Abstract
In bioassays for quality control of Bacillus thuringiensis (BT) formulations, the diet-incorporation method using silkworm (Bombyx mori) has been generally used since 1973. To make the silkworm assay simpler, we attempted a new procedure for preparing the incorporated diet; a powdery diet, BT solution and 2% agar solution were mixed in liquid form at 55°C and then allowed to soldify. The present study showed that the diet water content affected the larval development and BT susceptibility. An appropriate water content with no adverse effects was about 75%. Larval susceptibility to BT formulations differed between instars, and was high in the 2nd, 3rd and 4th instars, and low in the 1st and 5th instars. It decreased with days after ecdysis in the same instar. Larval mortality occurred after 2 days and then increased, but differed with exposure period and types of BT formulations.
