Screening of exotic and locally isolatedBacillus thuringiensis(Berliner) strains in Kenya for toxicity to the spotted stem borer,Chilo partellus(Swinhoe)

Abstract

Abstract Seventy‐one identified, exotic, B. thuringiensis strains, representing 31 H‐serotypes and 150 locally isolated B. thuringiensis strains, were screened for activity against second‐instar C. partellus larvae. Differences in the toxicity of different subspecies to C. partellus larvae were revealed in the screening tests. Bio‐assays were performed on selected, toxic B. thuringiensis strains, and varieties entomocidus, kenyae and kurstaki (obtained from the USDA collection) were found to be the most potent strains tested, having calculated LC50 values of 2.00 x 105, 5.10 x 105 and 6.30 x 105colony‐forming units per gram (c.f.u./g) of artificial diet respectively. The results further highlighted the differences in the activity of different B. thuringiensis subspecies, and showed that there was variation in the potency of strains within the same subspecies. Thus the origin of the strain appears to affect its toxicity to a particular target species. A number of highly toxic B. thuringiensis isolates were recovered from soils, insect frass and dead insect material collected in Kenya. Isolates A‐3, B1–1, A‐C‐2 and L1–6, recovered from dead Heliothis armigera larvae and insect frass, were the most potent strains recovered, with respective LC50 values of 7.00 x 104, 1.70 x 105, 2.30 x 105 and 2.60 x 105 c.f.u./g of artificial diet. These strains are therefore excellent candidates for use in a control programme against C. partellus.