Fall is approaching and a common garden pest, the fall webworm (Hyphantria cunea) can become noticeable on trees, causing unsightly larval nests covering entire branches, resulting in stress to the tree and severe leaf damage. Fall webworm are caterpillars that weave loose webbing around the tree’s outer foliage while feeding on leaves, compared to tent caterpillars that appear in spring and build their more opaque nests within the inner crotch of the branches. The webworm caterpillar is approximately one inch in length with a light greenish-yellow body and black to reddish head. Adults emerge later on as white moths with dark spots on their wings.
The best way to eliminate fall webworm is to remove the infected branches immediately, before the larvae hatch and take over the tree. If the caterpillars have already left the nest, it is recommended to spray with an organophosphate insecticide such as Acephate (contained in Orthene or Sevin) or Malathion. Acephate is both a foliar and soil systemic which keeps on working 10–15 days after application. Malathion is a foliar insecticide which is also commonly used, but note that Malathion may leave a residue. The best proactive method of killing overwintering larvae is to apply a dormant oil in early spring while the tree is dormant. Dormant oil is a more natural solution and works by smothering and killing the overwintering eggs.
In the spring, adult moths emerge and deposit eggs, continuing the life cycle of the caterpillar. These caterpillars may go through as many as eleven growth stages before leaving the web.
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