Bacillus thuringiensis (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 nontarget species, such as wildlife. Some formulations can be used on essentially all food Crops.

 

Bacillus thuringiensis (Bt) is used in insecticides. Its unusual properties that make it useful for pest control in certain situations. Bt is a naturally occurring bacterium common in soils throughout the world. Several strains can infect and kill insects. Because of this property, Bt has been developed for insect control. At present, Bt is the only "microbial insecticide" in widespread use.

The insecticidal activity of Bt was first discovered in 1911. However, it was not commercially available until the 1950s. In recent years, there has been tremendous renewed interest in Bt. Several new products have been developed, largely because of the safety associated with Bt-based insecticides.

 

Unlike typical nerve-poison insecticides, Bt acts by producing proteins (delta-endotoxin, the "toxic crystal") that reacts with the cells of the gut lining of susceptible insects. These Bt proteins paralyze the digestive system, and the infected insect stops feeding within hours. Bt-affected insects generally die from starvation, which can take several days.

Occasionally, the bacteria enter the insect's blood and reproduce within the insect. However, in most insects it is the reaction of the protein crystal that is lethal to the insect. Even dead bacteria containing the proteins are effective insecticides.

The most commonly used strain of Bt (kurstaki strain) will kill only The leaf- and needle-feeding caterpillars. In the past decade, Bt strains have been developed that control certain types of fly larvae (israelensis strain, or Bti). These are widely used against larvae of mosquitoes, black flies and fungus gnats.

More recently, strains have been developed with activity against some leaf beetles, such as the Colorado potato beetle and elm leaf beetle (san diego strain, tenebrionis strain). Among the various Bt strains, insecticidal activity is specific. That is, Bt strains developed for mosquito larvae do not affect caterpillars. Development of Bt products is an active area and many manufacturers produce a variety of products. Effectiveness of the various formulations may differ.

The specific activity of Bt generally is considered highly beneficial. Unlike most insecticides, Bt insecticides do not have a broad spectrum of activity, so they do not kill beneficial insects. This includes the natural enemies of insects (predators and parasites), as well as beneficial pollinators, such as honeybees. Therefore, Bt integrates well with other natural controls. For example, in Colorado, Bt to control corn borers in field corn has been stimulated by its ability to often avoid later spider mite problems. Mite outbreaks commonly result following destruction of their natural enemies by less selective treatments.

Perhaps the major advantage is that Bt is essentially nontoxic to people, pets and wildlife. This high margin of safety recommends its use on food Crops or in other sensitive sites where pesticide use can cause adverse effects.

Insects Controlled by Bt

Kurstaki strain (Biobit, Dipel, MVP, Steward, Thuricide, etc.):

• Vegetable insects

• Cabbage worm (cabbage looper, imported cabbageworm, diamondback moth, etc.).

• Tomato and tobacco hornworm.

• Field and forage crop insects

• European corn borer (granular formulations have given good control of first generation corn borers).

• Alfalfa caterpillar, alfalfa webworm.

• Fruit crop insects

• Leafroller.

• Achemon sphinx.

• Tree and shrub insects

• Tent caterpillar.

• Fall webworm.

• Leafroller.

• Red-humped caterpillar.

• Spiny elm caterpillar.

• Western spruce budworm.

• Pine budworm.

• Pine butterfly.

Israelensis strains (Vectobac, Mosquito Dunks, Gnatrol, Bactimos, etc.)

• Mosquito.

• Black fly.

• Fungus gnat.

San diego/tenebrionis strains (Trident, M-One, M-Trak, Foil, Novodor, etc.)

• Colorado potato beetle.

• Elm leaf beetle.

• Cottonwood leaf beetle.