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June 5, 2002

Bagworm, Thyridoteryx ephemeraeformis, eggs will be hatching in central and southern Illinois, so it is time to minimize damage. Bagworms attack many evergreen and deciduous trees and shrubs (128 plant species), including arborvitae, juniper, spruce, fir, pine, maple, box elder, linden, honey locust, and black locust. Eggs overwinter in old female bags, 1 to 2 inches long and covered with dried foliage from the host. Young caterpillars, or larvae (1/8 to 1/4 inch long), emerge in June and feed on foliage. The cater-pillars climb up the tree and spin threads, which they hang from until caught in breezes. The caterpillars float for long distances.

Bagworm eggs will continue to hatch for several weeks. Generally, 2 weeks after eggs hatch, bagworms quit migrating and feed on plant foliage. While feeding, they create a bag or pouch made of interwoven dead foliage, small twigs, and silk. Young caterpillars blend in with plant foliage. Large populations can completely defoliate plants, which may result in death of branches or entire plants. This is especially the case for evergreens, which don’t normally put out a flush of new growth following defoliation by bag-worms. As the caterpillars increase in size (3/4 to 1 inch long), the bags hang from trees like Christmas ornaments. Caterpillars feed for about 3 months. On some plants, female bags are mainly found at the top, male bags at the bottom, which may make it easier for females to effectively disperse the pheromone to attract males. In August, males pupate within the bags and emerge as winged adults–ugly black moths. It takes about 7 to 10 days for a pupa to become an adult, depending on temperature. Males then fly off to mate with females. Females never develop into winged moths and lack eyes, wings, legs, and antennae, never leaving the bag made during the larval stage; they remain inside bags and produce eggs. There is one generation a year in Illinois.

Management includes removal and/or the use of pest-control materials. If fewer than 30 bags are present, the bags (holding the overwintering eggs) can be removed before egg hatch. Bags should be placed in a plastic container and disposed of. Scout trees and shrubs regularly to assist in timing application of pest-control materials; apply about 2 weeks after hatching starts to allow all the bagworms to hatch and blow around. Apply in mid-June in southern Illinois and late June in central Illinois. Bagworms tend to occur in greater numbers north of I-80 after mild winters.

Pest-control materials for controlling bagworms include cyfluthrin (Tempo), Bacillus thuringiensis (Dipel and Thuricide), and spinosad (Conserve). 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. Research conducted with Clifford Sadof at Purdue University demonstrated that spinosad controls bagworms. Larger bagworms are more difficult to control, and the females feed less as they prepare for reproduction. Thorough coverage of all plant parts is essential, especially the top of trees. Besides the pest-control materials mentioned, research has shown that beneficial nematodes attack bagworms. When sprayed onto the bags, the nematodes infect the bagworms inside.

A sex pheromone, used in traps to lure male moths, may be used to interfere with mating behavior, reducing fertilization. Unfertilized eggs do not hatch.

Bagworms are susceptible to natural enemies, including the ichneumon parasitic wasps, Itoplectis conquisitor and Chirotica thryrifopteryx. Both wasps attack the pupae; however, bagworms are generally present at damaging levels before the wasps are effective. Parasitism of male bagworms is generally greater than females, as parasitic wasps tend to locate on the bottom of trees.

Author: Raymond A. Cloyd


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