PARAMETERS INFLUENCING POTENCY OF BACILLUS-THURINGIENSIS VAR ISRAELENSIS PRODUCTS

Bioassays of products based on Bacillus thuringiensis var. israelensis have been carried out according to standard protocols. These analyses revealed that the slopes of the log-probit transformed concentration mortality curves of various products were different from that of the i nternational standards (IPS82 for B. thuringiensis israelensis). For s tatistical reasons, this invalidates the tests. Products giving variou s slopes of the concentration mortality curves will obtain different p otencies when estimated at a LC90 level than when estimated at LC50 le vel, as normally done. The LC90 level is probably more relevant for th e field effect. Changing the median particle size of a product in a no n destructive way results in change of slope and LC50 and thereby pote ncy. Therefore, potency of a product as measured in these bioassays is not just a measure of the quantity of B. thuringiensis israelensis cr ystal protein present, but a function of product parameters like media n particle size. Biochemical methods for quantification of toxin can t herefore not relate simply to potency of the products obtained with th is method. It is suggested that standard protocols for bioassay may be changed to assure equal particle size of products and samples to obta in parallel dose response.