Development of entomopathogenic fungi as biopesticides for the management of Cowpea Aphid, Aphis craccivora Koch

Abstract

Cowpea (Vigna unguiculata L. Walp) is an important African indigenous vegetable in the tropics where it is considered as a key source of nutrients and income for small-holder farmers.The crop is drought tolerant and does well in poor soils because of its ability to fix atmospheric nitrogen. Cowpea is commonly used in soil fertility management and provides livestock fodder. However, the exploitation of the full potential of this crop is constrained by abiotic and biotic factors that lead to low yields in Africa. Among the biotic factor’s cowpea aphid (Aphis craccivora Koch) is a major limiting factor in cowpea production and leads to yield losses of up to 100% when infestations occur in early stages of the crop and when control measures are delayed or not implemented. Where cowpea is grown as leafy vegetable, presence of honeydew produced by aphid on leaves renders them unfit for human consumption further occasioning yield losses. Chemical of insect pests’ control is the most popular adopted management strategy but is associated with many negative effects on users, environment and beneficial organisms. This study evaluated pathogenicity of Metarhizium anisopliae, Beauveria bassiana and Isaria sp. isolates against A. craccivora. All the isolates were pathogenic to A. craccivora but M. anisopliae isolates ICIPE 62 and ICIPE 41, and B. bassiana ICIPE 644 outperformed all the other isolates by producing the highest mortalities of 90, 80 and 75% within the shortest time (LT50) of 3.3, 3.6 and 3.7 days, respectively at conidial concentration of 1 × 108 conidia ml-1 . Evaluation of dose-dependent mortality was done for the three isolates and ICIPE 62 produced the lowest concentration that killed 50% of the tested insects (LC50 = 2.3 × 106 ).Comparison of relative potency showed that ICIPE 62 was more potent than the other isolates and the same isolate produced the highest number of spores (4.5 × 107 ) on aphid cadavers 6 days post-treatment. In screenhouse evaluation oil and aqueous formulations of ICIPE 62 resulted in reduction of aphid population compared to control.Field evaluation of aqueous and oil formulations did not show any significant effects on the aphids in the first season characterized by heavy and frequent rain and lower temperatures but were able to highly reduce aphid population in the second season which was dry with reduced and infrequent rainfall and elevated temperatures. However, application of Duduthrin in both seasons did not reduce aphid population compared to fungal treated plots. Treatment applications did not confer yield benefits in season 1(wet) but did in season 2 (dry) where EPF treated plots produced more leaf yield compared to other treatments though grain yield in season 2 was not different among the treatments. Efficacy of combining intercropping of cowpea and maize and application of entomopathogenic fungi (EPF) was evaluated for three cropping seasons under field conditions. The performance of the EPF was compared to Duduthrin and untreated control in cowpea monocrop and cowpea-maize intercrop. In the first season which recorded high rainfall and cooler temperatures, the application of EPF and Duduthrin in either monocrop or intercrop did not reduce aphid populations. Similarly, in the second season with characteristic dry period with lower precipitation and higher temperatures, intercropping alone or application of Duduthrin in an intercrop did not reduce aphid population nor protect the crop from A. craccivora damage. However, at the same time, application of EPF in a maize-cowpeaintercrop reduced aphid population and protected the crop from aphid damage. The third season with average rainfall lower than seson 1 but higher than season 2, application of EPF in the monocrop and intercrop significantly reduced aphid population. Comparison of leaf yield showed that EPF treated cowpea-maize intercrop did not produce higher yields than other intercrop treatment (maize-cowpea intercrop treated with Duduthrin and the untreated maizecowpea intercrop) combinations in seasons 1 and 3. However, in season 2 (dry and hot) the cowpea leaf yield in cowpea-maize intercrop treated with EPF was similar to cowpea monocrop treated with EPF and higher than cowpea-maize intercrop treated with Duduthrin despite the monocrops having higher cowpea plant population. Cowpea grain yield in cowpeamaize intercrop was higher in season 2 among the intercrop treatment combinations.Application of Duduthrin did not increase leaf and grain yield production in all the three cropping seasons. This study has identified ICIPE 62 as a potential fungal-based biopesticide for the management of cowpea aphid. The study also demonstrated the efficacy of combining intercropping and application of EPF as a viable control strategy for A. craccivora under field conditions.However, the success of this approach depends primarily on optimal weather conditions.Adoption of these control strategies could significantly reduce reliance on chemical pesticides and confer more benefits to small-holder vegetable producers.