Bt powder is a natural insecticide made from a soil bacterium called Bacillus thuringiensis. It kills caterpillars and other leaf-eating larvae while remaining safe for humans, pets, and most beneficial insects. It’s one of the most widely used organic pest controls for vegetable gardens, and many formulations carry OMRI certification for organic agriculture.
How Bt Powder Kills Insects
Bt powder works entirely through ingestion. A caterpillar or other target larva has to eat plant material coated with the powder for it to take effect. Nothing happens from contact alone.
Once swallowed, the insect’s highly alkaline gut dissolves the protein crystals in the powder, releasing toxins called Cry proteins. These activated toxins bind to receptors on the gut lining and punch tiny pores through the cell membranes. The gut wall breaks down, allowing bacteria to flood into the insect’s body cavity. This causes a fatal infection, and the larva typically dies within two to three days of eating the treated foliage. You’ll often notice caterpillars stop feeding within hours, even though they don’t die immediately.
This mechanism is extremely specific. Mammals, birds, and fish have acidic stomachs that never activate the crystals, which is why Bt is harmless to nearly everything except the targeted larvae.
Which Pests It Controls
Different strains of Bacillus thuringiensis target different groups of insects. The strain matters, so check the label before buying.
- Bt kurstaki (Btk): The most common strain in garden powders. It targets caterpillars, including cabbage loopers, imported cabbageworms, tomato hornworms, grape leaf folders, and tobacco budworms. This is what most home gardeners need.
- Bt israelensis (Bti): Targets mosquito and fungus gnat larvae. Usually sold as granules or dunks for standing water rather than as a dust for foliage.
- Bt aizawai (Bta): Also effective against caterpillars, particularly wax moth larvae. Sometimes preferred for certain agricultural applications where Btk is less effective.
Bt powder does not kill adult butterflies, moths, beetles, aphids, or any sucking insects. It only works on larvae that actively chew and swallow treated plant tissue.
How to Apply Bt Powder
Timing is everything with Bt. Apply at the first sign of caterpillar activity, ideally right after eggs hatch and while larvae are still small. Young caterpillars are far more susceptible than mature ones, and once they’ve folded themselves inside leaves, the powder can’t reach them.
For dusting, apply with a hand duster, ground equipment, or even by shaking it from the container in calm air. Thorough coverage of leaf undersides is essential because that’s where most caterpillars feed. Commercial agriculture rates run 20 to 30 pounds per acre for crops like broccoli, cabbage, tomatoes, and lettuce, with heavier rates (up to 60 pounds per acre) for dense crops like cotton. In a home garden, a light, even coating on both sides of the leaves is the goal.
Repeat applications every seven days or after rain, since Bt powder breaks down quickly in sunlight and washes off with water. For heavy infestations or dense foliage that’s hard to coat evenly, use the higher end of the recommended rate. Apply in the early morning or late afternoon when UV exposure is lower, which gives the powder more active time on the leaves.
Do not mix Bt dust with other products. Some formulations are designed to be applied dry, and combining them can reduce effectiveness or create unintended chemical interactions.
Safety for People, Pets, and Pollinators
Bt powder has one of the strongest safety profiles of any insecticide. The protein crystals require a highly alkaline environment to activate, and since human and animal digestive systems are acidic, the toxins pass through without effect. You can