Bagworm
Contact: Eric Day, Manager, Insect Identification Laboratory
Publication 444-268, August 1996
Bagworm
Lepidoptera: Psychidae, Thyridopteryx ephemeraeformis
Plants Attacked
Juniper, arborvitae, other cedars, pine,
hemlock, spruce, Chinese elm, honeylocust, primarily. Also on
crabapple, maple, sycamore, box elder, willow, linden, poplar,
and many others.
Description of Damage
Increasing degree of leaf damage from
defoliation from June to late July and August. Stripping of leaves
usually most noticeable in the uppermost parts of trees and
shrubbery. Associated with presence of many spindle-shaped bags
up to 1 1/2" in length from late summer to spring.
Identification
Larvae enclosed in cone-shaped
bags consume
foliage. Insect seldom seen, except head of larva protruding
from the bag. The larva is mottled brown to black and encloses itself
in a bag spun from silkstrands. 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.
Life History
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.
Control
It is important to treat during mid-June. Small larvae
are more susceptible to insecticides. Larger larvae and molting
larvae are not easily killed. Insecticides with some residual
are preferred. Picking off and burning bags from fall until
spring will reduce populations, but is tedious. The presence of
bags during winter is a good indication of which plants need to be treated
the following year. One generation per year.
Remarks
Bagworm is a serious pest, capable of rapid buildup and
extensive defoliation. Annual surveys during fall, winter, or
early spring are important to detect infested plants before
serious damage results.