The most commonly used Bacillus thuringiensis (Bt ssp. kurstaki) controls moth and butterfly larvae. A. Bt must be sprayed during warm, dry conditions to thoroughly cover foliage where young caterpillars are actively feeding. B. Within about 1 day of consuming treated foliage, caterpillars become infected, relatively inactive, and stop feeding. C. An enlarged view of Bt in the gut of a caterpillar. The natural bacteria are rod-shaped and contain reproductive spores and protein toxin crystals (endotoxins), but the spores and protein crystals are separate components in some commercial Bt formulations, and these separate components and one whole bacterium (greatly enlarged) are shown here. D. Within several days of ingesting Bt, caterpillars darken and die, and their carcasses eventually decompose into a dark, liquidy, putrid mass.

Illustration by V. Winemiller.