer on cowpea plants(14). Moreover, Bacillus thuringiensis (Bt) effectively controls the pod borer(8). On pigeonpea, deltamethrin, cypermethrin, and fluvalinate, monocrotophos and endosulfan, dimethoate, fenvalerate, triazophos, permethrin are also effective against the borer(14). In this study, deltamethrin and

carbaryl were significantly more effective in terms of controlling LPB than the six other treatments (Table 3).

Losses in grain yield have been estimated to rang from 20 to 60 %(15). Pod yield losses ranging from 17-53 % resulted from LPB damage (Table 4). It is concluded that the application of insecticides during the flowering stage of cowpea plants significantly increased the pod yield and reduced the heavy losses caused by LPB (Table 4). With respect to the timing of

220 植物保護學會會刊 第 42 卷 第 4 期 2000

application, Ogunwolu established a threshold of 40 % larval infestation on flowers(10). However, as LPB larvae infestated to cowpea plants, the early instars feed on floral parts, whereas the later instars bore into the pod and feed on developing seeds, all are help them escaping from insecticides contact. Therefore, at the onset of flowering, the first application may need to be processed to prevent their population from establishing on cowpea plants.

LITERATURE CITED

1. Alghali, A. M. 1993. The effects of some agrometereological factors on fluctuation of the legume pod borer, Maruca testulalis (Lepidoptera : Pyralidae), on two cowpea varieties in Nigeria. Insect Sci. Applic. 14: 55-59.

2. Chang, T. C., and Chen, C. C. 1989. Observation of three lepidopterous pests attacking leguminous vegetables in Taiwan. Bulletin of Taichung DAIS. 24: 21-29. (In Chinese).

3. Chang, T. C., and Chen, C. C. 1993. Population fluctuation of major insect pests on kidney bean and the proper time of control. Bulletin of Taichung DAIS. 38: 11-22. (In Chinese).

4. Firempong, S., and Mangalit, H. 1990. Spatial distribution of Maruca testulalis larvae on cowpea, and a sequential sampling plan for estimating larval densities. Insect Sci. Applic. 11: 217-222.

5. Jackai, L. E. N. 1981. Relationship between cowpea crop phenology and field infestation by the legume pod borer, Maruca testulalis. Ann. Entomol. Soc. Am. 74: 402-408.

6. Jackai, L. E. N. 1982. A field screening technique for resistance of cowpea (Vigna unguiculata) to the pod-borer Maruca testulalis (Geyer) (Lepidoptera: Pyralidae) Bull. Entomol. Res. 72: 145-156.

7. Karel, A. K. 1985. Yield losses from and control of bean pod borers, Maruca testulalis (Lepidoptera: Pyralidae) and Helicothis armigera (Lepidoptera : Noctuidae). J. Econ. Entomol. 78: 1323-1326.

8. Karel, A. K., and Schoonhoven, A. V. 1986. Use of chemical and microbial insecticides against pests of common beans. J. Econ. Entomol. 79: 1692-1696.

9. Liao, C. T., and Chen, C. C. 1998. Distribution of Maruca testulalis Geyer (Leidoptera : Pyralidae) eggs and larvae on sesbania. Taichung DAIS. 59: 47-52.

10. Ogunwolu, E. O. 1990. Damage to cowpea by the legume pod borer, Maruca testulalis Geyer, as influenced by infestation density in Nigeria. Trop. Pest Manage. 36: 138-140.

11. Okeyo-Owuor, J. B., and Ochieng, R. S. 1981. Studies on the legume pod-borer, Maruca testulalis (Geyer)-I. Life cycle and behaviour. Insect Sci. Applic. 1: 263-268.

12. Okeyo-Owuor, J. B., and Oloo, G. W. 1991. Life tables, key factor analysis and density relations in natural populations of the legume pod borer Maruca testulalis Geyer (Lepidoptera: Pyralidae) in western Kenya. Insect Sci. Applic. 12: 423-431.

13. Okeyo-Owuor, J. B., Agwaro, P. O., and Simbi, C. O. J. 1983. Studies on the legume pod-borer Maruca testulalis (Geyer)-V. Larval population. Insect Sci.

Occurrence of the Legume Pod Borer, Maruca testulalis Geyer on Cowpea and its Insecticides Application Trial 221

Applic. 4: 75-81. 14. Shama, H. C. 1998. Bionomics, host plant

resistance, and management of the legume pod borer, Maruca vitrata - a review. Crop Prot. 17: 373-386.

15. Sigh, S. R., and Allen, D. R. 1980. Pests, diseases, resistance, and protection in cowpea, pp. 419-443. In: R. J. Summerfield & A. H. Bunting [eds.], Advances in Legume Science. and HMSO, London.

16. Sigh, S. R., and van Emden, H. F. 1979. Insect pests of grain legumes. Annu. Rev.

Entomol. 24: 255-278. 17. Taylor, T. A. 1967. The bionomics of

Maruca testulalis Geyer (Lepidoptera : Pyralidae), a major pest of cowpeas in Nigeria. J. West Afr. Sci. Assoc. 12: 111-129.

18.Taylor, T. A. 1978. Maruca testulalis: an important pest of tropical grain legumes, pp. 193-200. In: S. R. Singh, H. F. van Emden & T. A. Taylor [eds.], Pests of Grain Legumes: Ecology and Control. Academic Press. London.

222 植物保護學會會刊 第 42 卷 第 4 期 2000

摘 要

廖君達＊、林金樹 2000 豆莢螟在豇豆上的發生及藥劑防治試驗 植保會刊 42: 213-222.(彰化縣大村鄉台中區農業改良場)

豆莢螟是豇豆栽培期間的重要害蟲，主要危害花器及豆莢部位。本試驗在調查豆莢螟幼蟲在花器及豆莢上的分布及週年族群變動情形。結果顯示，豆莢螟幼蟲在不同高度之花器及豆莢內的垂直分布並無顯著性差異。多數的一、二齡幼蟲位於花器部位，三齡幼蟲分布於花器及豆莢部位無顯著差異，四及五齡幼蟲則多數位於豆莢。豆莢螟於豇豆春作的危害率顯著高於夏作及秋作，受害高峰期發生於五月下旬。試驗中以八種殺蟲劑用來評估對豆莢螟的防治效果。結果顯示，第滅寧及加保扶的效果最佳。硫敵克處理的豇豆，每公頃平均鮮莢產量可達 4950.9公斤為最高。豆莢螟危害，可造成 17 %至 53 %的產量損失。經過殺蟲劑的施用，能夠顯著地增加鮮莢產量及降低豆莢螟所造成的損失。

(關鍵詞：豆莢螟、發生、殺蟲劑、防治、豇豆)

*通訊作者。E-mail: liaoct@tdais.gov.tw