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Bagworm caterpillars within their cases made from plant foliage. (Photo: R. Childs)
A close-up photograph showing a bagworm larva partially outside its bag. (Photo: D. Swanson)
Dead foliage of an arborvitae displaying the old gray bags of a recent past bagworm infestation. (Photo: R. Childs)
A hedge of arborvitae that clearly displays the common dispersal pattern of a bagworm dinfestation. (Photo: R. Childs)
A close-up of the Snailcase Bagworm. (Photo: R. Childs)
A close-up of the Snailcase Bagworm on a dime to show relative size. (Photo: R. Childs)

Pest: Bagworm (Thyridopteryx ephemeraeformis (Haworth)

Order: Lepidoptera

Family: Psychidae

Host Plants

In the northeastern United States, Thuja (arborvitae) and Juniperus (juniper) are the two common host genera.


The larvae form "bags" or cases made from plant foliage and silk and these may appear to be normal plant parts to the untrained eye. Larvae will carry these cases while they defoliate the host plant. Injury can be quite severe when populations are high. Complete defoliation and plant death can result from the feeding activity of this pest. Given that the female does not fly, injury usually appears severe on one plant and then often radiates down a hedgerow from that one initially infested plant to others.

Life Cycle

Eggs over-winter within the female "bag" and hatch around mid-June in southern New England. Newly hatched larvae will disperse by crawling from the bag or they may balloon from the egg hatching site a short distance. Larvae will begin to feed and then construct the "bags" that they carry with them while they feed into late summer. Pupation occurs in late September into October in New England. Male moths will develop and appear moth-like but females will forever remain larval-like in appearance by not developing wings, legs or antennae. Males will seek out these females and mating occurs with the female never leaving her case. Soon thereafter, the female produces anywhere from 500-1000 eggs and then dies.


Given that this insect does not pupate until late September into October it is usually killed by frost, in the larval stage, in New England. It is not a "native pest" of this region. In fact, bagworms are only a continuous problem starting at the latitude that includes Maryland and ranging south to the Gulf of Mexico. However, occasionally bagworm is a serious pest in southern New England and it is due to several factors: New populations are usually brought in on nursery stock from more southern states and introduced into the landscape at planting time. They will cause a certain amount of injury during that growing season and if that region experiences a prolonged, warm autumn, the larvae will pupate and mating will occur during the following year. Given that each female can produce upwards to 1000 eggs, this pest can increase in numbers exponentially for the next growing season.

Management Strategies

If detected early, the few "bags" can be removed by hand. Otherwise, Bacillus thuringiensis (B.t.) is effective on younger larvae. Several chemical pesticides are also labeled for this pest.

Written by: Robert Childs
Revised: 10/2011