While the calendar says we haven’t even left spring yet (summer starts on June 20), we have already received our first fall webworm inquiry of the year. Even though their name implies that they would most enjoy the months of September and October, we can encounter early instars of the first generation of these pests at this time of year.
Fall Webworm Biology
This pest overwinters as pupae that are hidden in leaf litter below their host plants. The adults that emerge from these pupae are active in March and April; they are starkly white with occasional brown spots. The females of these moths will lay their eggs, and the resulting larvae emerge in late spring/early summer, feed, pupate, and emerge as adults in August. The nests they construct provide shelter and allow them to feed as a group.
The adult moths that emerge in the late summer will mate and lay their eggs, and a new generation of larvae will emerge to feed. This second generation tends to cause more damage and build even more noticeable nests, and they will overwinter to start the process again the next year.
There are two “types” of fall webworm caterpillars we can encounter. One is the blackheaded biotype and the other is a redheaded type. Both feed in similar fashions by building their nest over the tips of branches to feed on the newest foliage. However, blackheaded fall webworms tend to get started a month earlier than the redheaded types, while the redheaded types build a denser nest. Both types are hairy caterpillars.
Fall Webworm Hosts
Fall webworms have an extensive host list. Some of their preferred foods include sweet gum, American elm, mulberry, hickory, oak, and maple.
Fall Webworm Solutions
Fall webworms are usually a cosmetic issue, though they can cause harm to newly transplanted trees or young trees in the landscape. If you are noticing them now, the best solution is to simply remove the leaf or leaves that have webbing and caterpillars, and physically destroy the insects. Stomping works well. You can also blow nests from trees with a strong jet of water, poke them with a stick and twirl the stick around the inside to destroy their home, or prune out infestations. When the larvae are small, treating the foliage around their nest with Bt or Spinosad would be organic treatment options.
By Jonathan L. Larson, Entomology Extension Specialist