Issue 4, June 8, 2022
Bagworms
Bagworms (Family: Psychidae) are an insect that everyone has probably seen, even if they weren’t sure exactly what they were looking at. Bagworms are caterpillars that spin a small protective silk bag over their body. They decorate the bag with bits of leaves or other debris and drag the bag behind them as they walk, a behavior similar to that of hermit crabs. As the caterpillars grow, you may see bags up to 1.5 inches long.
In late summer, female bagworms fasten their bag to a tree or shrub, close the opening of the bag, pupate, lay eggs and die. The eggs overwinter inside the bag and hatch in late spring, usually late-May to early-June. When the young caterpillars emerge from the bag, they spin a strand of silk that is picked up by the wind and carries them to new host plants. This process is called ballooning and can continue for up to 2 weeks after egg hatch, until the caterpillars find a suitable host plant.
Left: Bagworms on arborvitae, Sarah Hughson, University of Illinois at Urbana-Champaign
Center: Bagworm on crabapple, Sarah Hughson, University of Illinois at Urbana-Champaign
Right: Close-up of a bagworm reaching out of the bag, Sarah Hughson, University of Illinois at Urbana-Champaign
Bagworms feed on arborvitae, juniper, cedar, spruce, white pine, crabapple and many other ornamentals. They are common on trees and shrubs near buildings or in parking lots.
Injury caused by bagworms is most obvious when bagworms are mature, in July and August. Deciduous trees that experience heavy bagworm feeding can refoliate, so the injury is mostly aesthetic and will not cause lasting damage. However, significant bagworm injury to evergreens can cause lasting damage and impact overall tree health. It is most effective to treat bagworms when they are young, so some landscape managers may choose to identify and record locations with bagworm damage in the summer and plan to treat the following season.
Apply treatments for bagworms two weeks after egg hatch (when catalpa is in full bloom). At this time, the young caterpillars have finished ballooning, settled onto a host plant to feed, and are most susceptible to treatment. Mature caterpillars are less susceptible to treatment and applications will not reach the insects after they close the opening of the bag and begin to pupate. Effective treatments for bagworms include: Btk (Dipel, Thuriside), Spinosad (Conserve), carbaryl (Sevin), acephate, and pyrethroids like cyfluthrin (Tempo) and permethrin (Astro).
One non-chemical control method that can be done anytime of year is hand picking the bags from trees and shrubs. It is especially easy to see bags on deciduous trees in the fall after the leaves have dropped. Just be sure to dispose of the bags away from the host plants. If a bag is simply dropped on the ground, the eggs inside are not killed and find their way to the host tree when they hatch. The bags can be stomped or disposed of in trash that is changed regularly.
Author:
Sarah Hughson