Bagworms

Facts, Identification & Control

Scientific Name

Thyridopteryx ephemeraeformis

Appearance

Bagworm moths complete their life cycle by going through four stages: egg, larva (caterpillar), pupa (cocoon) and adult. The easiest way to recognize bagworms, also sometimes referred to as evergreen bagworms, is by indentifying the insect’s bags which can be seen hanging from the plants it feeds on. The bag is composed of silk and fragments of the foliage from the host plant. Typically, bags are about one to two inches long and will increase in size as the bagworm larval stage grows. Each bag houses only one individual. Full-grown larvae are grayish colored and about one inch long. The adult male has wings and can fly about, but the adult female is worm-like and doesn’t have legs or wings. Being more or less immobile, she never leaves the bag she built while an immature larva.

Behavior, Diet & Habits

The bagworm’s preferred plant hosts are juniper, arborvitae, spruce, pine and cedar, but will also infest deciduous trees. Since female moths cannot fly, larvae are responsible for infesting other trees as they move from one host tree to another or are introduced via infested nursery plants. Bagworms are likely to be a greater problem in urban environments where the host plants are commonly planted together. Bagworm larvae grow and feed on the plant’s needles or leaves causing injury to the plant. More often than not, bagworm infestations generally go undetected until plant defoliation or a large number of bags become noticeable.

Bagworm eggs are deposited inside the female’s bag where they will overwinter. In the spring, the eggs hatch and each larva begins to construct its own case, where it will live throughout its larval and pupal stages. The larva will enlarge the case as it grows and moves about by partially emerging its head and legs to feed and move to other locations. Once the larval stage is complete, the insect enters its pupal stage, but stays within the bag. About one month later, the adult male moth will emerge and fly to the female’s bag where mating occurs. A female lays anywhere from 300 to 1,000 eggs inside the bag, and subsequently dies. Eggs will overwinter until the following spring. There is only a single generation of bagworms a year.

More Information

Inspecting plants for the presence of either old or new bags is the first step toward prevention and control of this defoliating pest. Be thorough when inspecting since the plant’s dense foliage may hide some or all of the bags. In addition to looking at the outside surfaces of the plant, pull back limbs and foliage and make sure there are no bags hidden close to the host plant’s trunk. Remove any bags you find and destroy them right away. Don’t simply drop them to the ground. Inspection can be done at anytime of the year, but the best times are the late fall or winter before eggs hatch and larvae begin to feed and disperse.

When there are too many bags to pick by hand, spraying the plant’s foliage with insecticides may be required. Always seek the advice and recommendations of your pest management professional (PMP) prior to using any insecticides for control. There are numerous insecticides that are effective, but there are important considerations such as what materials work best in your area, plus the best time to spray to maximize the material’s effectiveness. Your PMP is equipped with the equipment, products and expertise that will result in effective and safe bagworm control.