What Is Bt (Bacillus thuringiensis) and How Does It Control Caterpillars?

Bt (Bacillus thuringiensis) is a naturally occurring soil bacterium used as a biological insecticide. It's not a chemical poison—it's a microbial agent that specifically targets caterpillars (lepidopteran larvae) when they ingest it.

How It Works

1. Bt spores and crystalline proteins are sprayed onto foliage.
2. Caterpillars eat treated leaves and ingest the toxin.
3. In the alkaline gut of the caterpillar, the crystals dissolve and bind to receptors, punching holes in the gut lining.
4. Feeding stops within hours; death follows in 1–3 days from septicemia and starvation.

Key Advantages

• Highly specific: Harmless to humans, pets, birds, bees, lady beetles, and most beneficial insects.
• Organic-approved: OMRI-listed for organic gardening.
• Short residual: Breaks down in sunlight within days—no long-term residue.

Forms & Application

• Liquid concentrate (e.g., Thuricide, Dipel) – mix with water, spray to runoff.
• Wettable powder – similar, longer shelf life.
• Granules – for soil-dwelling pests like cutworms.

Apply when caterpillars are young and actively feeding. Thorough coverage of leaf undersides is essential. Reapply after rain or every 5–7 days during heavy pressure.

Limitations

• Only works on caterpillars that eat treated foliage (not borers inside stems).
• Does not control aphids, mites, beetles, or other non-lepidopteran pests.
• Resistance can develop with overuse—rotate with other controls (spinosad, hand-picking).

For your datura caterpillars, Bt is an excellent, targeted choice. Apply at first sign of chewing damage.