Factsheet: Least-toxic Control of Bagworms
Pest type: Insects
Bagworms are the larval stage of moths. They attach their bags to and feed on ornamental plants such as evergreens, causing defoliation. Bagworms look like green or brown bags about 1/2 to 3 inches long and hang from branches like ornaments.
The most common species in the East usually attaches to junipers, pine hemlock, cedar, black locust, maple, sycamore, elm, buckeye, box elder and willow trees. In the Southern California region, bagworms are found mainly on ash, pear, sycamore, willow and locust trees. In Florida and other gulf coast states, look for bagworms on citrus and other subtropical plants. In temperate climates, the larvae hatch and begin feeding in late spring to early summer.
The bagworm has a voracious appetite and can cause serious damage to trees. Bagworms commonly feed on coniferous plants and on some deciduous trees and shrubs. Common evergreen hosts include juniper, arborvitae, spruce, and pine. The earliest signs of bagworm injury in evergreens are brown or stressed needles at the tips of branches. This is caused by the tiny, first-stage bagworm caterpillars etching needle surfaces as they feed. Heavy infestations of older bagworms may completely defoliate a tree or shrub. These plants can be severely stressed or even killed if control measures are not taken. Infestations can be caught in the fall after their cases have turned brown. By catching an infestation early enough, picking them off, or using least-toxic management strategies, you can save the tree.
Bagworms also feed on numerous deciduous trees, as well as many ornamental shrubs, but heavy infestations are relatively rare. Since deciduous plants can grow new leaves, damage is usually not serious.
You can prevent or at least alleviate bagworm issues by planting flowers that attract bagworm-killing insects like parasitoid wasps and tachinid flies. A study published through University of Illinois at Urbana-Champaign found that planting asters and daisies near bagworm-infested trees and shrubs in urban landscapes promotes these beneficial insects, reducing bagworm populations. Flowering plants provide nectar and pollen for these natural enemy populations.
Birds, especially sparrows and finches, are also important predators of bagworms in late summer. By planting fruit and seed-bearing plants and providing shrubs for roosting, you can further prevent bagworm infestations.
Monitor all plants that have been infected with bagworms in the past. Begin searching for bags in the winter, as some will contain eggs that will hatch in the spring. Destroy any that you find. In May and early June, closely monitor all plants at least every other week.
When there are only a few bagworms you can handpick and crush them. This must be done effective during fall, winter or early spring before the eggs hatch.
Look at your product labels and try to avoid products containing those chemicals listed below:
(A = acute health effects, C = chronic health effects, SW = surface water contaminant, GW = ground water contaminant, W = wildlife poison, B = bee poison, LT = long-range transport)