Evaluation of the impact of cotesia flavipes Cameron (Hymenoptera: Braconidae) and indeginous parasitoids on stemborer populations in southwestern Kenya

Abstract

Data obtained from regular field samples throughout the life cycle of Chilo partellus in three experimental sites; Ungoye, Mbita point and Kuja River, were used to construct life tables. Key factor analysis was conducted to determine the key mortality factors.

Results showed little variation in the total inter-generation mortality expressed as 100rx at all the three developmental stages of C.partellus. The mean values obtained ranged from 89.35%-98.1%, 86.1 %-99.8% and 91.7% -96.03% for Kuja river, Mbita point and Ungoye respectively. The most common cause of mortality was classified as disappearance whereas parasitism contributed minimally to the C.partellus mortality at all the sites. Disappearance at Ungoye was the key mortality factor and was significantly correlated with the total generation mortality.

The regressions of each mortality factor on log density for each age interval was also computed for the Ungoye experimental site to test for density dependence. Mortality due to disappearance at the small instar stage (k2), mortality due to disappearance at the medium larval stage (k4) and mortality due to disappearance at the medium larval stage (k2), mortality due to disappearance at the medium larval stage (K4) and mortality due to parasitism at the pupal stage (K7) showed positive correlations with their respective log density, indicating that mortality in these developmental stages acted in a density dependent fashion.

In the farmer’s fields in southwestern Kenya, C. partellus was found to be the predominant stemborer species in all the seasons. Cotesia sesamiae was the dominant larval parasitoid, but the exotic parasitoid, but the exotic parasitoid C. flavipes had a seasonal parasitism rate, which ranged from 1.25% to 6.14%.