Prospecting for fine scale establishment of exotic stem borer pupal parasitoid (Xanthopimpla stemmator Thunberg) in Kenya

Abstract

Lepidopteran stem borers are an important constraint to cereal production in Sub- Saharan Africa. The exotic Chilo partellus (Swinhoe) is one of the most economically important stem borer pest causing extensive losses on cereal crops in Kenya. This pest has also displaced indigenous species of stem borers while expanding its range in warm, mid and high altitude areas. In order to exert control on various developmental stages of this pest, both Cotesia flavipes Cameron (larval endoparasitoid) and Xanthopimpla stemmator Thunberg (pupal endoparasitoid) were imported and released in 2002 in the Eastern region of Kenya. This study was conducted to assess the establishment status, spread and impact of X. stemmator on C. partellus following its release in Kenya. Stem borer sampling was done on farms where the biocontrol agents had been released and on transects radiating outwards from them every 15km to assess spread. A total of 100 maize plants were inspected for stem borer infestation and destructive sampling done on 10 maize stems per farm to collect immature stem borer stages. Emerging parasitoids and adult moths were identified, counted and recorded. Results of this study showed that C. partellus was the most dominant stem borer species (constituted 71.2%) followed by Sesemia calamistis and Busseola fusca (26.0 and 2.8% respectively). A decrease in overall stem borer infestation (22.47±7.42%) with no significant difference across distances from parasitoid release points (F=0.4; df=3, 51; p>0.05) was also recorded. Seven parasitoid species were recovered, the most abundant being C. flavipes and this coupled with a significant increase in parasitism (25.27±3.27%) (V=1213, p˂0.05) from pre-release levels. The parasitoid of interest, X. stemmator was not recovered. This suggests a failure to establish though there is need to sample alternative hosts before this is declared. Key words: Classical biological control, lepidoptera, pupal parasitoid, post release survey, Xanthopimpla stemmator, Eastern Kenya