Although we've known about Bt for 80 years, its popularity for use in the garden is fairly recent. This bacterium was first isolated in Japan around 1902. It was found to be fatal to silkworms and the Japanese had to control it to maintain silkworm production. 1911 it was rediscovered in Germany where it was used to control flour moth caterpillars. Little else was heard about Bt until 1938 where it was used to control the corn borer across Europe. With the start of World War II and the advent of chemical pesticides little or no research was conducted on Bt.

In 1962 a French scientist discovered a strain of Bt that was very effective against a family of caterpillar's such as cabbage worms. The strain of Bt used presently was formulated by a USDA scientist around 1970. This form of Bt has been found to control more than 200 caterpillar type insects as well as the pesky mosquito.

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Very effective

against Mosquitos

Bt is an insecticide with unusual properties that make it useful for pest control. It is a naturally occurring bacterium common in soils throughout the world. Several strains can infect and kill insects. Because of this property, Bt is the only "microbial insecticide" in widespread use.

How Does it Work?


Bt has to be eaten to cause mortality. The Bt toxin dissolves in the insect's gut and becomes active. The toxins then attack the cells of the insect. The Bt spores spills out and germinate in the insect, causing death within a couple days.

ImageExcellent results in controlling gypsy moth

The best time to apply BT is after all the eggs have hatched and while the caterpillars are very small. Bt bacteria nor the protein toxins have any effect on people, pets (unless you are raising caterpillars as pets!), livestock, or honeybees. In fact, there is a Bt product that is specifically registered to control the honeybee infesting caterpillar, the wax moth.

There is considerable misunderstanding about the effects of Bt sprays. Indeed, Bt sprays will kill most of the young caterpillars that may ingest it as they feed on treated plant leaves. Both damaging caterpillars, such as the gypsy moth as well as harmless butterflies and moths, can be killed. However, by avoiding sensitive areas that may serve as habitat for desirable or endangered butterflies and moths, unwanted effects can be avoided.

Caterpillars killed by Bt first stop feeding then drop to the ground and decay harmlessly. The Bt applied in a spray does not multiply or accumulate in the environment.

Bt is susceptible to degradation by sunlight. Most formulations persist on foliage less than a week following application. Some of the newer strains developed for leaf beetle control become ineffective in about 24 hours.

Bt products for caterpillar control are available from your local garden center. However, you will have to read the labels carefully because the Bt package looks just like regular insecticides. Common over-the-counter product brands include Dipel, Thuricide, and Caterpillar Attack.

Facts about Bt:

  • Bacillus thuringiensis (Bt) is a naturally occurring bacterial disease of insects. These bacteria are the active ingredient in some insecticides.
  • Bt insecticides are most commonly used against some leaf- and needle-feeding caterpillars. Recently, strains have been produced that affect certain fly larvae, such as mosquitoes, and larvae of leaf beetles.
  • Bt is considered safe to people and non-target species, such as wildlife. Some formulations can be used on essentially all food crops.

If you're looking for an organic control of certain insects in your yards and gardens, consider Bt - it might be just the garden helper you're looking for.