Bagworms

26 June 2007

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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It seems a little early for Christmas decorations so you might be wondering why there are new ornaments on your evergreens. The new larvae of bagworms are out and they are probably setting up housekeeping on a tree near you. The bags of bagworms (Lepidoptera: Psychidae, Thyridopteryx ephemeraeformis) are usually seen hanging from tree branches although they can be found on a variety of things including walls, picnic tables, and other structures. The bagworm occurs in the United States from New England to Nebraska and south through Texas. 

Host plants commonly used are juniper, arborvitae, cedars, cypress, pine, hemlock, spruce, elm, and locust. They are also found on crabapple, maple, sycamore, box elder, willow, linden, poplar, and many others. Often they are discovered following an increasing amount of leaf damage from defoliation between June to late July and August. The stripping of leaves is usually most noticeable in the uppermost parts of trees and shrubbery. This defoliation is associated with the presence of many spindle-shaped bags up to 1 1/2" in length from late summer to spring.

Larvae enclosed in the cone-shaped bags consume foliage and are seldom seen, except for heads of larvae as they protrude from the bag. The bagworm larva is mottled brown to black and encloses itself in its bag spun from silk. Bits of leaves and twigs from the host are incorporated in the bag during its creation. During June bags are difficult to see, since they start less than 1/4" in length. Bags increase to 1 1/2" by late summer. Adult males fly and are the only form occurring outside the bags. Females develop inside the pupal case within the bag where the eggs are laid.

Overwintering eggs begin hatching in early June. The tiny larva immediately begins to construct a tiny silken bag around itself and initiates feeding. Larvae and bags remain small (less than 1/4" long) for a few weeks and are not easily discovered. Feeding and molting continues until August when pupation occurs. Adults are active in late August and September. Only males leave the bag to mate with females which remain in the bags to lay overwintering eggs. Prior to molting and pupation, larvae attach the bag by silk strands to twigs and close the opening, protecting the larva.

Bagworms are difficult to control because they are often unnoticed until mature. Mature larvae will often pupate early if they detect pesticides on the plant foliage. Though there are a few known parasites and predators, they are often not adequate in urban habitats.

If the bags are few in number and easily reached they may be picked off the plant and squashed. This can be done easily in the late fall when deciduous foliage has been dropped or the bits of plant material on the bags turn brown and can be easily located on evergreens. Be sure to cut the attachment silk band so that the branch will not be girdled in the future. Dozens of additional wasp-induced galls occur on oak, rose, and brambles. Many of the insects are not well studied, and the galls do not have accepted common names. The wasps that emerge from the galls will not sting humans.

The bacterium, Bacillus thuringiensis (Bt), is effective against bagworms if it is used against young larvae. Applications should be made at the end of June after all the eggs have hatched and the larvae are through ballooning.

Insecticides are very useful for control of bagworms. Plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. Early sprays against young larvae are more effective than later applications. Products recommended for bagworm control are: acephate (Orthene), carbaryl (Sevin), dimethoate (Cygon), and malathion.

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For more information about landscape topics contact your local Virginia Cooperative Extension office. Many county Extension offices have a help desk that is staffed by Master Gardener volunteers. These volunteers are trained to answer questions about home and landscape pest problems. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.