Bagworm is an unusual caterpillar that gets its name from the bag-like structure in which it lives. Most of the insect’s life is spent within this protective bag, which is made of leaf fragments bound together with silk produced by the larva. The cocoons hang from branches, blending in with the plant itself. The bags look different depending on the host because the insects use the tree’s foliage to construct the cases. As such, bagworms can be difficult to see and often go unnoticed.
Feeding damage from bagworms on trees and shrubs looks like etching. As the caterpillars grow they will consume entire leaves and needles. A heavy infestation can completely defoliate a plant. If unmanaged for several consecutive seasons, defoliation can ultimately lead to plant death. Damage can be more severe on evergreens as they are often unable to produce new growth after loss of needles.
The spindle-shaped bag structures are the principal evidence of a bagworm infestation.
Tree Species at Risk
Eggs hatch from late-May through mid-June. Then, larvae begin feeding and constructing new bags. As larvae continue to feed and grow, they enlarge the bag to accommodate their increased body size. In late-July through early August, fully grown larvae secure their bags to a twig with silk and pupate inside. Bagworms then overwinter within their protective case.
More than 125 species of plants are at risk of infestation. Juniper, arborvitae, spruce, pine and cedar are common hosts. However, bagworms can attack lots of other conifers and deciduous plant. Favored species are readily identified now, during dormant season. From fall until spring, the presence of the bags is a red flag for future trouble from these pests.
Controlling Bagworm
Removing and destroying the bags can reduce light bagworm infestations on shrubs and small trees. This should be done in the dormant season before eggs hatch. Removing the egg sacs reduces the number of caterpillars that can emerge next spring. Now is a good time to look for these pests. Put in a little extra effort and you can help prevent a serious infestation.
On conifers and other plants with heavy infestations, management is frequently necessary. Any treatments should target the newly emerged caterpillars in the spring.
