African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 4, Num. 2, 1996, pp. 207-214

Leptinotarsa decemlineata (Coleoptera: Chrysomelidae) and natural enemy relations as influenced by straw mulch, Bacillus thuringiensis and esfenvalerate insecticide

B. ODONGO and R. FOSTER^1

Namulonge Agricultural and Animal Production Research Institute, P.O.Box 7084, Kampala, Uganda
^1 Department of Entomology, Purdue University, 1158 Entomology Hall, West Lafayette IN 47906, U.S.A

(Received 6 December, 1994; accepted 14 February, 1996)

Code Number: CS96058
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ABSTRACT

Studies were conducted to determine the effects of straw mulch, Bacillus thuringiensis var tenebrionis (Bt) and esfenvalerate (Asana) on Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), a pest of potato, and on natural enemies Podisus maculiventris (Say), Perillus bioculatus (F) (Hemiptera: Pentatomidae) and Lebia grandis Hentz (Coleoptera: Carabidae). Adult beetles were significantly (P=0.05) fewer on potato with or without mulch but which were sprayed with Bt and Asana than on the checks. Larvae and natural enemies were more on Bt and non-insecticide treatments than on plants in esfenvalerate plots. Treatment means, however, were not significant (P=0.05). Mulch and Bt suppressd beetle population and preserved good levels of indigenous beneficial insects. We recommend that mulch and Bt can be applied within an integrated pest management programme for L. decemlineata.

Key Words: Lebia grandis, Perillus bioculatus, Podisus maculiventris

Resume

Les recherches ont ete menees pour determiner les effets de paillis, de Bacillus thuringiensis var tenebrionis (Bt), et esfenvalerate (Asana) sur Leptinotarsa decemlineata (Say) (Coleoptere: Chrisomelidae), une peste de la pomme de terre, et sur les ennemis naturels Podisus maculiventris (Say), Perillus bioculatus (F) (Hemiptera: Pentatomidae) et Lebia grandis Hentz (Coleoptera: carabidae). Les scarabees adultes etaient significativement (P=0.05) peu nombreux sur la pomme de terre avec ou sans paillis arrosee de Bt et d'Asana que sous controles. Les larves et ennemis naturels etaient plus nonbreux sur Bt et traitements sans insecticides que sur les plantes en parcelles d'esfenvalerate. Cependant, les differences des traitements n'etaient pas significativement differentes (P = 0.05). Le paillis et le Bt ont supprime la population des scarabees et a preserve les insectes indigenes benefiques. Nous recommandons, par consequent, que le paillis ete le Bt soient utilises dans un programme de lutte integree contre la peste L. decemlineata.

Mots Cles: Lebia grandis, Perillus bioculatus, Podisus maculiventris

Introduction

Colorado potato beetle, Leptinotars decemlineata (Coleoptera: Chrysomelidae), is native to Mexico where it feeds on wild solanaceous plants (Harris, 1978). When potato was introduced and began to be commercially produced in North America in the early 19th century, L. decemlineata exploited this abundant food source and multiplied to become an economic pest of the crop (Gauthier et al., 1981). Pest management measures, with the most preferred one being pesticides, have evolved over the years to try to contain the pest. The pesticides, however, have triggered the beetle's naturally available physiological defence mechanism or pesticide resistance which have resulted into the neutralisation of most of the poisons used for the beetle control. A majority of the pesticides in use are non-selective and tend to destroy the natural enemies of the pest or leave toxic residues in the food chain. Integrated management approaches which employ multiple-control methods that are cost effective and environmentally friendly are being investigated to keep the pest below economic injury level. Among the cultural management measures is the use of mulch to manipulate the environment, to physically bar the pest from reaching potato, to delay initial colonisation, suppress weed growth, reduce water loss and invigorate plant growth (Chabossau, 1938; Casagrande, 1987). Promising biocontrol agents mainly parasites and microorganisms have also been identified. Potential parasites include Myriopharus doryphorae (Riley) (Diptera: Tachinidae), Edovum puttleri Grissel (Hymenoptera: Eulophidae), Perillus bioculatus (Fab.), Podisus maculiventris (Say) (Hemiptera: Pentatomidae) and Lebia grandis Hentz (Coleoptera: Carabeidae) (Biever and Chauvin 1992). Bacillus thuringiensis var tenebrionis (Bt) is a biological insecticide that selectively kills only the larvae and not adults of the Colorado potato beetle. Natural enemies of the pest are also preserved by Bt. The use of straw mulch for L. decemlineata management has been adopted by very few commercial growers mainly because the mulch tend to clog the machines used for harvesting the potato. The technology could, however, benefit small scale growers who mainly use hand hoes especially in the developing countries. Moreover, as growers and consumers become enlightened on issues of environmental preservation and on ways of producing pesticide-free potatoes, mulch technology could be an alternative method for the management of the pest. The objective of this study was to evaluate the effects of straw mulch and B. thuringiensis var tenebrionis on L. decemlineata management and on P. maculiventris, P. bioculatus and L. grandis, the natural enemies of the pest.

MATERIAL AND METHODS

A study was conducted in 1992 and 1993 at the O'Neall and Pinney Purdue farms in Indiana, U.S.A. A single commercial variety, "Kennebec" was planted during the two growing seasons. The experimental design was a split-plot with straw mulch and non-mulch in main plots and esfenvalerate (Asana XL 0.66 EC, DuPont Agricultural Products, Wilmington, Delaware), B. thuringiensis (M-Trak, Mycogen, San Diego California) and non-insecticide check constituting the sub-plots. Seed potatoes were planted at the O'Neall farm on 1 May, 1992 and 8 May, 1993 and at the Pinney farm on 5 May, 1992 and 10 May, 1993. Spacing between plants was 0.30 m and between rows 0.90 m. The size of a sub-plot was 15m x 8 rows. Main plots were replicated 4 times. Soil was hilled up under the plants at one month after planting to facilitate tuberisation. An average of 14.3 kg of straw mulch per sub-plot was placed between rows and plants at the O'Neall farm on 10 June, 1992 and 12 June, 1993 and at the Pinney farm on 16 June, 1992 and 22 June, 1993. Mulch was applied when potato plants were still free of the Colorado potato beetle infestations. One week after mulch application potato plots were irrigated to moisten the straw and to prevent the same from being blown away by wind. The first and the eighth rows were the border rows. Weeding was done with a hand hoe. Overhead irrigation was applied afterwards as needed. Bt was sprayed at the recommended rate of 4.6 L ha^-1 when first and second instars of L. decemlineata appeared. Aqueous suspensions of the biological insecticide were dispensed with a low pressure hand operated pump. Bt was applied on 15 June, 1992 and 23 June, 1993 at O'Neall farm and on 22 June, 1992, 30 June, and 29 July, 1993 at Pinney farm. Esfenvalerate insecticide was applied at 0.034 kg (a.i.) ha^-1 on the same days using a similar but different applicator from that used for spraying Bt. Esfenvalerate and Bt were applied in the cool evening hours when dew had just dried and when air was relatively still to confine the insecticides within the required plots.

Thirty randomly selected potato plants per sub-plot were inspected for 3rd and 4th instars and adults of the Colorado potato beetle. Three natural enemies, namely, P. maculiventris, P. bioculatus and L. grandis, were counted per plant. A plant was the sampling unit. The identity of the natural enemies was confirmed and specimen preserved at the Purdue Entomology Research Collection. All insect counts were averaged over the total number of plants to obtain mean counts per plant. In addition, mean counts per season plus site combination were computed with a SAS statistical computer program (SAS Institute, 1985).

Statistics. Friedman's Chi-square test for two-way classification was used to detect differences between the treatments. This statistic uses ranks, instead of the original data (Friedman, 1937; Steel and Torrie, 1980). The statistic was preferred over the parametric one because the data was abnormal. Attempt to stabilise the variance by transformations failed. For example, insect counts varied from very low to high depending on the different life stages of the insect. This situation, therefore, created an abnormality within the data. Briefly, a GLM statistical package in SAS was used to condense the variable means across sites and dates. The site-season combination served as blocks. Ranking of means within blocks was done by hand where the smallest value was assigned rank 1, the middle value rank 2 and the largest one had a 3. Means that tied acquired equal ranks. Friedman's Chi-square statistic was performed on the treatment rank totals averaged over mulch treatments. Analysis of overall means was also made on six mulch and insecticide combinations and four columns of site-season combinations. Results of this latter analysis is presented only for adult counts which showed significant treatment differences. Finally, overall means of variables for the site season combinations for mulch and insecticides were calculated. From these means, percentage differences of mulch over non-mulch and insecticides over non-insecticide check were computed.

RESULTS

Late larvae. Mulch-Bt combined treatment had 49% more larvae than mulch and non-insecticide control treatment, while mulch-Asana had 37% fewer larvae than the check (Table 1). Bt and non-mulch combined treatment had 20% fewer beetles than non-mulch and non-insecticide check. Esfenvalerate applied on non-mulched potato plants had 10% more larvae than non-mulch and non-insecticide check. Overall mean was 1.44 larvae 10 plants-1 for both Bt and the control. A reduction of 3% of larval counts was due to esfenvalerate compared to the control. Treatment means did not differ significantly (P=0.05).

Table 1

Table 2

Natural enemies. Podisus maculiventris were generally few being relatively more abundant in Asana and Bt treated sub-plots than in the control. There were fewer P. maculiventris counts from mulch than non-mulch treatments (Table 3). Asana treatment in non-mulched soil main plot had 48% and Bt 16% fewer P. maculiventris than non-insecticide. The natural enemies were not recorded at the O'Neall farm in 1993 due to very low prey counts (Tables 1 and 2). Asana reduced the beneficial insect more than Bt which in turn did not alter the counts relative to the check plots. Treatment differences were not significant. Perillus bioculatus was eliminated by esfenvalerate insecticide in both mulch and non-mulch plots (Table 4). Bt, however, suppressed the natural enemy by 4%. In non-mulch treatment, Bt had 49% more P. bioculatus than the control.

Lebia grandis was similarly fewer by 7% in esfenvalerate in mulch main treatment while Bt had 32% less of the beneficial insects compared to the check (Table 5). More insects were on esfenvalerate treated (+32%) and Bt (+2%) all in non-mulch treatment compared to the control. Treatment differences did not differ significantly (P=0.05).

DISCUSSION AND CONCLUSION

Leptinotars decemlineata were unusually few during the study period, even fewer than was the case in the growers' fields. This was because the experimental plots were located at newly opened land and away from other potato fields. Only pests that immigrated from distant fields managed to reach and colonise the plots.

Table 3

Table 4

Table 5

Perillus bioculatus feeds on late larvae and adults of L. decemlineata (Jeremy, 1980). Podisus maculiventris is a general predator and feeds on more that 60 insect species (Drummond et al., 1984). The major prey include the eastern caterpillar, Malacosoma americanum (Fabr.), Colephora serratella L. and tobacco budworm, Heliothis viscerens (F.) (Lopez et al., 1976). The wide host range available to P. maculiventris enables this insect to survive under difficult conditions of low prey density like during the pre-colonisation period, when potato is harvested or sprayed with an insecticide. The beneficial insects, therefore, are usually available in the vicinity or within the potato crops (Bjegovic, 1971). The impact of the natural enemies are improved when the pest population is reduced due to application of other integrated pest management operations like crop rotation, straw mulch and B. thuringiensis (Bt) (Puttler and Long, 1983; Foster et al., 1994).

A review of P. maculiventris releases in Europe showed that this predator usually arrived late in the spring in potato fields when the first brood of the beetles had already caused damage (Boiteu, 1988). The pests are active at lower temperatures than P. maculiventris signifying that a delay in early season colonisation by the adult beetles may benefit the predaceous stink bugs and enhance their potential to suppress the pest (Biever and Chauvin, 1992). Podisus maculiventris has provided satisfactory control of the Colorado potato beetle in routine releases in the former USSR (Boiteau, 1988). Drummond et al. (1984), who conducted a study on development and survival of P. maculiventris reared on different hosts including the Colorado potato beetle, concluded that the benefical insect generally had a lower development threshold than the Colorado beetle. This meant that a portion of the larval population could escape predation because the beetles could reproduce faster than its predator. However, such deficiencies could be compensated for by the presence of other seasonal predators and supplementing with other IPM measures.

This investigation showed that straw mulch and Bt reduced L. decemlineata and preserved the indigenous natural enemies. Although non-mulch and esfenvalerate treatments the usual growers' method of beetle control killed most of the beetles, the treatment also eliminated the natural enemies in a number of cases. Mulch and Bt can be components of an environmentally friendly integrated pest management package for preserving the natural enemies while at the same time suppressing the pest population to below economic injury levels.

ACKNOWLEDGEMENTS

Thanks are due to Drs. C. Oseto, R. Edwards, J. Neal, and G. Brust of Entomology Department, Purdue University for critically reviewing the work and to Dr. W. Nyquist and J. Santini of Agronomy/Biostatistics Department for their input with the statistical analysis. We are grateful to the government of Uganda, Manpower For Agricultural Development of the USAID and the World Bank for funding the study. The paper is published by permission from the Director General of the National Research Agricultual Organisation, Entebbe, Uganda.

REFERENCES

Biever, K.D. and Chauvin, R.L. 1992. Suppression of Colorado potato beetle (Coleoptera: Chrysomelidae) with augmentative releases of predaceous stink bugs (Hemiptera: Pentatomidae). Journal of Economic Entomology 85:720-726.

Bjegovic, P. 1971. The natural enemies of the Colorado potato beetle (Leptinotars decemlineata Say) and attempts on its biological control in Yugoslavia. Zast Bilza 21:97-111.

Boiteau, G. 1988. Control of the Colorado potato beetle, Leptinotars decemlineata (Say): learning from the Soviet experience. Bulletin of Entomological Society of America 38:112-124. Casagrande, R.A. 1987. The Colorado potato beetle: 125 years of mismanagement. Bulletin of Entomological Society of America 33:142-150.

Chabossau, F. 1938. Remaques sur Lebia grandis Hentz. Review of Agricultural Zoology 37: 165-171. Abstract.

Drummond, F.A., James, R.L., Casagrande, R.A. and Faber, H. 1984. Development and survival of Podisus maculiventris (Say) (Hemiptera: Pentatomidae) a predator of the Colorado potato beetle (Coleoptera: Chrysomelidae). Environmental Entomology 13:1283-1286.

Foster, R., Latin, R., Simon, J., Weller, S., Weinzierl, R., Taber, H. and Barret, B. (Eds.). 1994. Midwest Vegetable Production Guide for Commercial Growers. 123 pp.

Friedman, M. 1937. The use of ranks to avoid the assumption of normality implicit in the analysis of variance. Journal of American Statistical Association 32:475-477.

Gauthier, N. L., Hofmaster, R.N. and Semel, M. 1981. History of Colorado potato beetle control. In: Advances in Potato Management. Lashomb, H. and Casagrande, R.A. (Eds.). Hutchinson Ross, Stroudburg, Pa.

Harris, P.M. 1978. The Potato Crop. Scientific Basis for Improvement. Chapman and Hall, London. 55 pp. Jeremy, T. 1980. The introduction of Perillus bioculatus into Europe to control the Colorado potato beetle. EPPO Bulletin 10: 475-478.

Lopez, J.P., Ridgeway, R.L. and Pinnell, R.E. 1976. Comparative efficacy of four insect predators of the bollworm and tobacco budworm. Ibid 5:1160-1164.

Puttler, B. and Long, S. H. 1983. Host specificity tests of an egg parasite, Edovum puttleri (Hymenoptera: Eulopidae) of the Colorado potato beetle (Leptinotars decemlineata) (Coleoptera: Chrysomelidae). Proceedings of the Entomological Society of Washington 85: 384-387. SAS Institute. 1985. SAS User's Guide: Statistics. SAS Institute, Carey, N. C.

Steel, R.C.D. and Torrie, J.H. 1980. Principles and Procedures of Statistics. A Biometric Approach. Second Edition, McGraw-Hill Book co., NY. 633 pp.

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