# Bacillus thuringiensis

> A soil bacterium whose crystal proteins kill caterpillars after they eat treated leaves; subspecies kurstaki and aizawai are the strains used against budworms.

- Canonical: https://genetixbay.com/knowledge/glossary/bacillus-thuringiensis.html
- Also written: Bt, B.t., Btk, Bacillus thuringiensis kurstaki, Bt aizawai
- Glossary category: Pests and diseases · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

*Bacillus thuringiensis* (Bt) is a soil bacterium that forms crystal proteins toxic to specific insect groups. Once eaten, the [crystals](https://genetixbay.com/knowledge/glossary/trichome.md) dissolve in the alkaline gut of the larva, bind to gut receptors and punch holes in the gut wall; the larva stops feeding within hours and dies over a few days. Subspecies *kurstaki* (Btk) and *aizawai* target caterpillars; subspecies *israelensis* (Bti) targets fly larvae.

## In practice

Outdoor growers spray Btk or *aizawai* products against [corn earworm](https://genetixbay.com/knowledge/glossary/corn-earworm.md), loopers and other caterpillars. Timing is everything: it must be on the leaf or flower when young larvae feed, before they bore into [colas](https://genetixbay.com/knowledge/glossary/cola.md), and sunlight breaks it down within days, so repeat applications follow egg-laying. It has no contact action and does not harm bees, [predatory mites](https://genetixbay.com/knowledge/glossary/predatory-mite.md) or people at label rates. Whether and how late it may go on flowers depends on the product label and local rules; check what is registered for cannabis locally and respect the [pre-harvest interval](https://genetixbay.com/knowledge/glossary/pre-harvest-interval.md).

## Don't confuse with

- **Bti**: the mosquito and [fungus gnat](https://genetixbay.com/knowledge/glossary/fungus-gnat.md) strain, used as a soil drench.
- **Bt crops**: plants engineered to make Bt proteins; no such cannabis exists commercially.
- **Beauveria bassiana**: a fungus that infects on contact.

## Related terms

[Bti](https://genetixbay.com/knowledge/glossary/bti.md), [Caterpillar](https://genetixbay.com/knowledge/glossary/caterpillar.md), [Corn earworm](https://genetixbay.com/knowledge/glossary/corn-earworm.md), [Biological control](https://genetixbay.com/knowledge/glossary/biological-control.md), [Pre-harvest interval](https://genetixbay.com/knowledge/glossary/pre-harvest-interval.md)

## Read more

- [Caterpillars on cannabis: species, degree-days and control](https://genetixbay.com/knowledge/pests/caterpillars.md)

## Sources

1. Oregon State University Extension. Corn earworm management in hemp (EM 9626). https://extension.oregonstate.edu/catalog/em-9626-corn-earworm-management-hemp
2. Utah State University Extension. Corn Earworm (hemp). https://extension.usu.edu/planthealth/ipm/notes_ag/hemp-corn-earworm