Chapter III-3 - Bacteria: Bioassay of Bacillus thuringiensis against lepidopteran larvae

Abstract

This chapter discusses bioassay of Bacillus thuringiensis against lepidopteran larvae. Bioassays are principally used to assess the insecticidal activity of the protein toxins found in the parasporal inclusions of B. thuringiensis. Bioassays may also be employed to determine the role of spores and spore/toxin interactions in the activity of a particular isolate or fermentation run. Diet incorporation assays are an excellent way to determine activity and host range of bacteria, viruses and protozoa when a direct measure of activity is desired. Larvae should be similarly aged, free from pathogens or other contaminants and highly vigorous. Once the B. thuringiensis is extracted from the granules, it can be tested either through diet incorporation or it can be tested by the droplet feeding method. Concentration of B. thuringiensis should be tested to determine the optimal dosage for discerning differences among treatments. Inactivation of B. thuringiensis by sunlight is a well-known phenomenon and has hindered acceptance of B. thuringiensis products. The effect of different strains or formulations of B. thuringiensis on sunlight stability can be measured in the laboratory by using equipment. Rainfall also washes B. thuringiensis deposits off foliar surfaces. It is found that fermentation conditions and formulation may greatly affect the rainfastness of B. thuringiensis and procedures to measure this characteristic could be useful.