Evaluation of potency of native Bacillus thuringiensis against maize insect pests and aflatoxin producing fungi in Kenya

Abstract

Bacillus thuringiensis is the most used biological control agent to date. Among major constraints to maize production, safety and hence food sufficiency in Kenya is infestation, damage and contamination by insect pests. Maize grains are adversely damaged by Prostephanus truncatus which occasionally paves way for the growth of aflatoxin producing fungi. The focus of this study was to establish the toxicity of native Bt against adult P. truncatus, second instar larvae of Chilo partellus, Aspergillus flavus and Aspergillus parasiticus. Seven Bt isolates (i.e. KG 411, KG 12-0, KG 20, KG 420, KM 31, KM 12 and KM 24) caused over 50% mean mortality of P. truncatus at the first preliminary dose of 10mg/ml. With subsequent analysis of the efficacy of Bt against P. truncatus, isolate KG 411 was significantly more toxic to it at 95% confidence limit (p <0.001) than all the other Bt isolates. Bt isolate KG 411 had LD50 of 0.30mg/ml which caused 77.1% mean mortality of adult P. truncatus. Potency tests of the Bt isolates against the second instar larvae of C. partellus showed significant differences at 95% confidence limit (p<0.001), with isolate KM 12 causing the highest mean mortality of 76%. Evaluation of effects of spores and crystals produced by the Bt isolates on A. flavus and A. parasiticus showed that isolate KM 31 caused the highest inhibition of fungal growth. Only isolate KM 31 was potent against both P. truncatus and the two fungal species. However isolate KG 411 which was highly toxic against P. truncatus had no significant growth inhibition effect against the two fungal strains. This result demonstrates that native Bt could constitute an alternative biological control option for management of adult P. truncatus, A. parasiticus and A. flavus in maize protection.