Effectiveness of Entomopathogenic Fungus Beauveria bassiana Isolate on Callosobruchus maculatus (Coleoptera: Chrysomelidae) Mortality under Different Concentrations in Kenya

Callosobruchus maculatus Fab. (Coleoptera: Bruchidae), is a major field-to-store post-harvest pest of in the tropics and worldwide. They cause weight loss, decreased germination potential and reduction in commercial and aesthetic value as a result of physical contamination of grain by insects, eggs and excrement, decreased nutritional value. Entomopathogenic fungi have been employed in control of a number of storage pests. Nine B. bassiana isolates were evaluated for effectiveness in controlling C. maculatus in cowpea grain under controlled laboratory conditions. Mortality of the bruchids was evaluated stepwise where the most effective isolate concentrations against C. maculatus were assessed. Initial assessment involved determination of the most effective concentration among the isolates following dilution. The most effective isolate concentrations were then doubled and halved to identify the most effective dose rate. The isolates showed significant differences on the mortality of cowpea bruchids at the different concentrations. The halved rate from the most effective rate with BBC recording 85% and 76.7 % cumulative mortality at day 3 during the first and second season respectively. Isolate J35 had 98.2% mortality at 5 days after application during the first season and 88.3% during the second season, which was only lower to the formulated isolates BBC and BVT. At the optimal dose rate experiment, isolate J57 presented the highest cumulative mortality (25.5%) at 14 days after application while J59 recorded the lowest cumulative mortality at 11.8% at the same time. The combination of isolate J39 and RI showed the highest cumulative mortality of 40.4% and 44.4% during the first and second season respectively. Results obtained from this study support the importance of exploring the use of B. bassiana in control of C. maculatus during storage.