The MDA determines which product to use to treat gypsy moth by looking at the results of annual gyspy moth survey data, alternate life survey results, and size of the treatment area. Depending on the product, treatments are conducted to target either very young caterpillars or adult moths.
Foray and Btk
In areas that have many male moths captured and where egg masses are found during the alternate life stage survey, Foray 48B is used. The products has the active ingredient Bacillus thuringiensis kurstaki (Btk). It is used where there is evidence of a reproducing population, and is used against small caterpillars.
Foray 48B is applied in May-June. Since insect development is temperature dependent, an unusually cool or warm spring affects this timing. Treatments occur twice, about a week apart, to catch late-hatching caterpillars. Spraying normally takes place early in the day when low winds and high humidity allow the spray droplets to land on tree tops where they will be the most effective, and also because fewer people are active outside. A treatment site will typically be finished before children are walking to school or people are heading to work.
You can also read more about Bt and human health from the National Pesticide Information Center.