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. Bacterial agents, like Bt, are effective in controlling insects in the larva stage only. The larva stage in an insectâ€™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. The larval stage is the stage in which the insect appears worm or caterpillar-like. The Bt is applied to the foliage of plants infested with larva. If possible, apply the Bt to the underside of the leaf surface for two reasons: 1) Most larvas feed from the underside of the leaf surface. 2) Bt is broken down faster in sunlight. Application to the undersides of the leaf surfaces will prolong Bt's activity. High temperatures do not encourage Bt's breakdown.
After Bt spores are ingested by larvae, they grow and reproduce, meanwhile producing crystalline toxins. The crystalline toxins paralyze the digestive tract of the larvae causing it to cease eating. Death will follow, but time of death can range anywhere from 12 hours to 5 days after ingestion. This depends on the amount of Bt ingested, the size and variety of the larvae and variety of Bt used for control. 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, fruit worms, 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.
Pests such as ear worms and corn borers pose a big problem for organic farmers. Ear worms are found in the tips of corn, while borers chew their way into the side of the husk. Remember, organic insecticides don't work on insect eggs, so worms will continue to hatch even after applying Bt.