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June 7, 2006
Well, it is that time of year to be thinking about dealing with bagworms (Thyridopteryx ephemeraeformis) in the southern and central Illinois. Newly hatched caterpillars (or larvae) are difficult to detect because they blend in with plant foliage. The caterpillars climb to the tops of trees and dangle on 1- to 3-foot strands of silk. These strands eventually are caught in the wind and detach, becoming streamers that keep the caterpillars aloft for hundreds of feet to many miles, depending on updrafts and wind speed. This process is referred to as “ballooning.” Bagworms float until the silk catches on a plant or other object. It is important to note that caterpillars can balloon in the spring from nearby or even distant trees. Young caterpillars are small and cause only minimal damage to foliage, feeding on the epidermal and mesophyll layers, creating light areas on leaves. Avoid spraying an insecticide for at least 2 weeks after egg hatch, as this allows sufficient time for the caterpillars to complete the ballooning, settle down, and initiate feeding. An application during this time provides a high level of control. A second application may be needed a week or two later.

A female bagworm still hanging on a tree from last year may contain from 500 to 1,000 eggs. Newly hatched caterpillars have emerged from the bottom of the bags in late May, about 2 weeks earlier than usual. They should be hatching in northern Illinois in early June. Each caterpillar creates a tiny silk bag, or case, covered with material from the host plant it feeds on. The caterpillars remain in the bag for the rest of their life. Young caterpillars, 1/8- to 1/4-inch long, initially feed on the epidermal tissue on one side and mesophyll layer, causing leaves to appear whitish before turning brown. Young caterpillars typically start feeding at the top of trees and shrubs.

Older larvae are 3/4 to 1.0 inch long and consume entire needles or leaves—mainly stripping branches at the top of the tree. As caterpillars mature, and the food source declines, damage progresses down the plant. Stripped conifer branches usually die. A severe infestation can completely defoliate a plant, which may result in death of branches or the entire plant. This is especially true for evergreens that don’t normally put out a flush of growth following defoliation by bagworms. Deciduous trees and shrubs that have been infested generally produce new growth and are able to survive. Bagworm caterpillars feed for about 3 months. On certain plant species, female bags are found at the top, whereas male bags are located near the bottom of the plant canopy. This arrangement makes it easier for females to effectively disperse a pheromone that attracts males.

In late summer, around mid-August, bagworms pupate inside the bags. It takes about 7 to 10 days for bagworms to change from pupa to adult, depending on the temperature. The males, which are “ugly” black moths with clear wings, emerge through the bottom of the bag and fly off to mate with females. Females never develop into winged moths and lack eyes, wings, legs, and antennae: They just remain inside the bag, producing eggs before dying. Eggs are the overwintering stage. There is only one generation per year in Illinois.

Handpicking and destroying bags from fall through midspring is very effective in removing the overwintering eggs before they hatch. Bags should be placed into a plastic container and disposed of quickly.

Insecticides recommended for controlling bagworms include Bacillus thuringiensis var. kurstaki (Dipel or Thuricide), cyfluthrin (Tempo), trichlorfon (Dylox), and spinosad (Conserve). Insecticide applications are most effective on the young caterpillars. Older caterpillars, in bags that are at least 3/4-inch long, are more difficult to control. Also, females tend to feed less as they prepare for reproduction—thus reducing their susceptibility to insecticide sprays. The bacterium Bacillus thuringiensis is effective on young caterpillars, but the material must be ingested: So thorough coverage of all plant parts is critical. Spinosad works by contact and ingestion and is very effective in controlling bagworms. Cyfluthrin and trichlorfon are recommended for larger caterpillars. Again, thorough coverage of all plant parts is essential, especially the tops of trees where bagworms typically initiate feeding. As mentioned, insecticides should be applied about 2 weeks after eggs hatch. This allows the bagworms to blow around, permitting the caterpillars to complete the ballooning process. Insecticide applications made too early usually results in the need for a second application. With this year’s early egg hatch, it is recommended to apply at this time in southern Illinois; in mid- to late June in central Illinois; and in late June to early July in northern Illinois. Scouting trees and shrubs within 2 weeks after an application is helpful in making sure that no more bagworms have blown in and in evaluating control efforts.