Have you noticed trees containing unsightly, light-grey webs towards the ends of their branches? These webs contain the fall webworm, a perennial pest of several trees all across North America.
Fall webworms feed on over 85 species of trees in the US, according to UGA Extension horticulturist Bob Westerfield. Hickory, American elm, pecan, walnut, and various fruit trees are their preferred food species. Willow, persimmon, maple, sweetgum, cottonwood, and alder are also host trees.
Some other common pests, like the tent caterpillar, have nests that can be confused with those of the fall webworm. Tent caterpillars have smaller nests confined to the crotches, or branch intersections, of the host tree. Fall webworm nests are usually larger and will enclose leaves and small branches.
Damage done by fall webworms is usually minimal in most situations. The caterpillars consume the leaves that are surrounded by the nest. As the leaves are consumed, the nest slowly expands to include more leaves for food. They usually eat leaves late in the season and are concentrated in limited areas. Very little long-term damage is done to the host trees. However, here in the south we can have long summers, which can lead to multiple pest generations and extended defoliation. The biggest problem is the unsightly nature of the nests in landscape trees.
Fall webworms spend the winter months in their pupa stage. These pupae can be found mostly in the ground, but can also be located under loose bark and in leaf litter. Adults will emerge from late May to July. A layer of several hundred new eggs is deposited on the underside of the host tree's leaves. The eggs hatch in about a week, and the small mass of new caterpillars begins webbing over and skeletonizing entire leaves. Larvae mature in about six weeks. They then drop to the ground and pupate. Here in Georgia, up to four generations can be completed in a single season.
Although most fall webworm damage is not severe, repeated generations may cause defoliation severe enough to warrant control. Webworms tend to be subject to periodic population outbreaks. Population explosions last for a few years before natural control agents catch up and reduce the webworm activity.
In small and medium-sized trees, small nests can be pruned out. Scout for webworms to catch infestations while only a few leaves are affected by the nests. Small nests can be crushed. Don't feel tempted to burn nests while still in the tree, as this can cause unwanted branch injury.
Biological control can also be encouraged for fall webworm control. Over 80 species of predators have been documented for fall webworm. Wasps, birds, predatory stink bugs, and parasitic flies are the most important. These predators should be in good supply if you aren't regularly applying insecticides in your landscape.
Another biological control method is BT (Bacillus thuringiensis), a natural bacterial insecticide. BT is effective against fall webworms when applied to small caterpillars. Thoroughly cover leaves adjoining a nest. As the leaves are incorporated into the nest, the BT will be ingested and kill the caterpillars.
Conventional chemicals can also control webworm larvae. Wet down the nest and nearby foliage with a labeled insecticide. As the caterpillars walk on the leaves and eat them, they will contact the insecticide. Repeat applications may be needed for recurring generations.
Clark MacAllister is the Dawson County extension agent. For more information, call (706) 265-2442.