The main defense against a Spongy Moth (formerly Gypsy Moth) infestation is an aerial application of Bacillus thuringiensis var. kurstaki (Btk). Btk is used to reduce high populations of spongy moth caterpillars at sites that meet specific guidelines for treatment. Btk is a naturally occurring bacterium found in the soil and on plants and is not harmful to pets, birds, fish, plants, beneficial insects, or humans.