Gypsy Moth

Bacillus thuringiensis kurstaki (Btk)

Btk is a naturally occurring soil bacteria. It interferes with the gypsy moth caterpillars’ digestive system when consumed, eventually killing the caterpillars. Btk is cultured using foods such as potatoes as a growth hormone.

Btk only harms some species of moth and butterfly caterpillars, but with the timing of spray, it is primarily gypsy moth caterpillars and some tent caterpillars that are affected. Btk breaks down when exposed to sunlight within days of spray, so later-feeding species are not harmed.

We use a biological product that contains Btk. This product is Foray 48B, and it is listed for use in certified organic production or food processing by the Organic Review Materials Institute (OMRI). It is a commonly used product to battle gypsy moth caterpillars and by organic vegetable farmers up to the day of harvest.

Two applications of Btk, five to 10 days apart, are used to ensure the best chance of killing all or nearly all the gypsy moth caterpillars in newly established populations.

When will Btk spraying begin?

We usually begin spraying Btk in southern Wisconsin in May and move northward, finishing in early June. Planes typically take off at sunrise and will continue spraying as long at the conditions permit, typically through the morning. On occasion, spraying may occur in the early evening or during the afternoon if conditions are appropriate.

In urban areas during the school year, we will not spray between 7:15 and 8:30 a.m.

Why must planes spray so early in the day?

We need the high humidity and low winds of early morning to assure the spray does not evaporate or blow away. The planes continue spraying as weather conditions allow and/or when the area is finished. This may mean spraying into the late morning or afternoon, if possible.

Is Btk harmful to humans?

Numerous laboratory and field studies over the past 30 years have shown that Btk is not harmful or toxic to humans. People can stay indoors or leave the area until spray is done if they feel more comfortable.

Is Btk harmful to animals?

Btk will not harm animals, but the low-flying planes may frighten pets or livestock. It is recommended to keep animals inside, if possible.

Fast Fact sheet about Btk - 1 page PDF (You will need a PDF reader to view this document.)

For more information, call toll free 1-800-642-6684 and select option 3, or e-mail gypsymoth@wisconsin.gov.