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Bacillus thuringiensis (Bt)

Also known as Bt, Bti (Bt israelensis), Btk (Bt kurstaki)

Beneficial or harmlessBeneficial
Bacillus thuringiensis cells with insecticidal protein crystals Photo: Jim Buckman is credited and the original uploader is P.R.Johnston. · Public domain

In short

Bacillus thuringiensis is a soil bacterium sold as a biological insecticide. Each strain makes crystal proteins that are toxic only to certain insect larvae once eaten: kurstaki and aizawai for caterpillars, israelensis for fungus gnat and mosquito larvae. It has very low toxicity to people, pets, birds and fish.

How it looks

Not visible on plants. Under the microscope, a rod-shaped bacterium that forms spores and protein crystals. Sold as wettable powders, liquids or granules labelled with the strain name.

Signs

Larvae that eat treated leaves (or, for Bti, feed in treated mix) die in about one to five days after the toxin breaks down their gut lining.

What favours it

Works only on young, actively feeding larvae of the target group. Sunlight breaks it down rapidly on leaf surfaces, within roughly one to four days, so coverage must be renewed.

Prevention

  • Use the right strain for the pest: kurstaki or aizawai for caterpillars, israelensis for fungus gnats.
  • Treat when larvae are small and first seen rather than waiting for heavy damage.

Treatment

Start with the first step and move down only if the problem persists.

  1. For caterpillars, spray foliage thoroughly, including undersides, since sunlight breaks it down within days, and repeat as the label directs.
  2. For fungus gnats, drench the potting mix with a Bti product; combine with letting the mix dry more between waterings.
  3. Follow the product label for rate, interval and crop.

Pros and cons

Pros

  • Very selective: targets particular insect groups and spares most beneficial insects.
  • Very low toxicity to people, pets, birds, fish and mammals.
  • Breaks down quickly in the environment.

Cons

  • Works only when eaten by larvae of the target group, so it does not control adult insects.
  • Kurstaki strains also kill the caterpillars of desirable butterflies and moths.
  • Short-lived on leaves because of sunlight, so repeat applications are needed.

Look-alikes

Beneficial nematodes
Living worms that actively enter soil insects; Bt is a bacterial product that must be eaten.

Where you find it

Caterpillar-damaged vegetables and ornamentals; houseplants and seedlings with fungus gnats.

Sources

  1. Bacillus thuringiensis (Bt) General Fact Sheet — National Pesticide Information Center

Written in our own words from these sources. Check a product label before using any treatment.