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Oh, No! Not Bagworms

May 23, 2006
Well, it is that time of year, which you have been nervously anticipating—dealing with bagworms (Thyridopteryx ephemeraeformis), primarily in the southern and central portions of Illinois. However, once considered an insect pest only south of I-80, bagworms have now been found consistently the past several years north of I-80 and near the Wisconsin border. Is this due to global warming or the mild winters? We can only speculate at this time. What is important is that bagworms are here! I tend to have a more positive attitude regarding bagworms and think that they give trees and shrubs a “special appeal”—making them look like Christmas trees.

Newly hatched caterpillars are very difficult to detect because they typically blend in with plant foliage. Additionally, although we won’t admit it, as we get older our eyesight tends to diminish, which also makes it difficult to detect bagworms. In the spring, caterpillars climb to the tops of trees and hang out on 1- to 3-foot strands of silk. These strands eventually get caught on wind currents and detach, becoming streamers that allow the caterpillars to remain aloft for hundreds of feet to several miles, depending on wind speed (or velocity) and the occurrence of updrafts. This process is often referred to as ballooning. The caterpillars float through the air until the silk catches onto a plant or other object. It is important to note that caterpillars can balloon from nearby or even distant trees. Young caterpillars are small and cause only minimal damage to foliage. They feed on the epidermal and mesophyll layers, creating light areas on leaves. In general, it is recommended to avoid spraying any insecticides for at least 2 weeks after egg hatch to allow adequate time for the caterpillars to complete the ballooning process, settle down, and initiate feeding. An insecticide application during this time maximizes control of bagworms resulting in higher mortality levels. A second application is typically required a week or two later.

A female bagworm still hanging on a tree from last year contains from 500 to 1,000 eggs. Newly hatched caterpillars emerge from the bottom of the bags in late May or early June, depending on geographic location. Each caterpillar creates a small silk bag, or case, covered with material from the host plant it has fed upon. Caterpillars remain in the bag for the remainder of their life. Young (early-instar) caterpillars are 1/8 to inch in length and initially feed on the epidermal tissue on one side and the mesophyll layer, causing leaves to appear white before turning brown. Young caterpillars typically initiate feeding at the top of trees and shrubs. Why do you think they do this?

The older or mature caterpillars are 3/4 to 1 inch long and consume entire needles or leaves, primarily stripping the branches at the top of the tree. As the caterpillars mature, and the nutrient quality of the host declines, they migrate downward, feeding on lower foliage. Entire branches of conifers may die if stripped of foliage by the caterpillars. A severe bagworm infestation can completely defoliate a host plant, which may result in death of branches or the entire plant. This is especially true of evergreens, which don’t normally produce a flush of new growth following defoliation by bagworms. In contrast, deciduous trees and shrubs typically produce new growth and are thus able to survive an infestation of bagworms. In general, bagworm caterpillars feed for about 3 months. On certain host plant species, female bags are located at the top, whereas male bags are distributed near the bottom of the plant canopy. This arrangement allows the females to effectively disperse pheromones, which attracts the winged males and increases the possibility for mating and fertilization of the eggs (“Sex in the Tree,” as opposed to “Sex in the City”).

In late summer (around mid- to late August), caterpillars develop into a pupal stage inside the bags. Bagworms take about 7 to 10 days to change from a pupa to adult; however, this is dependent on temperature. The males, which are “ugly” black moths with clear wings, emerge through the bottom of the bag and disperse to mate with females. Females never develop into adult moths because they lack eyes, wings, legs, and antennae. The females remain inside the bag, producing eggs before dying. The eggs are the overwintering stage of this insect, and there is one generation per year in Illinois.

Handpicking and destroying bags from fall through mid-spring is very effective in removing the overwintering eggs before they hatch. Bags can be placed into a plastic container with soapy water or into a sealed Ziploc bag and then disposed of. We don’t recommend placing the bags in a container with kerosene and then lighting it!

Insecticides recommended for control of bagworms include Bacillus thuringiensis subsp. kurstaki (Dipel or Thuricide), cyfluthrin (Tempo), trichlorfon (Dylox), and spinosad (Conserve). Insecticide applications are most effective on the young caterpillars. Older caterpillars in the bags are inch long and are more difficult to control. In addition, females tend to feed less as they prepare for reproduction, which reduces their susceptibility to insecticide sprays. The bacterium Bacillus thuringiensis is very efficacious on young caterpillars; however, the material must be ingested, so thorough coverage of all plant parts is essential. Spinosad (Conserve) works by contact and ingestion, and is extremely effective in controlling bagworms. Cyfluthrin (Tempo) and trichlorfon (Dylox) are typically recommended for the larger caterpillars. Again, thorough coverage of all plant parts is essential, especially the tops of trees, where bagworms commonly initiate feeding. As previously mentioned, insecticides should be applied about 2 weeks after egg hatch. This allows the caterpillars to complete the ballooning process. If insecticides are applied too early, then a second or third application may be needed. In general, it is recommended to apply insecticides in late May to early June for southern Illinois; mid- to late June for central Illinois; and late June to early July for northern Illinois. Scouting trees and shrubs 2 weeks after applying an insecticide will be helpful in determining if additional bagworms have blown in and allow you to evaluate the effectiveness of insecticide applications.