Well, it is that time of year to be thinking about dealing with . . . . bagworms (Thyridopteryx ephemeraeformis), in the southern and central portions of 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 (“hang out”) 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 (or velocity). This process is referred to as “ballooning.” Bagworms float until the silk catches on an object or plant. It is important to note that caterpillars can balloon in the spring from nearby or even distant trees. The young caterpillars are small and cause only minimal damage to foliage. They feed on the epidermal and mesophyll layers, creating light areas on leaves. It is recommended to avoid spraying an insecticide for at least 2 weeks after egg hatch, as this allows sufficient time for the caterpillars to complete the ballooning process, 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.

Female bagworms still hanging on trees 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 out in northern Illinois in early June. Each caterpillar creates a tiny silk bag, or case, covered with material from the host plant it is feeding on; the caterpillar remains in the bag for the rest of its life. Young caterpillars are 1/8 to 1/4 inch long and initially feed on the epidermal tissue on one side and the 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 in. long and consume entire needles or leaves—mainly stripping the branches at the top of the tree. As the caterpillars mature, and the food source declines, their damage progresses downward on the plant. Stripped conifer branches usually die. A severe bagworm 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. Infested deciduous trees and shrubs 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, male bags near the bottom of the plant canopy. This arrangement makes it easier for females to disperse effectively a pheromone, which 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, “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 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 in. long, are more difficult to control. Additionally, 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 previously 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 result in the need for a second application. With their early egg hatch this year, 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 will be helpful in making sure that no additional bagworms have blown in and in evaluating control efforts.