Global menu

The Green pages

Bacillus thuringiensis subsp. kurstaki (Btk)

Btk may be used to control the tomato hornworm caterpillar (Manduca quinquemaculata)
Photo: flickr (Greg Gilbert)
Tomato hornworm caterpillar (manduca quinquemaculata)
  • Tomato hornworm caterpillar (manduca quinquemaculata)
  • Imported cabbageworm caterpillar (Pieris rapae)

The information below is intended only as a guide. Always read product labels carefully.

Product description

Bacillus thuringiensis subsp. kurstaki, known as Btk, is a bacterium used to control caterpillars (moth and butterfly larvae).

Insecticides with Btk as the active ingredient are available as liquid concentrates to be diluted before use.

Target pests

Pesticides with Btk as the active ingredient may be used on ornamental plants, fruits and vegetables to control a variety of caterpillars, including:

  • tent caterpillars
  • imported cabbageworm
  • tomato hornworm
  • gypsy moth
  • oblique-banded leaf roller

NB: The product label lists the plants on which the pesticide may be used, along with the target organisms. 

How it works

Btk produces spores and crystals that affect the digestive system of caterpillars when they eat it. This causes them to stop feeding within a few hours and to die within 2 to 5 days. Note that the product has no effect on adult insects (moths and butterflies).


Before preparing the mixture, shake the concentrate container well. During application, it is also important to shake the solution to keep the product well mixed.

The prepared solution should be sprayed on the foliage of infested plants, ideally on a cloudy day. For best results, it should be used on young larvae in the early stages of an infestation. The treatment is also most effective when applied when caterpillars are feeding actively.

It is best to delay treatment if rain is forecast in the next 24 hours. The product could be washed off before the larvae have time to ingest it.


The pesticide may irritate the skin and eyes, so it is best to wear gloves, long sleeves and pants and goggles when handling and applying it.

Do not use near any body of water or wetland, or dump any pesticide or rinse your equipment there, as this will contaminate the water. Never dump pesticides down sewers.

Some precautions should be taken in storing the product. It should ideally be kept in a cool place (between 4° and 15°C) to maintain its effectiveness. It should also be used within 12 to 24 months of the manufacturing date marked on the container. Although it is not toxic to humans, it should be kept out of reach of children.

Add this