I’m sure by now you have noticed trees containing unsightly, light-grey webs towards the ends of their branches. These webs contain the fall webworm, a yearly pest of several trees all across North America.
Fall webworms feed on over 85 species of trees. Hickory, American elm, pecan, walnut, and various fruit trees are their preferred food species. Willow, persimmon, maple, sweetgum, cottonwood, and alder are also favored host trees.
Damage done by fall webworms is 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. However, this year fall webworm levels seem to be exploding even worse than last year.
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.
Adult webworms 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 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, as we have seen in 2016 and 2017. Population explosions last for a few years before natural control agents, such as birds, 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.
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.
Beat the nests down with a stick, or spray them with a sharp blast from the water hose to expose the webworms to natural predators.
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 near a nest. As the leaves are incorporated into the nest, the Bt will be ingested and kill the caterpillars.