Host Plant dependent toxicity of Bacillus thurrngiensis var. kurstaki strains and Cry toxins to Plutella xylostella (Lepidoptera: Plutellidae)

The toxicity of Bacillus thuringiensis var. kurstaki strains and toxins to six-day old larvae of the diamondback moth, Plutella xylostella was evaluated by leaf disc bioassays using the different host plants viz., cauliflower, cabbage, broccoli, knol khol, mustard and Chinese mustard. The progeny of laboratory reared insects fed for a generation on the different host plants was used for bioassay. The four-day median lethal concentration of B. thuringiensis strains (mg l−1) for six-day old larvae varied depending upon the host crops; 6.7-fold (LC50 0.011–0.074 mg l−1) for HD-1, 7.4-fold LC50 (LC50 0.29–2.15 mg l−1) for HD-73, 48.3-fold (LC50 0.003–0.145 mg l−1) for Cry1Aa and 79-fold (LC50 0.001–0.079 mg l−1) for Cry1Ab. B. thuringiensis var. kurstaki strains and toxins also inhibited pupation. The six-day median pupation inhibitory concentration ranged for each strain or toxin depending up on the host crops viz., HD-1 0.0005–0.011 mg l−1 (22-fold), HD-73 0.013–0.63 mg l−1 (48.5-fold), Cry1Aa 0.00008–0.0067 mg l−1 (84-fold) and Cry1Ab 0.0002–0.0022 mg l−1 (11-fold). The inhibition of pupation could be considered a highly sensitive and easy criterion of assay. The toxicity of B. thuringiensis var. kurstaki is influenced by the host crop interaction with test insect and toxins or strains per se. These studies also emphasize a need for crop-specific field doses of B. thuringiensis and endotoxin expression in transgenic crops for maximal control of P. xylostella.