Inspect Landscape Evergreens for Bagworm Damage
submitted by Sarah Browning, Nebraska Extension Educator
Early spring is a good time to inspect evergreen trees and shrubs in your landscape for signs of bagworm damage. Look for bare branches, where the foliage has been eaten away, and/or browning of foliage. A closer look may show small, tan, oval-shaped, cocoon-like structures hanging down from your plant’s branches. These are evidence that your plant was attacked by bagworms last summer.
Bagworms are a insect native to the United States and common in eastern Nebraska. Bagworms feed on many species of trees and shrubs, but are most common on evergreen trees and shrubs. Juniper, arborvitae, pine, and spruce branches may be killed if completely defoliated. Entire plants can be killed if an infestation goes uncontrolled.
Usually bagworm infestations build up slowly over several years. The insects and their cocoons can be hard for gardeners to spot since they look so much like a natural part of the plant. Last year’s cocoons are easier to spot in early spring before evergreens begin their new growth.
Bagworms also feed on shade, orchard, and forest trees of nearly every kind, as well as many ornamental shrubs and perennials, however severe attacks are unusual. Since deciduous plants regrow new leaves each year, the defoliation caused by bagworm feeding is usually not serious, however, the growth of small or newly planted trees could be slowed by heavy leaf feeding.
Bagworm Identification and Lifecycle
The adult male bagworm is a small, furry gray moth with clear wings; the adult female does not have wings and never leaves the bag she constructs during feeding. The larva is a brown or tan caterpillar with black markings. Bagworms overwinter in the egg stage inside the female bags, which are fastened to twigs. There may be as many as 300-1000 eggs in a single bag.
Since the female bagworm cannot fly, local populations can build up to damaging levels as succeeding generations of insects emerge. Eggs hatch in late May and early June, and larvae feed until late August or early September. There is one generation per year.
After hatching, the larvae emerge from a hole at the base of the mother’s bag and spin down a strand of silk. The tiny insect is often blown by wind to nearby branches or plants. Once a suitable host is found the new insect immediately begins to form a new bag over its body. Initially the young insect’s bag is about 1/8 inch long, but at maturity will grow up to 2 inches long. By mid-August the mature larvae attach their bag to a branch with a strong band of silk and begin to pupate. Adult males emerge in September.
How can bagworms be controlled? Handpicking the old cocoons is the best way to control light infestations on small plants. Be sure to remove the bags before eggs begin to hatch in May. Destroy bags by burning, immersing them in kerosene or by crushing. If bags containing eggs are discarded on the ground, the larvae will still hatch and may find their way back to your landscape plants.
Otherwise, plan to apply an insecticide to control this year’s small larvae in late June. Bacillus thuringiensis, BT, is available at nurseries and garden centers as Dipel or Thuricide. It’s very effective at controlling the insects without damaging other beneficial insects. It also has very low toxicity to birds and mammals.
Other insecticides labeled for bagworm control include acephate, carbaryl, cyfluthrin, malathion and permethrin. Affected plants must be thoroughly covered with the insecticide so it is ingested by the insects as they feed.
The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office