By Bob Westerfield
University of Georgia
Spiders aren’t the only web-weavers. Fall webworms weave webs, too. Their webs, spun in shade trees and ornamentals, leave plants defoliated and landscapes unsightly.
Webworms enclose leaves and small branches in their light gray, silken webs. Persistent infestations of individual trees may cause limb and branch dieback.
Native to North American and Mexico, fall webworms feed on more than 100 species of forest and shade trees. In the eastern U.S., pecan, walnut, American elm, hickory, fruit trees and some maples are preferred hosts. In some areas, persimmon and sweetgum are favored, too. In the west, alder, willow, cottonwood and fruit trees are commonly attacked.
Newly hatched larvae immediately spin a silken web over the foliage on which they feed. As larvae grow, they enlarge the web to enclose more foliage. On heavily infested trees, webs may enclose several branches.
Full-grown larvae may reach 1 inch or more in length. Larvae are covered with long, silky gray hairs arising in tufts from orange-yellow or black tubercles. Their head color varies from red to black.
This pest overwinters in the pupal stage. Pupae are usually in the ground but can be found in old nest remains, under loose bark and in leaf litter.
The adults emerge from late May into July. Adult moths have wingspans between 1.4–1.7 inches. The bases of the front legs are orange or bright yellow. In the southern part of its range, the moth is white with dark wing spots. Those in the northern range are usually pure white and were once thought to be a separate species.
Adults appear in most areas from May to August and deposit their eggs in hair-covered masses of several hundred each, usually on the underside of host leaves. In southern states, adults can emerge as early as mid-March and produce up to four generations in a year.
Eggs are usually deposited in single or double layers on the undersurface of leaves. The mass is lightly covered with scales from the female’s abdomen.
The eggs hatch in about a week and the small mass of caterpillars cover single leaves and strip them clean. As the caterpillars grow, they web over more leaves and eat the entire leaf.
The larvae mature in about six weeks. Then they drop to the ground to pupate. Moths emerge over an extended period in two generations.
Though the webs are very ugly, most trees damage is insignificant. However, in southern states multiple generations of attack can severely defoliate trees, so control measures are needed.
Small nests can be pruned out of small to medium trees. Monitor trees early to find nests when only a few leaves are affected. These small nests are easily crushed. Don’t burn or touch the nests in trees. That can cause tree damage.
Bacterial insecticide Bt (Bacillus thuringiensis) is quite effective against fall webworms. Apply it to the nest. Thoroughly cover leaves next to nests. When worms eat sprayed leaves, they ingest the Bt.
Most standard insecticide spray applicators blow nests out of trees with a strong jet of insecticide mix. While this generally works, often more spray is used than is needed. Find nests early, wet the nest and cover nearby foliage.
Extensive nests in tall trees are hard to spray with ground equipment. Treat these with an injection or a translocated systemic applied to the soil for root uptake.
(Bob Westerfield is the consumer horticulturist with University of Georgia Cooperative Extension.)