Abstract
Lethal activity and growth inhibitory activity of four commercial products of Bacillus thuringiensis (Dipol, Toarow CT, Bacilex and Thuricide) were investigated with the second instar of the silkworm, Bombyx mori, using a diet incorporation method. Six other experimental formulations of B. thuringiensis containing different amounts of the same single delta-endotoxin as the active ingredient were also investigated in a similar way. Lethal activity (LC50) was evaluated based on mortality on day 7 and growth inhibitory activity (EC50) based on weight gain for 2 d. The ratios of LC50/EC50 ranged from 1.6–2.6 among the four products and 10.1–11.6 among the six experimental formulations. It was revealed that the ratio of LC50/EC50 differed among products containing different toxins but not among formulations containing the same toxin. A linear correlation was observed between EC50 and protein content (%) of the formulations with the same toxin as observed between LC50 and protein content, indicating the possible utilization of EC50 instead of LC50 in silkworm assay for the quality control of B. thuringiensis formulations. Results also showed that an evaluation based on EC50 is 2–10 times more sensitive and time-saving than that based on LC50.
