Grass bagworm (Eurukuttarus confederata) is a species of bagworm that infests various grasses. The larval stage of the insect constructs a spiny case that measures ¼ of an inch to 1 inch in length. The case is composed of plant material and silk. It is initially flush with color, but turns brown as the grass decomposes. While the insect is seldom destructive to plants, it is considered an aesthetic nuisance that can plague landscapes and gardens when abundant.
Distribution & Habitat
Grass bagworm is common across the eastern United States. The insect’s range extends west to Kansas and Nebraska. Grass bagworm is most prevalent in urban environments.
Grass bagworm primarily infects grasses. It may also occasionally infest conifers.
The eggs produced by grass bagworm are small, and colored a dull white. The larvae appear similar to grubs. The have a glossy black head, and a dense, pale yellow body that is mottled with black. When mature, the larvae may extend up to 1 ½ inches in length. The adult males are small, growing up to ¾ of an inch in length. They feature two pairs of wings, and three pairs of legs. The wings are transparent, with black borders. When fully extended, the insect’s wingspan measures approximately 1 inch. The males’ body is laden with fur-like structures. The adult females remain in their larval form. As such, they lack wings and legs. The bags the insect weaves can reach between 1 ½ and 2 ½ inches in diameter. In late fall, the larvae reconfigure their bags, causing them to resemble pine cones.
The eggs overwinter in the bags woven by the females. The mummified husks of the females generally envelop the eggs. From late spring to summer, the eggs hatch, revealing masses of tiny larvae. Each larva crafts a case using a combination of silk and grass blades. During this period, some larvae will cling to silken threads of their making, and spin makeshift balloons that enable them to be transported by air currents to nearby plants, which they readily infest. Most larvae navigate across the plant they have hatched onto, and select a suitable location to begin weaving their bags. The larvae feed from within the bags, consuming grasses, lichens, mosses, weeds, and other plants. The larvae continue to feed for 8 to 10 weeks. As the larvae develop, they increase the size of their bags, contributing fresh plant material to them.
Once mature, the females ascend onto fences, buildings, furniture, and tree trunks, and attach their bags to them with a thick strand of silk. The larvae enter the bags, and hang in an inverted position to pupate. By mid-June, the adult males emerge from their bags. The females position themselves over a small opening at the bottom of the bag, and emit a pheremone that attracts the males. The males navigate to the females, and mate. Upon mating, the females deposit 300 to 1,000 eggs each within the bags. The adults die shortly thereafter. The eggs overwinter, and hatch the following spring.
Symptoms of Infestation
The bags produced by the larvae can generally be observed on different surfaces in mid-summer. Infested grass will turn brown. Plants that have been infested by the larvae may be partially defoliated. The silk secreted by the larvae can girdle branches, causing them to die back.
- Grass bagworm inflicts minimal damage to host plants. As such, control is not generally required.
- When necessary, pesticides can be utilized to control the larvae. Applications must be performed in spring, as the eggs begin to hatch. Spray the entire plant, ensuring that the branches are thoroughly drenched.
- Individual bags can be plucked from plants, and destroyed.
Photo courtesy of Lorenzo Magnis CC-by-2.0