Issue 13, July 30, 2010


Bagworms continue to be numerous throughout the state with many bags being about one and one-half inch long and others much smaller at about three-fourths to one inch long. Certainly, the caterpillars in the smaller bags are still controllable with Bacillus thuringiensis kurstaki (Dipel, Thuricide, many others). The larger ones as well as the smaller ones can still be easily controlled with a spray of spinosad (Conserve), cyfluthrin (Tempo), and other labeled pyrethrins. Btk is not reliably effective on older caterpillars. Older insecticides such as carbaryl (Sevin) and malathion, which are marginally effective on younger caterpillars, will provide almost no control at this time.

Bagworms can also be controlled on shrubs and smaller trees by handpicking. Although rarely economic for professionals, clientele may wish to do this. Picked caterpillars should be smashed, dropped into rubbing alcohol or soapy water, or otherwise eliminated. If live bags are dropped under the tree, many will crawl back up into the branches. Similarly, those just thrown into the trash are also likely to escape to attack other plants. These insects have a broad enough host range that they are likely to find a suitable host.

Typically, bagworms pupate in southern to central Illinois around mid-August ranging to early September in central to northern Illinois, depending on the year. Through this growing season, we have been about two weeks ahead of a normal year, so pupation may already be occurring in southern Illinois. Bagworms that have pupated will have the bag closed at the top, and there will be no green foliage on the bag. The associated photo shows pupated bagworms, including one from the previous year and one with a protruding male pupal case. Pupated bagworms are not controllable except by handpicking. Pupation has not started in central and northern Illinois, although it may start soon.

Male bagworms pupate after five larval instars, but female bagworms complete six larval instars before pupating. This means that female caterpillars feed several more days than males and that a pyrethroid spray when part of the population has pupated may still provide excellent control for the coming year. Several years ago, I sprayed bagworms in that situation with Tempo in central Illinois on Labor Day weekend. A couple of weeks later, I noticed male pupal skins protruding from the ends of the bags. This indicated that the male caterpillars had already pupated when I sprayed, and that the male moths had emerged. However, many bags contained dead larvae. The following year, the number of infested arborvitae had dropped from 120 trees to four. I had missed the male caterpillars, but had killed the female caterpillars before they had pupated.--Phil Nixon

Phil Nixon

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