Issue 15, August 28, 2009


Bagworms have about finished feeding for the year. Pupation occurs from mid-August to early September, depending on the temperature. With cooler temperatures this year, pupation may not have occurred yet in central and northern Illinois. Pupation should have occurred in southern Illinois. To determine whether pupation has occurred, fresh foliage around the top of the bag and an open top indicate an actively-feeding caterpillar susceptible to control. If they are closed at the top as in the associated photo, insecticidal control is not practical.

Even though feeding damage has essentially ceased for the year, caterpillar control now can result in reduced infestation next year. Several years ago I sprayed about 120 infested arborvitae on Labor Day weekend after the males had pupated, but the females had not. The next year, only four trees had detectable infestations. Male bagworm caterpillars go through five larval instars, whereas females go through six. This results in the females spending an extra week or so as caterpillars. They grow bigger during this time, resulting in female bags tending to be slightly larger than those of the males. I use this information when checking overwintered bags for viable eggs in the spring.

To achieve control this late in the season, cyfluthrin (Tempo, Bayer Advanced Multi-Insect Killer), bifenthrin (Onyx), or permethrin (Astro, Eight Insect Spray) will provide the best control. B.t.k. is not likely to be very effective on these large, older caterpillars.--Phil Nixon

Phil Nixon

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