A naturally occurring soil bacterium, Bacillus thuringiensis, is usually called Bt (“bee-tee”). A biological_insecticide product, it makes larvae--the worm or caterpillar stage of various pest insects--fatally ill when they eat something that has Bt on it. Unlike many other insecticides, both chemical and botanical, Bt has no effect on the environment. It is a nontoxic “bio-rational” insecticide that kills most leaf-eating caterpillars on trees, shrubs, flowers, and vegetables without harming other types of insects, fish, birds, animals, and humans.

Some forms of Bt can be stored for up to 3 years.
Bt is the active ingredient in a number of insecticide products that control worms and caterpillars, mosquitoes, potato beetles and flies. Sprayed or dusted on leaves of plants when caterpillars are actively feeding on the foliage, it enters their system as they eat. There the bacteria release crystals of a toxin that paralyzes the digestive tract, causing the caterpillars to stop feeding within two hours. Within a day or two, the caterpillars die. Properly used, Bt will kill from 70% to 90% of targeted pests. Any survivors are usually eaten by birds and other natural predators.

For information about actual products go to Yardener's Tools Shed for the file Bt Products