Bacillus thuringiensis (Bt) is a naturally occurring, soil borne organism that has gained recent popularity for its ability to control certain insect pests in a natural, environmentally friendly manner. Bt kills leaf-eating caterpillars by invading their digestive system. It is sold in both powder and liquid forms under a number of trade names including Dipel, Condor and Biotrol.

Bacterial agents, like Bt, are effective in controlling insects in the larva stage only. The larva stage in an insect's life cycle is the stage during which most of the feeding occurs. Since Bt must be ingested to work, the insect must be controlled during the larval stage (in which the insect appears worm or caterpillar-like).

Bt is applied to the foliage of plants infested with a leaf or needle eating larva. Apply Bt to the underside of the leaf surface, because that is where most larva feed. Also, Bt breaks down quickly when exposed to sunlight.

There are different strains or varieties of Bt available that have been selected for the control of specific insects. Bt variety kurstaki (BTK) controls the European corn borer, tomato hornworms, fruitworms, cabbageworm, cabbage looper, spring and fall cankerworm, spruce budworm, and other caterpillar-like larvae. Bt variety san diego (BTSD) controls early larvae of the Colorado potato beetle. Bt variety israelensis (BTI) controls mosquitoes, black flies and fungus gnats.