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.

Pests it manages:

• 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 to 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.

The good:

• 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.

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.