Screening of Local Bacillus thuringiensis Isolates for Toxicity to Chilo partellus, Sesamia calamistis and Busseola fusca in Kenya
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Stem borers are a major source of pre-harvest maize crop losses in Kenya and many Sub-Saharan African countries. This menace needs to be addressed if food security is to be realized in this region. Seven local isolates of Bacillus thuringiensis (Bt) strains were isolated from soils collected from Kakamega and Machakos districts in Kenya. They were screened for toxicity against 1st and 2nd instar larvae of Chilo partellus, Sesamia calamistis and Busseola fusca through laboratory bioassays on artificial and natural diets. On farm Bt toxin potency trials were carried out only in Machakos using isolate 1M which was isolated from the area. The various isolates showed differences in their toxicity to the three stem borers. Isolates 1M and VM-10 (from Machakos district) were found to be the most potent against C. partellus with larval mortalities of 100 % within 72 h. Their LD50 values were 0.004 mg/ml and 0.04 mg/ml respectively. The most toxic isolates against S. calamistis were, 44M, VM-10 and 1M, with larval mortalities of 73%, 64% and 62% respectively after 72h at a concentration of 8.6 mg/ml through artificial diet bioassays on 1st instar larvae. Isolates 44M and K10-2 showed high toxicity against B. fusca with larval mortalities of 20% by artificial diet bioassays and 44% by maize leaf bioassays respectively. Leaf disk bioassays with all the insect species showed higher larval mortalities than those done with the artificial diet bioassays indicating the larval preference of natural diet. However leaf disk bioassays with B. fusca recorded higher larval mortalities with sorghum than maize leaves. Field trial results obtained from Machakos district using a biopesticide made from isolate 1M indicated that it was highly effective in stem borer control.