Bagworms should be treated at this time throughout Illinois. We recommend waiting for a couple of weeks after egg hatch for the larvae to stop ballooning and settle down to feed. In central Illinois, the bagworms are about 3/8 inch long, but little damage has occurred. Egg hatch occurred up to a month ago. Bacillus thuringiensis kurstaki (Dipel, Thuricide), spinosad (Conserve), cyfluthrin (Tempo), or other labeled pyrethroid should be effective against the larvae. Realize that these insects tend to feed heaviest at the top of the tree and work their way down, so be sure that the top of the tree is treated.
Gypsy moth pheromone flakes were applied in Carroll, Cook, DuPage, Kane, Will, and Winnebago counties in northern Illinois during the fourth week of June, weather permitting. Inclement weather may result in the applications’ being finished in the next week. Maps of northeastern Illinois application sites can be viewed at http://www.urbanext.uiuc.edu/gypsymoth. Pheromone flakes are applied by airplane in areas where gypsy moth populations are low. Gypsy moth males follow a pheromone emitted by the female to find her for mating. When pheromone flakes are applied, the area is saturated, so that male moths are unable to find females for mating. Unmated females lay unfertilized eggs that do not hatch. In areas with higher infestations, male gypsy moths blindly encounter females often enough that there is little reduction in caterpillar numbers the following spring.
Fall webworms are numerous in southern Illinois. These are gregarious, yellowish caterpillars that live in a silk web spun over the end of a branch. There are two generations per year in the southern half of the state, with the second generation appearing in August and September. Because the caterpillars are always in the web, pruning out the web is an effective control. Bacillus thuringiensis kurstaki (Dipel, Thuricide), spinosad (Conserve), cyfluthrin (Tempo), or other labeled pyrethroid should be effective, but high spray pressure is needed to penetrate the silk tent.