Comparison of Commercially Produced Bacillus thuringiensis var. thuringiensis with Two Bioassay Techniques Based on Toxicity Units

Two bioassay techniques for the standardization of commercially produced Bacillus thuringiensis var. thuringiensis Berliner were simultaneously compared. A microbial insecticide preparation containing spores and toxic by products of this insect pathogen has been used as a standard for bioassays in the laboratory of Nutrilite Products, Inc., for over 10 years against saltmarsh caterpillar, Estigmene acrea (Drury), and for over 1 year on the cabbage looper, Trichoplusia ni (Hübner). While the median lethal dose (LD50) of the standard varied from day to day in both bioassays, the slope of the standard was constant. Similarly, the ratio of standard LD50 to unknown LD50 determined on different days was constant. There were no significant differences between the toxicity unit values as determined by the 2 bioassay techniques.