Bagworms (Thyridopteryx ephemeraeformis) are the insects that appear as "cocoons" on many different species of trees -- and inanimate objects -- at this time of year. They have been feeding since late May but do not become noticeable until they cause considerable damage and become quite large. They are very difficult to control at this point because they are well protected from insecticides by the dense bags they construct and because they are not feeding heavily at this point in their life cycle.
While many insects are very host-specific, bagworms are generalists. They feed on more than 100 species of trees and shrubs, including arborvitae, crabapple, honeylocust, juniper, maple, oak, pine, spruce, sweet gum and sycamore.
The damage to conifers takes longer to heal because most of them do not produce new growth from old wood. Recovery happens as they continue to grow from their tips, and eventually new growth will cover the damage. It can take years for them to regain their appearance.
These native insects overwinter as eggs in the bags of female adults. The larvae hatch out from mid-May to early June and immediately begin feeding and constructing bags from silk they produce and bits of leaves from their host plants. They are quite tiny when they first hatch and carry their bags upright, making them look as though they are wearing dunce caps.
Larvae continue to feed and grow through the summer months, sticking their heads out of their bags to feed and move about on host plants. They begin to pupate in August by securely attaching themselves to twigs and/or inanimate objects. Then they seal up their bags and re-orient themselves so they are facing downward. They are no longer feeding when they pupate.
Adult bagworms are moths, although females lack wings and remain grub-like; they never leave their bags. Adult males are nondescript, charcoal-gray moths with clear wings that hatch out of their bags and fly to mate with the females in late summer and early fall. Males die after mating and females die after laying 500-1,000 eggs in each bag.
They fact that females do not fly allows large populations to build up on host plants in a short period of time. Very tiny larvae can be blown in the wind, and they can crawl from tree to tree when plants are relatively close together. They are also spread on infested nursery stock.
Controls include removing the cases from infested plants by hand, especially between now and when they hatch next spring. Male cases can be left to weather off the plants because all the eggs are in female bags. It's easy to tell the difference: Male bags tend to be smaller than female bags, and the pupal case often extends out of the bottom of the bag where the male emerged as an adult. If in doubt, pick it.
Hand removal is the only effective option at this time of year. The silk bagworms use to attach themselves to twigs is very strong and usually has to be cut with scissors or hand pruners to remove it without damaging the plant.
If plants are too large for hand removal, spray applications are best directed at very small larvae. Because they are caterpillars, small larvae are well controlled with least toxic products such as Bacillus thuringiensis or Bt (Dipel, Thuricide, others) and spinosad (Captain Jack's Deadbug Brew, Monterey Garden Insect Spray, others). Larger bagworms can be controlled with carbaryl (Seven, others); cyfluthrin (Bayer Advanced PowerForce Multi Insect Killer); and malathion.See all posts