Bacillus thuringiensis potency bioassays against Heliothis armigera, Earias insulana, and Spodoptera littoralis larvae based on standardized diets

Bacillus thuringiensis screening programs based on the official potency bioassay using third-instar larvae and on a neonate bioassay were developed for Heliothis armigera, Earias insulana, and Spondoptera littoralis. In these bioassays, the diets were standardized to be suitable, with minor modifications, for feeding of the three lepidopterans. The bioassay protocol was based on determination of the LC50 of the microbial standard HD-1-S-80 in the insects susceptible to B. thuringiensis var. kurstaki strains. This was followed by preliminary screening of B. thuringiensis strains at the LC50 of the B. thuringiensis standard. The B. thuringiensis strains causing 100% mortality at this LC50 in the larvae were selected for potency determinations. The neonate bioassay was suitable for accurate determinations of potencies also in S. littoralis--a representative of insects weakly susceptible to the HD-1 standard. The role of the official and the neonate bioassays in developing microbial control programs is discussed.