Laboratory evaluation of toxicity of Bacillus thuringiensis subsp. kurstaki to larvae of mulberry silkworm, Bombyx mori L.

The toxicity of Bacillus thuringiensis subsp. kurstaki to different larval instars of mulberry silkworm, Bombyx mori L. was evaluated. The per cent larval mortality in all the instars of B. mori was higher at higher concentrations of B.t. but decreased with the decrease in B.t. concentrations. Higher mortality percentages were observed in the early instars as compared to later instars. The mortality rate decreased with the increase in age of the larvae. The LC50 was 1.40 x 101, 4.20 X 102, 1.0 x 103, 2.0 x 105 and 6.30 x 106 spores ml−1 for the first, second, third, fourth and fifth instar larvae, respectively. Longer incubation period was observed at lower concentrations of B.t. Higher concentrations of B.t. were associated with reduced pupation, higher pupal mortality, higher malformed adult emergence and lower emergence of normal adults in all the instars.