The biological pesticide used in the Gypsy Moth Suppression Program is a naturally occurring bacterium that is found in the soil. Bt or Btk is Bacillus thurengiensis, variety kurstaki. It has been used for over 40 years and has been tested extensively by the EPA. Btk is narrow spectrum, that is, it is only toxic to moth and butterfly caterpillars in the early stages of development. It is non-toxic to humans, other mammals, birds, fish, frogs, turtles, and beneficial insects such as bees, ladybugs and other insects. Bt must be ingested by gypsy moth caterpillars that are less than an inch long to be effective. Since gypsy moth caterpillars feed in the canopy, Bt is applied by airplane or helicopter. It is broken down by sunlight after about seven days, so there is only a short window of time when the toxin is effective. Bt is commercially formulated and sold under a variety of names, such as Dipel, Foray, Thuricide and Bactur.

more info: http://ohioline.osu.edu/hyg-fact/2000/2174.html