POTENCY OF PRODUCTS BASED ON BACILLUS-THURINGIENSIS VAR. ISRAELENSIS - INTERLABORATORY VARIATIONS

Six quality-control laboratories in 4 countries independently bioassay ed aliquots of a flowable formulation of Bacillus thuringiensis var. i sraelensis (B.t.i.) against the international standard powder IPS-82. All laboratories substantially followed World Health Organization or U .S. Department of Agriculture standard protocols. Significant differen ces were found in resulting potency values between laboratories. Facto rs that may have influenced results, such as age, stage, and strain of larvae used, amount and type of food provided to larvae, and processi ng of samples were examined. Use of different rearing temperatures, di fferent strains of Aedes aegypti L., or late 3rd instars vs. the recom mended early 4th instars did dot explain the inconsistencies. The slop e of the dose-response curve of the IPS-82 powder was influenced by pa rticle size, which varied with the nature and duration of sample homog enization. Laboratories using low-intensity processing obtained a grea ter slope in the dose-response curve for the flowable product than for the powder standard. The type and quantity of food provided to larvae affected susceptibility. Larvae fed an excess of protein-rich food be came 4th instars in 3 days and were less susceptible to B.t.i than tho se fed smaller quantities of carbohydrate-rich food that became 4th in stars in 5-7 days. Overall, deviations from standard protocols with re gard to larval stage, holding temperature, and lighting regime may not be as important as differences in sample processing and pretest reari ng conditions. The need to improve standardization in these areas, whi ch are not clearly specified in current protocols, is discussed.