Issue 7, June 6, 2011


Bagworm eggs are hatching in southern Illinois and will soon be hatching in central and northern Illinois. Due to the ballooning of young larvae, it is most effective to wait for a couple of weeks after egg hatch to apply larval control sprays.

Bagworm is a solitary, tent-building caterpillar. Eggs overwinter in the dead female's body within her spindle-shaped tent (bag) on the tree. After hatching, the larvae emerge through the lower end of their mother's bag. They immediately spin a conical tent around them and cover it with bits of foliage, bark, or anything else that is handy. They climb to the top of the tree, appearing as moving, one-eighth inch long, conical hats. They then spin out and dangle at the end of a two to three foot long strand of silk that they attach near the top of the tree. Winds detach these strands and carry the associated larvae up to several miles. This process is called ballooning. With the female moth being larvaform and wingless, ballooning by young larvae is the principle method of dispersal for this species.

The larvae hatch out over a span of several days and balloon repeatedly for a couple of weeks before settling down to feed in earnest. Although some feeding occurs during the ballooning period, it is primarily window-feeding and not very damaging nor noticeable. Their feeding on the foliage appears initially as light areas of foliage which turn brown as the exposed cells die. Probably because bagworms have to try to feed on wherever the wind blows them, they feed on many kinds of trees and shrubs. Not only evergreens such as arborvitae, spruces, Eastern white pine, Eastern red cedar, and other junipers are damaged, but they also feed on deciduous hosts including crabapple, oaks, maples, and hackberry.

One application of Bacillus thuringiensis kurstaki (Dipel, Thuricide, others), spinosad (Conserve), cyfluthrin (Tempo), indoxacarb (Provaunt) or other effective insecticide provides season-long control if applied after the larvae finish ballooning. This should be during the third week of June in southern Illinois, about July 4 in central Illinois, and in the second week of July in northern Illinois. Because the larvae start feeding at the top of the tree and work their way down, sprays need to reach the top of the tree to provide complete control.

The larvae otherwise continue to eat and grow through the summer, increasing the size of their tents (bags) as they get larger. Because of this process of continual growing, an actively feeding bagworm will have green foliage at the top of the bag that has only recently been clipped off and applied to the outside of the silk tent. This aids in determining insecticide control efficacy as the foliage at the top of the bag soon turns brown if the caterpillar is dead.

Fully-grown larvae pupate between early-August and mid-September, depending on the weather and locale. They will typically pupate in the early part of that range in southern Illinois and towards the end of that range in northern Illinois. Bags of pupating bagworms are closed at the top and are brown because the larvae are no longer feeding. Within a couple of weeks, black one-half inch long, clear-winged male moths emerge to mate through the lower ends of the bags with adult female moths that are wingless, remain in their bags, and are similar in appearance to the larvae. Mated females fill their bodies with 300 to 1000 eggs and die. The eggs hatch in late spring of the following year.

Because the eggs overwinter in their mothers' bags on the trees, hand-removal and destruction of the overwintering bags up to egg hatch provides effective control. On average, every other bag will contain eggs. Due to ballooning, complete bag removal does not guarantee a larva-free ornamental, but typically the number of subsequent larvae on the tree is substantially reduced. (Phil Nixon)

Phil Nixon

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