Compatibility and interaction between Bacillus thuringiensis and certain insecticides: perspective in management of Tuta absoluta (Lepidoptera: Gelechiidae)

Bacillus thuringiensis var. kurstaki (Berliner) (Bt) has been suggested as a biological control agent for Tuta absoluta (Meyrick). The objective of this study was to determine the interaction between abamectin, azadirachtin, indoxacarb, chlorantraniliprole, dichlorvos and metaflumizone with Bt. Effect of recommended doses of the chemical insecticides on colonisation of Bt was also investigated in culture medium. Except for metaflumizone, none of the chemicals tested reduced the colonisation of Bt compared with control. Interaction between Bt and the chemical insecticides on 2nd-instar larvae was also assessed. In interaction tests, Bt was applied at LC₅₀ level, 0, 12, 24 or 36 h after treating the larvae with LC₁₀ or LC₂₅ of the chemical insecticides. An antagonistic effect was observed in all treatments where Bt was applied immediately after the chemical insecticide. Also, antagonism was observed when treatment with Bt was done 12 h after azadirachtin and metaflumizone applications. Applying Bt 12 and 24 h after treatment with LC₂₅ of chlorantraniliprole, dichlorvos and abamectin resulted in synergism. But, synergism with LC₁₀ of dichlorvos and abamectin was observed only after 12 h. Additive effect was observed in the rest of the time and concentration combinations. Based on the results obtained, simultaneous use of the chemical insecticides tested and Bt is not recommended for T. absoluta control; and an appropriate time interval should be taken into consideration accordingly.