What is Bacillus thuringiensis (Bt)?

Bacillus thuringiensis, usually referred to as "Bt" (pronounced “bee-tee”), is a naturally occurring bacterium that is lethal to most leaf-eating caterpillars on trees, shrubs, flowers, and vegetables.

Bacillus thuringiensis variety kurstaki (B.t.k.) is the general purpose material that homeowners buy when they see just the words Bt or Bacillus thuringiensis without the variety name attached. This is the type of Bt that is appropriate for home landscape use to kill caterpillars. As you can see on the left, there are also varieties designed to kill mosquitoes or potato beetles.

Bt is harmless to all other insects, animals, and humans. It comes in powder form for use as a dust or, diluted with water, for use as a foliar spray. It is also available in liquid form. It is sprayed or dusted on leaves of plants that are under attack by caterpillars so that the pests will ingest the bacteria as they eat the leaves. The Bt causes paralysis of the caterpillar's digestive tract, causing it to stop feeding within two hours. Within a day or two, certainly no longer than 72 hours, the caterpillars die.

For information about how to use Bt in the home landscape see “Using Bacillus thuringiensis”.