This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Bacillus thuringiensis, often abbreviated as Bt, is a naturally-occurring bacteria that makes pests sick when they eat it. There are two strains commonly used as natural pesticides.
Bacillus thuringiensis kurstaki (Btk) gives excellent control of leaf-eating caterpillars such as cabbage worms and tomato hornworms, but has no activity against insects that do not eat treated leaves. After the insects eat the bacteria, their guts rupture and they die. Bt is therefore one of the safest natural pesticides you can use in terms of controlling caterpillar pests of vegetables or fruits without harming beneficial insects.
Bacillus thuringiensis israelensis (Bti) can be useful in controlling fungus gnats in greenhouses or houseplants, or for preventing mosquito problems in standing water that cannot be drained or controlled with fish.
Most caterpillars seen eating leaves can be controlled by Bt when applied at the proper time. In vegetable gardens, armyworms, cabbage worms, diamondback moths, melon worms, corn earworms, green cloverworms, pickleworms, tomato fruitworms, tomato hornworms, grape leafrollers, grapeleaf skeltonizers, salt marsh caterpillars, and various webworms and budworms are candidates for treatment with Bt.
Sunlight degrades Bt after a few hours, so it is best applied late in the day so it can be consumed during the nightly feeding. Keep in mind that your objective is to place the substance where the caterpillars will eat it. In the case of corn earworms, this means squirting the Bt solution into the tips of young ears of corn. When using Bt to control leaf-eating pests, repeat treatment every seven to 10 days, or until it is no longer needed.
Always follow label directions for diluting concentrated products of Bt and other natural pesticides. Some Bt products include genetically modified strains; products listed by the Organic Materials Review Institute (OMRI) include only naturally occurring forms.
Mix only as much concentrate as you will need. If not used within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot. Store Bt pesticides in their original containers on a high shelf, out of the reach of children or pets, in a cool place where temperatures will not exceed 100 degrees Fahrenheit. Under good storage conditions, powdered or other dry Bt products may last five years, while liquid products should be replaced after two to three years.
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