Spongy Moth Treatments: Bacillus thuringiensis kurstaki

​​​​​​​​​On most sites, we use a biological product that contains Bacillus thuringiensis kurstaki, or Btk. Btk is a naturally occurring soil bacteria that interferes with the spongy ​​moth caterpillars’ digestive system, eventually killing the caterpillars. The product is called Foray 48B. Foray 48B is commonly used by organic vegetable farmers up to the day of harvest. It harms only some species of moth and butterfly caterpillars if they are feeding when we spray. Because of the timing, it is primarily spongy moth caterpillars and some tent caterpillars that are feeding. In areas where desirable species might be feeding, we use different treatments.

​When do you spray Btk?

We usually begin spraying Btk in southern Wisconsin in May and move northward, finishing in early June. Planes typically take off at sunrise and continue spraying as long at the conditions permit, typically throug​h the morning. We may spray in early evening or during the afternoon if conditions are right. We spray each site twice, five to 10 days apart, because not all caterpillars hatch and begin feeding at exactly the same time.

​Why do 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, or until ​the area is finished. This may mean spraying into the late morning or afternoon, if possible.

Is Btk harmful to humans or animals?

​Numerous laboratory and field studies over the past 30 years have shown that Btk is not harmful or toxic to either humans or animals. The bacteria is occurs naturally in soil, so gardeners and animals routinely come in contact with it. 

It does have an odor that some people find unpleasant, and it may coat your vehicles if they are outdoors. 

The low-flying planes may frighten pets or livestock, so we recommend that you keep them inside if possible.

If you are uncomfortable with the idea of spraying, please get on our list for updates of our plans so you can stay indoors or leave the area if you prefer:

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