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. 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. The larval stage is the stage in which the insect appears worm or caterpillar-like.
It should be applied to the foliage of plants infested with larva. If possible, apply the Bt to the underside of the leaf surface for two reasons. Most larvas feed from the underside of the leaf surface and Bt is broken down faster in sunlight. Application to the undersides of the leaf surfaces will prolong Bt's activity.
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.
Do NOT kill tomato hornworms if they appear like the worm in this photo. The eggs on the worms back are Trichogramma wasp eggs. They are killing the worm by sucking its internal juices. Then they will hatch into more wasps and kill more worms.
