Impact assessment of an exotic parasitoid on stemborer (Lepidoptera) population dynamics in Kenya

A ratio-dependent host-parasite interaction model with a linear trend was d eveloped to analyze the impact of the exotic parasitoid Cotesia flavipes (C ameron) (Hymenoptera: Braconidae) on stemborer population dynamics in the s outhern coastal area of Kenya. The time dependence of the host and that bet ween the host and parasitoid were tested using autocorrelation and cross-co rrelation functions. The criterion for measuring the parasitoid impact was the difference between the model predictions with and without the parasitoi d. The results indicated that the suppressive impact of the parasitoid on s temborer densities began in the short rains season of 1997-1998 in the nort hern part of the coast zone but was only seen in the southern part of the c oast beginning with the long rains season of 1999. The density reduction of the total stemborer complex, which includes one alien and two native speci es, by C. flavipes was 1.05 +/- 0.25 larvae per plant in the north coast an d 0.62 +/- 0.58 larvae per plant in the south coast during the long rains s eason of 1999, The reduction of the exotic stemborer Chilo partellus (Swinh oe) (Lepidoptera: Crambidae) by C. flavipes was 0.99 +/- 0.25 larvae per pl ant in the north coast in the long rains season of 1999, or a 52.94 +/- 13. 37% decrease of the C. partellus density. In the south coast. the reduction was 0.83 +/- 0.41 larvae per plant or similar to 33.07 +/- 16.33% decrease in the C. partellus density.