Skip Navigation

Bagworms
THE “MOBILE HOME” MOTH

(From 1997 Newsletter By Bailey Walten)
Thyridopteryx ephemeraeformis is an unusual insect in that it lives, feeds, and breeds inside its own portable house. Commonly known as a “bagworm”, this moth caterpillar hatches from its egg in early June and immediately begins constructing a home (bag) using its silk and interweaving leaves and stems from its host plant. This insect is extremely well disguised and is often mistaken for being part of the plant on which it is feeding. Once safely sheltered, it begins to feed on the foliage of its host. The newly hatched larva is very tiny (1/16 inch) and first feeds on top of the leaf with its bag pointing upwards. Initial damage is slight leaving small brown spots in the leaf’s epidermis. However; as the bagworm grows, it consumes entire leaves (or needles depending on the type of host) while increasing the size of its now vertically suspended bag. A severely infested plant will appear to be wriggling as the worms stick their heads and legs out from their bags which twitch back and forth while feeding and moving about the plant. Large deposits of fecal matter (frass) will collect under the host as the foliage is consumed. If the bagworms devour all the leaves before they have fully developed, they will crawl to a neighboring plant and resume feeding. This observer once saw bagworms that had exhausted their food supply and still hungry, had crawled through a window air conditioning unit and entered a home in their desperate search for more food. One can often see a domino affect on a conifer hedge as one plant after another is denuded. This damage is particularly destructive to evergreens because once defoliated, they will not recover.

Bagworms usually finish feeding by mid to late August when their bags are 1 to 2 inches long. The larva then attach the bags with strands of silk to a branch, seal themselves inside, and pupate. The male will emerge in 7-10 days as an adult moth with clear, one inch span wings. He will fly to an adult female who remains in her bag and has pupated into a yellow-white blob without wings, legs, eyes, and antenna. After mating the male will die in several days and the female will decay after laying 1000-1500 eggs which overwinter inside her bag. These eggs will hatch the following year thus completing the one generation per year cycle. In the deep south, there is no dormant period so continuous generations occur throughout the year.

Widely distributed east of the Rockies, this specie of bagworm is one of our most serious pests. Known to attack at least 128 different plants, it is a perennial problem and particularly destructive to ornamentals such as arborvitae, leyand cypress, blue spruce, junipers, white pines, and other coniferous hosts in our area. Deciduous hosts can also be severely defoliated by these worms. Honey locust, maple, sycamore, elm, oak, willow and many shrub varieties may be attacked. Damage is usually not as devastating on deciduous plants because defoliation occurs later in the season after enough food reserves have been stored for refoliation the following season. Control is best achieved by spraying the infested plants in June and again in July to kill later hatching caterpillars. Picking the bags off the trees in the winter and “destroying them” will also help to reduce their populations the following year. However; do not rely on picking alone because well-hidden bags can be easily missed and some may have fallen down into the ground litter. Do not confuse the bags for cones or other plant parts. **Failure to treat promptly when damage is first noticed can allow these voracious feeders time to irreparably harm valuable plant material.**

Plants weakened by bagworm infestations should definitely be fertilized to stimulate new growth and increase vigor. Any plants or individual limbs that have been denuded and fail to refoliate the next spring should be removed.

Bagworm populations were high last year and the MARYLAND COOPERATIVE EXTENSION SERVICE is predicting heavy damage this season from large amounts of over-wintering eggs laid last year. Be on the lookout come June because the BAGWORMS ARE COMING!!!