Abstract
In bioassays for quality control of Bacillus thuringiensis formulations, the silkworm, Bombyx mori, has been generally used as a test insect in Japan. The median lethal concentration (LC50) of a test sample to the 3rd instar silkworm is usually calculated based on mortality at 5 days after treatment and is compared with that of a standard sample. In the present study, the total weight of 2nd or 3rd instar silkworm survivors at 48 h after treatment was measured instead of mortality and per cent growth reduction was obtained by comparison of larval weight gains in treatments against those of controls. Relationships between logalistic concentrations of test sample and per cent growth reduction by probit showed a straight line and the median growth reductive concentrations (EC50) were calculated by probit analysis. EC50 values of test samples between the 2nd and 3rd instar silkworms at 25°C were similar of about 10 times lower than the LC50 values. EC50's in the 2nd instar silkworms at 20 and 25°C were similar. The present study demonstrated that the bioassay based on growth reduction in the 2nd instar silkworm was more sensitive and speedy than one based on mortality for the quality control of B. thuringiensis formulations, reducing the test period from 5 to 2 days at 25°C.
