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September 11, 2006
We have had several recent inquiries about treating for bagworms. At this time of year, the caterpillars are full grown, with spindle-shaped bags approaching 1-1/2 inches long. The bags are covered with dead foliage from the host tree. Common hosts include eastern red cedar and other junipers, arborvitae, spruce, crabapple, and oak.

Bagworms are typically controlled with insecticidal sprays of Bacillus thuringiensis ‘kurstaki’ (Dipel, Thuricide), spinosad (Conserve), trichorfon (Dylox), cyfluthrin (Tempo), permethrin (Astro), or other pyrethroids. All of them except Btk also work well on older caterpillars. Before treating, be sure that the caterpillars have not pupated, that they are still actively feeding. When feeding, the caterpillar sticks its head and first few segments of its body out of the bag. There also is green foliage at the top of the bag. Pupated bagworms have the top of the bag tied shut with silk. Spraying insecticide on pupated bagworms has no effect.

Bagworms typically pupate in southern Illinois from early to mid-August, in central Illinois from mid-August to early September, and in northern Illinois in early September. This year, we have had reports in central and northern Illinois of actively feeding bagworms into the first week of September, so they still may not have pupated, particularly in northern Illinois.

Male bagworms go through five larval instars and typically pupate a few days before the female caterpillars, which have six larval instars. Be sure to look at a number of bagworms to see if they are actively feeding, because if the females are still feeding and are controlled, it greatly reduces the number of eggs that are laid and the resulting larvae that hatch next spring.

If the bagworms have already pupated, one can still remove the bags and destroy them. About every other bag will contain a female, each of which produces 300 to 1,000 eggs that overwinter in the bag on the tree. This bag removal can be done through the winter until June, when the eggs hatch.