Gypsy moth outbreaks can be delayed or reduced so that trees and people are less stressed. The most common suppression treatment is a spray of Bacillus thuringiensis, commonly called Bt. This bacterial insecticide kills caterpillars that eat it within a week of spraying. Bt is found naturally in soil and degrades within a week as the sun hits it. Bt does not harm people, animals, birds or fish.
Sprays also don t affect gypsy moth caterpillars outside the treated area.
Sometimes spray programs are repeated for one to three years when the gypsy moth population is in the outbreak stage. Once the gypsy moth caterpillar population has collapsed in an area, spraying can stop for several years. The rapid migration west of the gypsy moth in the United States is caused by people unknowingly moving egg masses that are attached to cars, firewood, nursery plants and outdoor furniture. Wind can move male moths and young caterpillars, but it doesn t spread the moths rapidly. There are regulations requiring the inspection of logs, nursery plants, Christmas trees and outdoor household items which help reduce the number of accidental introductions, but it won t stop them all.