Controlling Bagworms in Trees
By Ron Honig
Homeowners with evergreen trees should take note that it is time to consider treating bagworms in trees if infestations exist. Bags made of silk and debris from clipped foliage are hanging on trees this spring with this season’s crop of bagworm eggs. Around mid-May small larvae hatch from the bags and continue their lifecycle of feeding on tree foliage.
The bags have been humorously called “Nature’s Christmas ornaments” but if left unchecked, bagworm larvae will continue to multiply to numbers that can significantly defoliate evergreens or whatever tree species the worms have invaded.
The caterpillars Thyridopteryx ephemeraeformis, more commonly known as bagworms, are easy to recognize once they have begun to construct their bags which become their home and hiding place throughout the year.
Life Cycle
Bagworms over-winter as eggs in last year’s bags hanging on the tree or anywhere else they can find a place to attach. From mid-May through June, larvae hatch and exit the bags. The larvae often suspend themselves by strands of silk as they move from the old bag to a new branch. They immediately construct new miniature silk-lined bags and then begin feeding.
Bags are often decorated with bits of foliage. As the larvae grow in size, so do the bags. Larvae remain in the bags to feed and move about with only heads and thoracic (legged) segments protruding from the front. When it’s time to molt, the front of the bag is closed temporarily. Afterward, it is reopened and the larvae continue to forage. The back of the bag remains open to allow the elimination of wastes.
By mid- to late-August bagworm feeding is complete. Larvae anchor bags to branches and seal themselves inside. Researchers say the larvae then turn themselves to face the posterior opening of the bag and enter the pupal stage. Pupation is completed in two to three weeks.
Just before moths emerge, male pupae work their way through the posterior openings, protruding slightly. Male moths are black with clear wings. Female moths remain in pupal cases within the female bags. Lacking wings, legs, antennae, mouthparts, and functional eyes, females are nothing more than soft, white, slug-like, egg-filled sacks according to entomologists.
Males then move to the female bags where mating occurs while the female larvae remain in the bag. After copulation, females deposit eggs (up to 1,000 per female) into their pupal cases and seal the bags. Eggs will remain in the bags throughout the winter months.
Hosts
Bagworms are best known as pests of eastern red cedar, junipers, arborvitae, spruce, and pines. When infestations are heavy however, bagworms will attack a number of broadleaf trees and even clover, ragweed and nightshade plants according to entomologists.
Control
Hand-picking bags off of trees during the winter months can help reduce the number of hatching larvae in May, but if the infestation is heavy or if the trees are tall, controlling bagworms culturally can be impractical. Pulled bags need to be disposed of and not just dropped on the ground.
The use of a well-timed insecticide treatment program can reduce feeding larvae to low levels. Depending on the amount of management and effort the homeowner wishes to expend determines the treatment timing.
If the homeowner only makes one insecticide application, it should be applied in late June to early July.
If the owner wishes to control the bagworms as much as possible, then two insecticide applications are recommended: once in late May and then again at the end of June.
Many products are labeled for bagworm control including the insecticides Permethrin, Cyfluthrin, Acephate, and Spinosad. The particular product label will provide the recommended rates for mixing. Large jobs or treating tall trees might best be done by lawn care professionals.
Homeowners wishing to maximize bagworm control, should plan their first treatment at the end of May or early-June.
