The label is the law. Read your product's label before you mix or apply; the rates here come from the labels listed under Sources.
- Active ingredient
- Bacillus thuringiensis subsp. kurstaki (spores and insecticidal toxins)
- Chemical class
- Microbial biopesticide; each Bt strain's toxin is specific to a group of insects.[1]
- Mode of action
- IRAC Group 11A: Microbial disruptors of insect midgut membranes: Bacillus thuringiensis[3]
- Signal word
- Bonide Thuricide (Bacillus Thuringiensis (Bt) Moth Larvae (Caterpillar) Control): Caution[2]
How it works
A caterpillar has to eat Bt. Inside its high-pH gut the spore proteins become active toxins that break down the gut lining, and the larva dies of infection and starvation within 1 to 5 days; young larvae are most affected.[1]
The label says caterpillars stop feeding and die within 2 to 3 days of eating it.[2]
Why coverage and timing matter
Sunlight breaks Bt spores down fast; in one study more than 80% could no longer grow after one hour of sun.[1]
Spray both upper and lower leaf surfaces thoroughly, without heavy runoff, and keep the mix agitated.[2]
Where it is used
The label covers residential vegetable gardens, fruits, nuts, shade trees and ornamentals, and the product may be applied up to the day of harvest.[2]
Apply at the first sign of caterpillars, repeat weekly as needed, and reapply after heavy rain; begin before larvae roll themselves into protective leaf folds.[2]
Do not apply it through an irrigation system, and do not keep diluted spray longer than 12 hours.[2]
Products and registrations
| Product | Registrant | EPA Reg. No. | Active % | Formulation | Signal word |
|---|---|---|---|---|---|
| Bonide Thuricide (Bacillus Thuringiensis (Bt) Moth Larvae (Caterpillar) Control)[2] | Bonide Products, LLC | 4-226 | 15.0% | liquid concentrate | Caution |
Label rates
| Product | Use | Pests | Rate |
|---|---|---|---|
| Bonide Thuricide (Bacillus Thuringiensis (Bt) Moth Larvae (Caterpillar) Control) | Vegetables, fruits and nuts | Cabbage loopers, imported cabbageworms, diamondback moth, tomato hornworms, leafrollers, tent caterpillars | 1–4 tsp per gal of water[2] 1 to 4 teaspoons per gallon depending on crop and pest (for example, cabbage looper 2–4, diamondback moth 1–2, tomato hornworm 1–3). Treat when eggs or newly hatched larvae first appear. |
| Bonide Thuricide (Bacillus Thuringiensis (Bt) Moth Larvae (Caterpillar) Control) | Shade trees and ornamentals | Bagworms, cankerworms, gypsy moth, tent caterpillars, elm spanworm, fall webworm | 4 tsp per gal of water[2] For gypsy moth, spray when caterpillars are small (2nd or 3rd instar) and repeat in two weeks. |
How to apply it, step by step
1. Protective gear
- Mixers and applicators must wear a dust/mist-filtering respirator rated at least N-95, R-95 or P-95; repeated exposure to high concentrations of the microbial proteins can cause allergic sensitization.[2]
- Avoid breathing the spray mist and keep it off your skin, eyes and clothing; wash with soap and water afterward and wash work clothes before reuse.[2]
2. Before you spray
- Bt kills only caterpillars (moth and butterfly larvae) that eat it, so spray when eggs or newly hatched larvae first appear, before larvae hide in folded or rolled leaves.[2]
- Spray in calm weather with no rain forecast for 24 hours, and don't let it run into storm drains or gutters.[2]
3. Mixing
- Shake or stir the bottle well, then add it to water in the jar or tank while stirring: 4 teaspoons per gallon for leaf-eating caterpillars on shade trees and ornamentals, and the label's crop table (mostly 1 to 4 teaspoons per gallon) for fruits and vegetables.[2]
- A sticker such as molasses may be added; don't keep diluted spray longer than 12 hours.[2]
4. Applying
- Spray upper and lower leaf surfaces thoroughly, but not to heavy runoff, and keep the mix agitated as you go; don't apply it through an irrigation system.[2]
- Repeat weekly as needed and after heavy rain; for spongy (gypsy) moth, spray while caterpillars are small and repeat in two weeks.[2]
5. After you spray
- Caterpillars stop feeding right away and die within 2 to 3 days. It may be used up to the day of harvest.[2]
First aid
- This label prints no first-aid steps; it warns that the product is harmful if swallowed or inhaled and causes moderate eye irritation, so keep the label with you if you need to call a poison control center or doctor.[2]
Storage, spills and disposal
- Store and carry it upright, in a cool, dry place children and pets can't reach.[2]
- Put an empty bottle in the trash or recycling; for a partly full one, call your local solid waste agency, and never pour unused product down a drain.[2]
From the safety data sheet
- The SDS lists the concentrate as not classified under the workplace (GHS) rules; the pesticide label hazard statement it reproduces carries the signal word Caution: harmful if swallowed or inhaled and causes moderate eye irritation.[4]
- If it gets in the eyes, rinse right away with plenty of water and get medical attention if pain, blinking or redness continues; on skin, take off affected clothing and wash the skin with mild soap and water, then rinse with warm water.[4]
- If someone swallows it, have them rinse their mouth, don't make them vomit, and get emergency medical help; if someone breathes in spray mist, move them to fresh air and let them rest.[4]
- The label text in the SDS says people mixing or applying it must wear a NIOSH-approved N-95, R-95 or P-95 dust/mist respirator, because repeated exposure to high concentrations of microbial proteins can cause allergic sensitization.[4]
- Soak up spills with clay, diatomaceous earth or another inert absorbent and keep it out of drains and waterways; store it in a cool, dry place out of children's reach, since storage above 90°F can weaken it.[4]
Safety and environmental hazards
People
The label signal word is CAUTION: harmful if swallowed or inhaled and causes moderate eye irritation. NPIC explains that Bt toxins need the high-pH gut of an insect larva to activate and are destroyed by the acid of the human stomach, and that eating Bt proteins has not been shown to cause cancer, mutations or birth defects.[2][1]
Pets
The label says to store it where children and pets cannot get to it. Mammals lack the gut enzymes and receptors that activate Bt toxins.[2][1]
Bees and pollinators
NPIC reports that a pure Bt strain tested on honeybees was practically nontoxic; the Thuricide label says it has no harmful effect on honeybees or lady beetles when used as directed. Because kurstaki toxin targets caterpillars, butterfly larvae on treated plants can be killed too.[1][2]
Fish and water
NPIC reports Bt is practically nontoxic to birds, fish and shrimp.[1]
Common questions
- How much Bt per gallon of water?
- The Thuricide label calls for 1 to 4 teaspoons per gallon on vegetables and fruit depending on the pest, and 4 teaspoons per gallon on shade trees and ornamentals.[2]
- Is Bt safe to use on vegetables right before harvest?
- Yes. The label allows application up to the day of harvest.[2]
- Will Bt kill butterflies?
- Bt kurstaki targets the caterpillars of moths and butterflies, so butterfly caterpillars that eat treated leaves can be killed along with the pests.[1]
- Why didn't Bt work on my caterpillars?
- It works best on young larvae and has to be eaten, and sunlight breaks it down quickly, so spray early, cover both leaf surfaces and reapply after rain.[1][2]
Sources
- National Pesticide Information Center, “Bacillus thuringiensis General Fact Sheet.” npic.orst.edu/factsheets/btgen.html (accessed 2026-10-06)
- U.S. EPA Pesticide Product Label System, “Bonide Bacillus Thuringiensis (Bt) Moth Larvae (Caterpillar) Control (Thuricide) label, EPA Reg. No. 4-226 (EPA-stamped, Apr. 29, 2016).” www3.epa.gov/pesticides/chem_search/ppls/000004-00226-20160429.pdf (accessed 2026-10-06)
- Insecticide Resistance Action Committee, “IRAC Mode of Action Classification Scheme, Version 11.5 (Feb. 2026).” irac-online.org/documents/moa-classification/ (accessed 2026-10-06)
- Bonide Products, LLC, “Bonide Thuricide Bacillus Thuringiensis (BT) Safety Data Sheet (Bonide Products, revised November 2, 2020; the SDS prints no EPA number, but Thuricide is a registered brand name of EPA Reg. No. 4-226, the page's product).” cdn.shopify.com/s/files/1/0533/3847/2647/files/BND802_MSDS.pdf (accessed 2026-10-19)