Hand picking is the best method because it is the cheapest, safest, and most environmentally friendly. The best time to make applications is the middle of June with a product containing Bacillus thuringiensis (Bt) which is commonly available at most garden centers or box stores. Another product equally effective is spinosad (active ingredient) and also commonly available at the previously mentioned places. This time of year the bags need to be cut open to see if there is a caterpillar or a pupa (cocoon) inside the bag. If there are pupae, then treatments will not be effective. Control is only obtained through caterpillars consuming insecticide treated foliage. Sprays do not penetrate the bags. If they are pupae they not eathing; thus not controlled. If there are caterpillars things such as sevin or pyrthroids (active ingredient ends in -thrin) are able to provide control if coverage is adequate. Mid-August to early September they are pupae and sometime in August to the end of October they emerge to mate and lay eggs inside the bags. Thus, removing bags removes next year's population. Bags can be removed from September through May to drastically reduce numbers found the following year.