SILKWORM ASSAY FOR BACILLUS-THURINGIENSIS FORMULATIONS USING DIET INCORPORATION METHOD .1. EVALUATION BASED ON LARVAL MORTALITY

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