Bagworm

The bagworm, Thyridopteryx ephemeraeformis (Haworth) is the larval stage of a moth native to Pennsylvania that is reported to feed on over 100 different plants. It is most common in the southern part of the state, where populations can build up rapidly and become serious pests. On pine trees, its cone-shaped bags are often mistaken for cones, which go unnoticed until the infestation is severe. Bagworms spread slow because the female is unable to fly, however, bagworms can be windblown or crawl to other host plants and can also spread through infested nursery stock.

This insect is most easily recognized by the case or bag that the caterpillar forms and suspends from ornamental plants on which it feeds. The bag is made of silk and bits of host foliage. These materials are interwoven to disguise and add strength to the case. When the larva is mature, the bag may be 30 to 50 mm long. Young larvae hatching from the eggs are approximately two mm long, glossy black on the back and dull amber on the undersurface of their bodies.

Habits: Caterpillars have chewing mouthparts. A wide range of boadleaf and evergreen trees and shrubs serve as hosts for bagworm species, including arborvitae and other ornamental conifers, box elder, cedar, cypress, elm, fruit and nut trees, juniper, live oak, locust, maple, persimmon, pines, salt cedar, sumac, sycamore, wild cherry, willow and many other ornamental plants. Leaves may be damaged by having the outer layer of cells (epidermis) removed by small caterpillars or all tissues but major leaf veins removed by larger caterpillars. Infested plants develop more bagworms each year because female stages do not fly. They may become abundant enough in some years to completely defoliate their host plant. Nearby host plants can remain unaffected. Completely defoliated evergreen species such as arborvitae and juniper, can be killed. During leaf-feeding, the caterpillars emerge from the top of the bag and hang onto the host plant with their legs and sometimes with a silken thread.

The bottom of the bag remains open to allow fecal material (frass) to pass out of the bag. During molts and pupation, caterpillars seal the bags. Dispersal of bagworms to new host plants occurs when young caterpillars hanging from silken threads are spread by wind or perhaps by birds. Bags can be removed from host plants by hand. During the winter months, bags contain remains of female moths and eggs produced by them. During the late spring and summer, bags will contain caterpillars that can be removed. Males are best reared from bags after caterpillars pupate. Males can also be attracted to lights.

Life cycle:Bagworms spend the winter as eggs inside the female’s bag. Several hundred eggs may be laid and overwinter in a bag. Since some bags contain only males, not all bags examined will contain eggs during the winter. The eggs begin to hatch in late April to mid May. Upon hatching, the young larvae crawl out of the bottom of the bag and start to feed and construct silken shelters over their bodies. These young bagworm larvae are highly mobile in their search for food plants; walking or using wind currents to disperse. Their bags, at this time,consist of little more than spun silk and dust particles. As the larvae feed and grow, they continue to enlarge the exterior of their bags with pieces of twigs and foliage,bits of bark, shed skins and excrement. The bags offer camoufl age and even repel rainwater. Being hard to wet, the bags are highly impervious to pesticide sprays, which seldom penetrate to reach the larvae.

Feeding and growth usually continue until August,when the larvae are full grown and the bags are about 21/2 inches long. At this time, they stop feeding and loop strands of silk around a twig and become fi rmly attached. After the top of the bag is closed, the larvae reverse their position in the bags so their heads are facing downward. They then change into the pupal (resting) stage. The male moths emerge about four weeks after larval feeding has ceased. The female never leaves the bag to mate. After mating, she deposits a mass of eggs inside the bag. The female, in most cases,then drops to the ground and dies. The eggs remain in the bag throughout the winter and into spring. There is only one generation of bagworms each year.

Damages:Bagworms attack both deciduous and evergreen trees. Some of the more common evergreen host plants include arborvitae (Thuja), fir (Abies), hemlock (Tsuga), juniper (Juniperus), pine (Pinus) and spruce (Picea). Deciduous host plants include black locust (Robinia pseudoacacia), honeylocust (Gleditsia triacanthos), sweetgum (Liquidambar styraciflua) and sycamore (Platanus occidentalis).The cone-shaped bags, which they form, easily identify bagworms. These are carefully interwoven using silk and bits of leaves and twigs from the host plant resulting in a well-disguised covering. The tops of the young larvae are shiny black and their body undersides are dull amber. When fully grown, the bagworms are a dull, dirty, gray with darker markings toward the head. The adult male develops into a moth that can fly, but the female remains grub-like and stays inside the bag.

Control: The bagworm may be managed on small shrubs and trees by handpicking or cutting the bags from infested plants during late fall, winter, or early spring, before egg hatch. Dispose of the bags so that this pest will not reenter your landscape. A number of natural enemies attack the larval and egg stages of the bagworm. Apparently, natural enemies are responsible for bagworm population changes from year to year. The species of parasites that attack this insect can be observed by collecting dozens of bags in the winter and placing them in a fine screen cage. Observe the cage frequently during April and May. These natural enemies should emerge from the bagworm and can be seen flying around inside the cage. A sex pheromone has been identified that when used in traps to lure the male moths, has successfully interfered with this pest’s mating behavior.

When bagworms are too numerous to handpick, an insecticide application may be indicated. Several registered insecticide formulations are labeled for bagworm management. These products should be applied from early to mid-June while the larvae are small. Otherwise, treatments will not be as effective against larger larvae. However, to avoid damaging valuable plants, apply the material only to plants that are specified on the label. Be sure to follow all insecticide label directions.

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