Bacillus Thuringiensis - Pests it Manages, Pros & Cons, How to Use

Bacillus thuringiensis — Bt for short — is a naturally occurring soil bacterium and a caterpillar specialist. Caterpillars eat the sprayed leaf, stop feeding within hours and die a day or two later. It does nothing to anything else, which makes it one of the safest controls going.

Which pests does it manage?

• Caterpillars — cabbage white, cabbage moth, loopers and their relatives

The garden strain of Bt (kurstaki) affects only caterpillars. It won't harm bees, ladybirds, pets, people or your other plants.

How do I use it?

1. Mix to the label and spray onto the leaves the caterpillars are eating.
2. Cover both sides of the leaf — the caterpillar has to eat it for it to work.
3. Spray late in the day; Bt breaks down in sunlight.
4. Reapply weekly while caterpillars are active, and after rain.

What's good about it?

• Completely specific to caterpillars — safe for bees, pets, people and beneficial insects.
• Accepted for organic gardens.
• One of the safest sprays you can use on food crops.

What's the catch?

• Only works on caterpillars — no use against aphids, beetles or bugs.
• Caterpillars have to eat it, so timing and coverage matter.
• Breaks down quickly, so it needs regular reapplication.

For a caterpillar problem on food plants, Bt is about as safe as it gets. For the bigger picture, see our guide to managing pests safely.

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Image: Merle Shepard, Gerald R. Carner & P.A.C. Ooi, CC BY 3.0, via Wikimedia Commons.