Adult male evergreen bagworm moths are furry and look a lot like blackish bees with long, tapering abdomen tips. They have comblike antennae and usually have clear wings (which is very unmothlike), since they lose most of their wing scales as they squeeze out of their larval cases. Adult females lack wings and antennae; they look a lot like caterpillars or maggots and usually do not leave their bags.
Sometimes the brittle, brownish, segmented pupal case remains protruding from the bottom tip of a male’s empty bag, after he has emerged.
The larvae themselves are rarely seen; they are blackish or brown and live in distinctive conical or spindle-shaped bags on the host plant and only poke their heads out to feed. They retreat into the case for safety when not eating.
Similar species: There are nearly 30 species in the bagworm moth family in North America north of Mexico. All have wingless (or nearly wingless) adult females that do not leave their bags, and the males are usually drab blackish shades. The larvae of all create protective cases out of plant materials or other debris.
Wingspan of males: about 1 inch. Females don’t have wings. Bags may reach about 2½ inches long.
Habitat and Conservation
This is the familiar bagworm well-known as a pernicious pest on evergreens and many other trees and shrubs in eastern North America. The winged male moths are rarely seen, since they only survive for a few days, but you might see them at lights in late summer and fall, August through October — mostly in September. The wingless females and larvae are confined to their bags and are therefore easier to locate. Interestingly (but not happily for landscapers), the larvae can travel across ground for considerable distances between plants before pupating.
Bagwarm larvae eat the leaves and soft stems of many types of trees and shrubs, including evergreens. In Missouri, they are most commonly noticed on eastern red cedar and on the various junipers and arborvitaes used in landscaping.
The adult moths in the bagworm family only live for a few days and do not eat.
The bagworm moth family, Psychidae, has a worldwide distribution of about 1,000 species, some of which are economically important. About 30 are found in North America north of Mexico. All have larvae that live in bags and mature females that are flightless.
Bagworms have a fascinating life cycle. Like other moths, they progress from egg to caterpillar (this species has 7 caterpillar instars, or stages), and full-grown caterpillars pupate, then become sexually mature adults.
In the case of bagworms, however, the eggs, caterpillars, and adult females don’t leave their protective bags or even fully leave their pupal casing, which complicates matters slightly: The males must seek out the females. Receptive females emit pheromones (scents that attract the opposite sex), and a male, finding a female’s bagworm bag, must extend and poke his abdomen into the female’s case in order to mate with her. The female deposits her hundreds of eggs into her own bag and dies within a few days. If she doesn’t drop onto the ground when she dies, her dried-up body may remain with the eggs until they hatch in late spring the following year.
The tiny, newly hatched caterpillars may stay on the same plant, if there is enough foliage to support them, or they may disperse themselves by “ballooning” on the wind via a strand of silk, much like spider hatchlings do. When a young bagworm finds a suitable food plant, it eats and starts constructing its protective case. It can take all summer to reach maturity, at which the caterpillar is about 1 inch long. In late summer, they pupate and turn into their adult forms.
Landscapers and homeowners don’t find bagworms pleasant. Large infestations can cause considerable damage to a host shrub or tree, weakening it or simply making it look horrible. If the host plant is young, small, or already struggling for some reason, a bagworm infestation can kill it.
For nurseries and garden centers, even small numbers of bagworms can cause enough damage to nursery stock to make them unappealing to customers and thus unsalable.
There are plenty of resources online to help you combat bagworms in your yard. You can pick them by hand, if the numbers are low. Try to remove them in spring before the eggs hatch. You may try Bacillus thuringiensis or an insecticide on young larvae, but these usually only work well if you apply them before the larvae create their protective bags.
Because bagworms typically do not move very far from their mother’s food plant, and because a female can lay hundreds of eggs, infestations of bagworms often occur on individual plants or groups of plants, while nearby plants may have only a few bagworms. It also means that the same host plant may be “hit” by bagworms year after year.
The tough protective bags prevent many predators from bothering bagworms, but there are several species of ichneumon wasps and other parasitoids that lay eggs on and eat up bagworms. One of these ichneumons is Itoplectis conquisitor, a species that also zaps spruce budworm and some other problematic moth species. Additional bagworm predators include wasps and hornets, mice, woodpeckers, and sparrows.