2005-09-07 / Fields & Yields

Home & Garden Tips

Fall webworm management
By Roosevelt McWilliams Burke County Extension Agent

The fall webworm is most often discovered when the light gray, silken webs on the

trees in late summer and early fall are o b s e r v e d . Webworms e n c l o s e leaves and s m a l l branches in

their nests,

unlike the tent caterpillars that make a smaller nest in the crotch of branches. This pest is native to North America and is common from Canada into Mexico.

Fall webworm larvae have been known to feed on over 85 species of trees in the United States. Pecan, walnut, American elm, hickory, fruit trees and some maples are preferred hosts. Persimmon and sweetgum are also readily attacked while willow, cottonwood, and alder are only occasionally attacked.

This pest usually eats leaves late in the season and the nests are generally concentrated to limited areas. Because of this, little real damage is done to most trees. However, the nests can look very unsightly and multiple generations in long summers can lead to significant defoliation.

This pest overwinters in the pupal stage. Pupae are usually in the ground but can be located in old nest remains, under loose bark and in leaf litter. The adults emerge from late May into July. The eggs are usually deposited in a single or double layer of several hundred eggs 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’ web over additional leaves and finally are able to eat the entire leaf. The larvae mature in about six weeks, at which time they drop to the ground to pupate. In southern states, adults can emerge in mid-March and up to four generations can be completed.

Though the webs are unsightly, damage to most trees is considered to be insignificant. In southern states where several generations of attack can severely defoliate trees, control measures may be needed. This pest tends to go through periodic population explosions. Outbreaks every four to seven years may last for two to three years and then natural control agents greatly reduce the activity.

Small nests can be pruned out of small to medium trees. You should monitor the trees early to detect the nests when only several leaves are involved. These small nests can be easily crushed. Do not burn or torch the nests in trees as this may do additional damage to the tree.

The bacterial insecticide, Bt, is quite effective against fall webworms if it is applied when the larvae are small. Use formulations with UV protectants and thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the Bt will be ingested.

Most applicators attempt to blow the nest out of the tree with a strong jet of insecticide mix. While this generally works, more material is often used than is needed. Locate nests early and merely wet the nest and cover nearby foliage. As the larvae walk on the nest surface or incorporate new foliage, they will contact the insecticide. Second applications may be needed if additional generations occur.

Extensive nests may occur in tall trees and are difficult to spray with ground equipment. These trees can often be treated with translocated systemic applied to the soil for root uptake or injected.

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