How does Bacillus thuringiensis (Bt) work?

Last updated: October 2026

Bacillus thuringiensis (Bt) is a soil bacterium whose spores carry proteins that are toxic to insect larvae only when eaten¹. In the alkaline gut of a susceptible larva, the protein crystals dissolve, bind to receptors on gut cells and form pores that break down the gut lining^1,2. The larva stops feeding and dies within 1 to 5 days^1,3. Each strain targets a narrow insect group¹.

What is Bacillus thuringiensis?

Bacillus thuringiensis, usually shortened to Bt, is a bacterium found naturally in soils throughout the world¹. To reproduce it forms spores, and those spores carry proteins that are toxic to insect larvae when the larvae eat them¹. Because it comes from a natural source, Bt is classed as a biopesticide¹.

Bt has been registered for use in pesticides by the U.S. EPA since 1961 and is found in over 130 registered products, sold as sprays, dusts, granules, concentrates and pellets¹. Some of these products are approved for organic agriculture¹. The same family of insecticidal proteins is used worldwide both in sprayable formulations and in transgenic crops that produce the proteins themselves².

In North America, interest in Bt peaks in June, when young caterpillars and the first mosquito larvae appear, which is when Bt works best. In the Southern Hemisphere, the same window falls around December and January.

How does Bt kill insect larvae?

Bt works only after it is eaten. When a susceptible larva swallows the protein crystals, they dissolve in its gut². The gut must have a high pH, 9.0 to 10.5, to activate the toxin¹. Gut enzymes cut the proteins into their active form, which binds to specific receptor proteins on the surface of the midgut cells². The bound toxins then form pores in the cell membranes and break down the cells of the gut lining².

With its gut lining destroyed, the larva dies of infection and starvation, usually within 1 to 5 days¹. Leafrollers, for example, stop eating within hours after feeding on a sprayed leaf, even though they die several days later³. Young larvae are the most affected¹.

That chain of steps is also why Bt is so selective. Its specificity rests on the toxin's fit with receptor proteins in the larval gut². The human stomach is acidic and destroys the toxins, and mammals lack both the enzymes that release them and the gut receptors they bind to¹.

Which Bt strain do you need?

Strain matters more than brand. Each type of Bt toxin is specific to one target insect family, so a product made for caterpillars will not control mosquito larvae, and the reverse¹.

How do you use a Bt spray on caterpillars?

Spray early and spray thoroughly. University of California IPM guidance on leafrollers says Bt is most effective when larvae are small, under 1/2 inch long, that thorough coverage is required, and that control usually takes more than one application³. Caterpillars must eat treated foliage to be killed³. For hornworms on tomatoes, UC IPM lists Bt among the options effective against smaller larvae, alongside handpicking⁵.

Bt does not last long on leaves. On leaf surfaces it is broken down by ultraviolet light, weather and plant enzymes, and typical foliage half-lives are 1 to 4 days¹. In one study, more than 80% of spores could no longer grow after one hour of sun exposure¹. For tent caterpillars, UC IPM recommends a second application about 7 to 10 days after the first because of this short persistence, and notes that the spray has to penetrate the silk webbing⁴. Follow the product label for rates and intervals.

Not every caterpillar needs treating. UC IPM notes that natural enemies normally keep hornworm populations in check, and that healthy plants tolerate some leaf loss to tent caterpillars, which generally do not warrant an insecticide^5,4.

How is Bti used for mosquito and fungus gnat larvae?

Bti, short for Bacillus thuringiensis subspecies israelensis, produces toxins that target the larvae of mosquitoes, black flies and fungus gnats⁶. The U.S. EPA has registered five Bti strains in 48 products for residential, commercial and agricultural use, mainly against mosquito larvae⁶. It is applied to standing water where larvae live, including flower pots, tires and bird baths around homes, and larger water bodies such as ponds and irrigation ditches⁶. EPA reports no documented resistance to Bti as a mosquito larvicide⁶. EPA also advises eliminating standing water, even tiny amounts, so mosquitoes cannot breed⁶.

For fungus gnats (Bradysia species), Colorado State University Extension states that the larvae may be killed by Bti applied as a drench to the growing medium, though Bti formulations sold for fungus gnats are generally unavailable through retail outlets⁷. Check that the label of any Bti product allows the use you intend; some products may be used on certain drinking water, such as cisterns, and others may not⁶.

Does Bt harm bees, beneficial insects, fish or people?

Bt strains have shown little to no toxicity to lacewings, predatory beetles, parasitic wasps or earthworms that are not their targets, and EPA concluded that the tenebrionis, israelensis and kurstaki strains are low in toxicity to bees¹. A few studies have found harm to non-target moths¹, and UC IPM notes that Bt kills butterfly as well as moth caterpillars that feed on sprayed foliage⁴.

In water, the tested strains were practically nontoxic to trout, bluegill and sheepshead minnows¹. A 2023 systematic review of Bti mosquito control, covering 95 articles, nevertheless found a consistent negative effect on the abundance of non-biting midges (Chironomidae) and crustaceans⁸.

For people, Bt is low in toxicity when eaten or inhaled, although some Bt products have caused eye and skin irritation¹. EPA exempts Bt from food residue limits because of its low toxicity¹. Follow the product label for protective equipment and re-entry.