Bt: what it kills and what it harms
Bt is a soil bacterium sold in several strains, and each strain kills a different group of insects. Kurstaki and aizawai kill caterpillars, israelensis kills mosquito and fungus gnat larvae, tenebrionis kills Colorado potato beetle larvae, and galleriae is used against Japanese beetles. All of them must be eaten to work. For the beneficials sold for release, it is about as safe as a pesticide gets: UC IPM rates it none or low for predatory mites, predators and parasites, and it gives the kurstaki and israelensis strains no bee precaution. Its catch is the other side of that selectivity: it works only on the right pest, at the right stage, and it breaks down in sunlight within days.
Also sold or listed as: Bacillus thuringiensis, Thuricide, Monterey B.t., DiPel, XenTari.
At a glance
- Active
- Bacillus thuringiensis (strains kurstaki, aizawai, israelensis, tenebrionis, galleriae)
- Residue
- None to short; spores break down in sunlight within days
- Safe
- 5 of 5 beneficials
What it works on
- Caterpillars (kurstaki)Cabbageworms, loopers, hornworms, tent caterpillars, bagworms, webworms and spongy moth. Young larvae are killed best. See Bt kurstaki.
- Beet armyworm (aizawai)Kentucky Extension says most Bt products fail on beet armyworm, but XenTari, an aizawai strain, works.
- Fungus gnats and mosquitoes (israelensis)Kills larvae in compost and standing water, and does nothing to the adults. Sold as Gnatrol and Mosquito Dunks. See Bti.
- Colorado potato beetle (tenebrionis)Kills young larvae only. The one product on sale, Novodor FC, is a farm pack. See Bt tenebrionis.
- Japanese beetle (galleriae)Sprayed on leaves as beetleGONE! and sold as grubGONE! granules for lawns, where university results are mixed. See Bt galleriae.
- Aphids, mites, whiteflies, adult mothsBt is not a control for these. It has to be eaten, and each strain targets a particular group of larvae (NPIC); sap-feeders and adult moths do not eat treated leaves.
Effect on beneficials
- Aphidius colemaniUC IPM's greenhouse and nursery table rates Bt kurstaki "none" for parasites and Bt aizawai low (L → safe). The tomato, cucurbit and cole-crop tables all rate kurstaki low for parasites. This is a group rating for parasites; no Aphidius-specific trial was found. — UC IPM floriculture table (2022); UC IPM tomato, cucurbit and cole-crop tablesSafe — low toxicity, under 25 % effect
- Encarsia formosaRated "none" (kurstaki) or low (aizawai) for parasites by UC IPM (L → safe). This is a group rating; no Encarsia-specific trial was found. — UC IPM floriculture table (2022)Safe — low toxicity, under 25 % effect
- Phytoseiulus persimilisUC IPM rates kurstaki, aizawai and israelensis "none" for predatory mites in greenhouse and nursery crops, and the cucurbit and cole-crop tables rate kurstaki low (L → safe). That rating is mostly from the western predatory mite. — UC IPM floriculture, cucurbit and cole-crop tablesSafe — low toxicity, under 25 % effect
- Amblyseius cucumerisRated "none" for predatory mites for all three Bt strains in UC IPM's greenhouse and nursery table (L → safe). This is a group rating, not a cucumeris trial. — UC IPM floriculture table (2022)Safe — low toxicity, under 25 % effect
- Chrysoperla carneaUC IPM rates kurstaki, aizawai and israelensis "none" for predators other than mites, and the vegetable tables rate kurstaki low for general predators (L → safe). NPIC reports little or no toxicity to lacewings, predatory beetles and parasitic wasps. — UC IPM floriculture and tomato tables; NPIC Bt fact sheet (2022)Safe — low toxicity, under 25 % effect
No data never means safe: treat an unrated pairing as caution and try it on a small area first. See every product side by side →
How it works
Bacillus thuringiensis makes crystal proteins that are toxic only after an insect eats them. In the right insect's gut they break down the gut lining. Feeding stops quickly, and the larva dies over the next few days: 2–3 days on the Thuricide label, 1–5 days according to NPIC. It has to be eaten to work (NPIC; Monterey B.t. label).
Which insects it kills depends on the strain, so check the strain before you buy. kurstaki and aizawai kill caterpillars (moth and butterfly larvae). israelensis kills the larvae of mosquitoes, gnats and some other flies. tenebrionis kills beetle larvae, chiefly Colorado potato beetle. galleriae is a beetle-active strain sold against Japanese beetle adults and lawn grubs. The home-garden caterpillar sprays, Thuricide and Monterey B.t., are both kurstaki.
Each strain has its own page: Bt kurstaki for caterpillars, Bti for fungus gnats and mosquitoes, Bt tenebrionis for potato beetle, and Bt galleriae for Japanese beetle. This page covers what they share, and where they differ, on safety for beneficials, bees, butterflies and pets.
It does not last. UV light, weather and leaf enzymes break the spores down; in one study their half-life on soybean leaves was under a day (NPIC). That is why the labels say to repeat weekly and after rain.
It is not entirely inert. NPIC notes that a few studies found non-target moths were harmed. In a 2025 Spanish study, Bt cut the egg load of a parasitoid of the tomato leafminer Tuta absoluta without raising adult mortality.
Using it
- Products
- Kurstaki for home gardens: Bonide Thuricide (EPA Reg. 4-226) and Monterey B.t. (EPA Reg. 70051-106-54705). Both are strain SA-12.
- Rate
- Thuricide: ½–4 tsp per gallon, depending on the crop and pest. For example, 1–3 tsp for tomato hornworm, and 4 tsp for bagworm and tent caterpillar on shade trees and ornamentals. Monterey B.t.: 4 tsp per gallon (2 fl oz in 3 gallons), covering 1,000 sq ft.
- Interval
- Weekly while caterpillars are feeding, and again after heavy rain (Thuricide). Monterey: every 5–7 days while they are active, more often for heavy infestations, and again after heavy rain.
- Days to harvest
- Both kurstaki labels allow spraying up to the day of harvest. Neither gives a re-entry interval.
- Mixing
- Use diluted spray within 12 hours (Thuricide) or 24 hours (Monterey). Spray both sides of the leaves, and time it for young larvae.
Cautions
- Bees
- UC IPM gives kurstaki and israelensis bee precaution III: no bee precaution unless the label requires one. It gives aizawai (XenTari) II: keep it off flowering plants unless sprayed between sunset and midnight. Both home kurstaki labels claim no harmful effect on honeybees. EPA rates the kurstaki, israelensis and tenebrionis strains low in toxicity to bees (NPIC), but the 2016 Trident (tenebrionis) label says it may be toxic to bees exposed to direct treatment and not to apply it while bees are visiting. Some studies of aizawai products found effects on bees, possibly from other ingredients (NPIC).
- Butterflies
- Kurstaki is toxic to many moth and butterfly caterpillars, not only pests (Thuricide label). The Thuricide label forbids use within 300 feet of habitat of endangered or threatened moths and butterflies. Galleriae sprayed on milkweed killed 97–100% of young monarch caterpillars in Kentucky trials, so keep it off milkweed.
- Pets and people
- Bt is very low in toxicity to people and other mammals and does not cause disease in them (NPIC). The Thuricide label carries a CAUTION signal word: harmful if swallowed or inhaled, a moderate eye irritant, and mixers and applicators must wear a dust/mist respirator rated at least N-95, R-95 or P-95. It warns that repeated exposure to high concentrations of microbial proteins can cause allergic sensitisation.
- Fish and water
- NPIC reports that kurstaki, israelensis, tenebrionis and aizawai strains are practically non-toxic to trout, bluegill and sheepshead minnow. Water fleas (Daphnia) showed moderate toxicity to kurstaki and israelensis, and aizawai seemed highly toxic to them, possibly from impurities left from production. The Mosquito Dunks label allows use in fish habitats and water gardens (see Bti).
- Plants
- No plant-injury warning appears on the kurstaki labels.
Use instead
- Bt is usually the gentle option, so the better choice is usually the right strain. For caterpillars use Bt kurstaki, for fungus gnats Bti, and for potato beetle larvae Bt tenebrionis.
- For large hornworms, hand-pick them: Bt kills small larvae best. See tomato hornworm.
- For lawn grubs, the grub nematode Heterorhabditis bacteriophora is the other biological option Minnesota and Purdue Extension list besides Bt galleriae granules. Minnesota calls results for both inconsistent.
- For aphids, whiteflies or mites, Bt does nothing. Look at insecticidal soap or the natural enemies on each pest's page.
Other products
Sources
NPIC, Bacillus thuringiensis General Fact Sheet (Gervais et al., reviewed May 2022)
UC IPM, Relative toxicities of pesticides used in floriculture and nurseries to natural enemies (2022)
UC IPM, Relative toxicities tables for tomatoes (2016), cucurbits (2016) and cole crops (2020)
Bonide Thuricide (Bt kurstaki SA-12) label, EPA Reg. 4-226 (2016 notification)
Monterey B.t. label, EPA Reg. 70051-106-54705 (2014 version)
Univ. of Kentucky ENTFACT-308, Beet Armyworm in Kentucky (Bessin)
Urbaneja-Bernat et al. 2025, Pest Manag. Sci. 81:6445 (pesticide side effects on a Tuta parasitoid)
Redmond et al. 2020, Pest Manag. Sci. 76:472 (Bt galleriae and Japanese beetle; monarch larvae)
Drafted from the cited sources, then checked claim by claim against those sources on 23 September 2026.
Always follow the label. Labels are legally binding; this page is not.