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June 9, 2004

It is time to think about dealing with the notorious bagworms, Thyridopteryx ephemeraeformis, in southern and central Illinois. Young bagworm larvae have been found in Champaign-Urbana as of May 25, 2004. Newly hatched larvae or caterpillars are difficult to see 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 are caught in the wind and detach, becoming streamers (this process is referred to as “ballooning”) that keep the caterpillars aloft for hundreds of feet to many miles, depending on updrafts and wind speed. Bagworms float until the silk catches on an object or plant. It is important to note that caterpillars are likely to balloon in the spring from nearby or even distant trees. The young caterpillars are small and as such cause minimal damage initially to foliage. They feed on the epidermal and mesophyll layers, creating light areas on the leaves. Waiting to spray an insecticide for at least 2 weeks after egg hatch allows sufficient time for the caterpillars to complete their ballooning and settle down and feed. One application during this time provides a high level of control. A second application may be required a week or two later.

Female bagworms hanging on trees from last year can contain 500 to 1,000 eggs. Newly hatched caterpillars emerge from the bottom of the bags around June, depending on environmental conditions. Each forms a tiny silk bag or case covered with material from whatever host it is eating and resides in this bag for the rest of its life. The young caterpillars are 1/8 to 1/4 inch long and initially feed on the epidermal tissue on one side and mesophyll layer, causing leaves to appear whitish before turning brown. The young caterpillars start feeding at the top of trees and shrubs.

Older larvae are 3/4 to 1 inch long and consume entire needles or leaves, mainly stripping the branches at the top of the tree. As they age and the food source declines, the caterpillars and their damage progress down the plant canopy. Stripped conifer branches usually die. A severe bagworm infestation can completely defoliate plants, which may result in death of branches or the entire plants. This is especially the case for evergreens, which don’t normally put out a flush of growth following defoliation by bagworms. Deciduous trees and shrubs that have been infested generally produce a new flush of leaves and survive. Bagworm caterpillars feed for about 3 months. On some plants, female bags are mainly found at the top, male bags on the bottom, which may make it easier for females to effectively disperse a pheromone to attract 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 remain inside the bag, producing eggs before dying. The eggs are the overwintering stage. There is one generation per year in Illinois.

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

Insecticides recommended for control of bagworms include Bacillus thuringiensis var. kurstaki (Dipel or Thuricide), cyfluthrin (Tempo), trichlorfon (Dylox), and spinosad (Conserve). Insecticide sprays are effective on the young caterpillars. Older ones, in which the bags are at least 3/4 inch long, are difficult to control; and females feed less as they prepare for reproduction. The bacterium Bacillus thuringiensis is effective on young caterpillars, but the material must be ingested--so thorough coverage of all plant parts is essential. 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. As mentioned, insecticides should be applied about 2 weeks after hatching has initiated, to allow all bagworms to hatch and blow around, permitting the caterpillars to complete the ballooning process. Treating too early generally results in the need for a second application. It is best to wait until mid-June in southern Illinois and late June to early July in central Illinois. Scouting trees and shrubs a week or two after application will be helpful in making sure that no more bagworms have blown in and in evaluating control efforts.

In addition to the recommended insecticides, research has shown that certain species of entomopathogenic nematodes (Steinernema carpocapsae) attack bagworms. When the nematodes are sprayed onto the bags, they infect the female bagworms inside. The bags provide a humid environment conducive to nematode activity. It is important to apply the nematodes before females lay eggs. A sex pheromone, used in traps to lure male moths, may be used to interfere with mating behavior and reduce fertilization; unfertilized eggs do not hatch.

Author: Raymond A. Cloyd


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