The fall webworm (Hyphantria cunea) is native to North America and – here’s a switch – it turns out that we accidentally introduced this pest to Europe and Asia in the mid 1940′s. It’s actually one of the few insect pests that we have exported.
Fall webworms are often confused with tent caterpillars since they both build web nests in trees. However, tent caterpillars appear in the spring and fall webworms usually appear in the late summer and early fall. They also have very different styles when it comes to building their nests. Tent caterpillars build compact nests in the crotch of two branches, while the fall webworms build large silken webs which surround the foliage at the tips of branches. As the caterpillars exhaust the leaf supply within the web, they enlarge it to incorporate a new supply of fresh green foliage and the nest expands along the branch.
Fall webworms overwinter in the pupal stage usually in the soil, mulch, or leaf litter at the base of the tree. The adults are white moths that emerge in March and April. After mating, the female moths lay hundreds of eggs in a mass on the underside of leaves. Small caterpillars hatch out after about seven days and immediately begin building a web and feeding on the leaves of the tree.
Fall webworms attack many different species of trees including pecan, walnut, hickory, many types of fruit trees, and the eastern redbud.
There are two races of the fall webworm in North America, a blackheaded race and a redheaded race. The webworms on this redbud are the blackheaded race.
These caterpillars have a voracious appetite and have consumed most of the tissue between the leaf veins, leaving behind webs filled with brown skeletonized leaves and green frass (their droppings). This is not at all attractive in the landscape.
In general, webworm feeding doesn’t hurt the tree because the damage is usually localized to just a few branches and since it occurs later in the season, the tree has had time to store food. The damage is mostly cosmetic and the webs will eventually weather away over the winter. However, if the tree is under stress due to drought, poor nutrition, or repeated defoliation from multiple generations of caterpillars, then its health could be in jeopardy and control measures may be warranted.
In many cases, the unsightly nests of fall webworms can simply be removed from small to medium sized trees by pruning out the infested branches. Another solution is to destroy the webs by winding them up on a stick or tall pole if they are within reach. Any caterpillars that are not killed in the process are left exposed and will often be eaten by birds and other predators.
If you discover the caterpillars while they are still small, you can control them by spraying a liquid formulation of the bacteria, Bacillus thuringiensis (Bt). Bonide Thuricide (BT) Liquid, applied to the foliage right next to the nest, will kill the caterpillars when they eat the leaves and will not harm beneficial insects. Bonide Captain Jack’s Deadbug Brew, another naturally derived organic insecticide, is also an effective control when sprayed on the foliage adjacent to the web.
The trick with these sprays is to cover the foliage that the caterpillars will be eating. The foliage and caterpillars inside the nest are protected by the web and spraying the nest is usually not effective. Always read and follow the label directions when spraying any pesticide.
Bonide products can be found in most full service garden centers.