Bagworm, Thyridopteryx ephemeraeformis, is native to North America and is a major insect pest of trees and shrubs in landscapes and gardens. The larva or caterpillar life stage causes damage by feeding on plant foliage (leaves). Bagworms are often associated with conifers such as arborvitae, bald cypress, cedar, fir, juniper, pine, and spruce. However, bagworms also feed on deciduous broadleaf trees and shrubs, including: black locust, crabapple, elm, hackberry, honey locust, linden, maple, oak, poplar, rose, sycamore, and willow.

Biology: Bagworms overwinter as eggs within the female pupal cases located inside the bags. Eggs hatch and the larvae (caterpillars) emerge from the bags in mid-May through early June. However, temperature influences the timing of bagworm emergence, with cooler temperatures delaying egg hatch and bagworm activity. Bagworm larvae emerge from bags over several weeks. When young larvae are about 1/8 to 1/4 inches (3 to 6 mm) in length, they produce a fine strand of silk for ballooning (wind dispersal) to nearby host plants. Ballooning is the primary means of dispersal for the young larvae. After finding a suitable host plant, bagworms settle and begin feeding. While feeding, bagworm larvae construct silken bags that are covered with fragments of twigs and foliage from the host plant. The larvae remain in the bags with the head and thorax protruding while feeding on plant foliage. As the larvae increase in size, they cause progressively more plant damage. The bags expand to accommodate the growing larvae and allow them to withdraw when disturbed. Bagworm larvae can move among adjacent plants in landscapes and gardens when plant foliage is touching. Bagworm bags are brown and easy to see in contrast with surrounding foliage. The bags hang from branches like Christmas tree ornaments and can be mistaken for pine cones or other plant structures. The bags protect larvae from natural enemies such as birds. Bags can be found attached to buildings, garbage containers, and decks.

From mid-August to early September when bags are about 1 to 2 inches (25 to 50 mm) long, the larvae stop feeding and seal the top of the bags. The larvae then enter the pupae stage. For the most part, female bags are located at the tops of trees and shrubs, and male bags near the bottom. Females remain in pupal cases within the bags but do not develop into moths. Females do not have wings, legs, antennae, or functional mouthparts. Males transition into black moths with clear wings that are approximately 1/2 inch (13 mm) long. Males follow the sex pheromone emitted by females, using their feathery antennae to locate the female bags. Male bags have a transparent papery brown pupal case protruding from the bottom. Once the male locates a female bag, he inserts his abdomen into the hole at the bottom of the bag and mates with the female. After mating, the male flies away and dies, and the female remains in the bag. The female lays between 500 and 1,000 eggs, which are retained in the pupal case inside her body. Females eventually die and the eggs inside the bodies of dead females are protected within the bags over the winter. There is one generation per year.

Damage: Bagworm larvae or caterpillars have chewing mouthparts that are used to feed on plant foliage. Feeding damage can affect the aesthetic quality of trees and shrubs. Bagworm larvae feed for approximately three months. Larvae feed intensively and cause more damage as they increase in size. If not managed, excessive bagworm populations can defoliate trees or shrubs. However, the extent of defoliation depends on plant size and type. Conifers are particularly susceptible to damage from larval feeding because they do not regenerate new growth after the foliage has been consumed. Extensive feeding for several years in succession may result in complete defoliation and can kill well-established trees and shrubs.

Management: Bagworms can be managed with proper cultural practices and well-timed insecticide applications.