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Bagworms

June 18, 2003

It is time to be thinking about dealing with bagworms (Thyridopteryx ephemeraeformis) in southern and central Illinois. Bagworms attack a wide range of evergreen and deciduous trees and shrubs (128 plant species), including arborvitae, juniper, eastern red cedar, spruce, fir, pine, maple, box elder, linden, crab-apple, hackberry, oak, and black locust. The young caterpillars or larvae emerge from overwintering eggs in June and start feeding on plant foliage.

Female bags hanging on the trees from last year contain 500 to 1,000 eggs. Newly hatched larvae emerge from the bottom of the bags in June. They form tiny silk bags covered by whatever host they are eating. The young larvae are difficult to see because they blend in with plant foliage. The larvae climb high into a tree and dangle on 1- to 3-foot strands of silk. These strands are caught in the wind and detach, becoming a streamer that keeps the larvae aloft. Bagworms can float for long distances until the silk catches on an object. Many land on inhospitable places, such as roads or buildings, where they are likely to be killed. The rest catch onto trees and shrubs, then climb to the top of a plant and repeat the ballooning process or settle down to feed. Bagworms typically start feeding at the top of plants.

The young caterpillars are 1/8 to 1/4 inch long and initially feed on the epidermal tissue on one side and the mesophyll, leaving other epidermal tissue intact. Leaves appear whitish before turning brown. The young caterpillars create a cone-shaped bag, or case, that they carry with them for the rest of their life.

Older larvae (3/4 to 1 inch long) consume entire needles or leaves, mainly stripping the branches at the top of the tree. As they age and the food source declines, the larvae and their damage move down the plant canopy. Stripped conifer branches usually die. A severe infestation of bagworms can completely defoliate plants, which may result in death of branches or entire plants. This is especially likely for evergreens, which donít normally put out a flush of growth following defoliation by bagworms. Deciduous plants that have been attacked 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 at the bottom, which may make it easier for females to effectively disperse the pheromone to attract males.

In late summer, around mid-August, bagworms pupate inside the cases. It takes about 7 to 10 days for bagworms to change from pupa to adult, depending on the temperature. The males are ugly, black moths with clear wings, emerging through the bottom of the bag. Males fly off to mate with females. Females never develop into winged moths and lack eyes, wings, legs, and antennae; they remain inside the bag made during the larval stage, producing eggs before dying. The eggs are the overwintering stage. There is one generation a year in Illinois.

Handpicking and destroying the bags from fall through midspring is effective in removing the over-wintering eggs before they hatch. Bags should be placed in a plastic container and disposed of. It is important to note that larvae are likely to balloon in the spring from nearby or even distant trees.

Insecticides recommended for controlling bagworms include Bacillus thuringiensis kurstaki (Dipel or Thuricide), cyfluthrin (Tempo), trichlorfon (Dylox), and spinosad (Conserve). Insecticide sprays are effective against the young larvae. Larvae, in which the bags are 3/4 inch long, are very difficult to control. The bacterium Bacillus thuringiensis is effec-tive 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. Larger bagworms are more difficult to control, and the females feed less as they prepare for reproduction. Cyfluthrin and trichlorfon are recommended for larger larvae. Again, thorough coverage of all plant parts is essential, especially the tops of trees.

Insecticides should be applied about 2 weeks after hatching starts, to allow all bagworms to hatch and blow around, allowing the larvae to complete the ballooning process. Treating too early typically requires a second treatment to control larvae that later balloon onto trees and shrubs. Wait until mid-June in southern Illinois and late June to early July in central Illinois to treat. It is best to scout trees and shrubs a week or two after treatment to be sure that more bagworms have not blown in and to evaluate control.

Actively feeding bagworms always have small pieces of green foliage around the bagís top, and these pieces eventually dry and turn brown within a couple of days. Older larvae with no green foliage around the top have most likely pupated, and the use of insecticides is not warranted. Even after treatment, bagworms hang on the plant. In addition to the recommended insecticides, research has shown the certain species of entomopathogenic nematodes (Steinernema carpocapse) 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, 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 of females, as parasitic wasps tend to locate on the bottom of trees.


Author: Raymond A. Cloyd

 

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