BAGWORM–
A Pest of
Urban-Forest
Trees and
Shrubs

 Lacy L. Hyche

The bagworm is appropriately named. The “worm” (larva) spends its entire life in a case or bag (Photo 1) that it constructs with silk and leaf fragments from host trees. The bag is characteristic of the species, and identifies the insect and ultimately betrays its presence. Larvae, with bags attached (Photo 2), feed on foliage of a wide variety of trees and shrubs, often causing serious damage to valuable ornamentals. AAES studies on tree insects of Alabama provide information on the life cycle, habits, and importance of this unusual caterpillar in urban and suburban tree landscapes.

Arborvitae, junipers, cedar, cypress (Arizona), pines, and similar needle-bearing evergreens are the favorite hosts of the bagworm. Deciduous broadleaf trees are also attacked, and it is common to find bagworms feeding on black locust, honeylocust, maple, boxelder, sweetgum, sycamore, various oaks, and several other hardwoods.

Photo 2. Late-stage bagworm larva feeding on an ornamental evergreen.

Photo 3. A full-grown larva inside its silk-lined bag.

The full-grown larva inside the bag (Photo 3) is 1 to 1.25 inches long; the abdomen is brown and the head and thorax yellowish white with dark spots. Larvae reach maturity in late summer, usually by about mid-August in the Auburn vicinity. At this time, feeding and movement cease, and bags are fastened securely to twigs with a band of silk (Photo 1). Finished bags are 1.75 to 2.25 inches long. Bags of females are usually larger than those of males. Pupation occurs inside completed bags, and adults transform from pupae by late August or early September.

Bagworm adults are moths, yet they bear little or no resemblance to typical moths. The male (Photo 4, left) is black, about 5/8-inch long, and resembles more a large fly than a moth. The female (Photo 4, right) is grub-like, nearly an inch long, and possesses no eyes, antennae, functional legs or wings, or any other physical likeness to a moth. Winged males emerge from bags and fly to female bags to mate. Wingless females remain inside the bags and mating takes place through an opening in the lower end of the bag. The mated female deposits 400 to 1,000 eggs into the pupal case (Photo 5) from which she emerged, then exits the bag and dies.

Photo 4. Bagworm adults: male, left, and female, right.

Photo 5. Female pupal case filled with eggs.

Eggs overwinter in the old bags and hatch in spring. Hatch usually occurs in April in South Alabama and in late April and early May in northern Alabama. Newly hatched larvae move to new foliage and construct new bags. Larvae feed throughout spring and summer, enlarging bags as they grow. Only one bagworm brood is produced each year.

The severity of injury caused by the bagworm depends on the amount of foliage lost and type of tree, i.e., evergreen or deciduous. A single complete defoliation of an evergreen usually kills the tree. Deciduous trees, if healthy otherwise, can usually produce new foliage and survive. On valuable ornamentals, control of bagworm may be necessary. Control recommendations are available from extension specialists and county extension personnel.