The aerial application of Btk (Bacillus thuringiensis variety kurstaki) is used to reduce high populations of Spongy Moth caterpillars at sites that meet MDA requirements for spraying. Bt is a naturally occurring bacterium found in the soil and is not harmful to pets, birds, fish, wildlife, plants, beneficial insects, and humans. Bt is applied when the caterpillars are young (usually in May) to insure the greatest impact in reducing numbers. Alternative mechanical techniques, such as tree banding and egg mass scraping, also reduce caterpillar numbers. The Suppression Program recommends the use of a combination of methods.