By Aaron Ashbrook

LSU AgCenter Entomologist

Step outside just about anywhere in Louisiana right now, and you’re likely to spot dozens of hairy, white caterpillars crawling around. Sometimes, you may even see one fall from overhead, seemingly out of the sky.

If you look up, however, you’ll probably notice webs encasing tree leaves and branches. These webs are the handiwork of those fuzzy caterpillars that have been everywhere lately. They’re called fall webworms (Hyphantria cunea), a relatively common defoliator of tree leaves.

While the ever-present caterpillars are a nuisance and their webs are unsightly, you shouldn’t worry about them hurting your trees in most cases. Generally, trees can tolerate webworm activity without suffering long-term damage.

Let’s take this opportunity to learn a bit more about fall webworms.

The caterpillars can be variable in coloration, with either black or red heads. They once were thought to be two different species, but they actually are the same, just with different “morphs.” The bodies of the larvae can be yellow to dark gray and have long and short hairs. Both caterpillar morphs have two cream-colored stripes running the length of their body. In Louisiana, the red-headed morph has four generations a year, and the black-headed morph has five generations each year.

The full-grown black morph larvae are yellow or light green in color, whereas the red morphs are yellow to dark orange with red bumps with white hairs. Red-headed morphs are more common in the southern U.S.; however, their populations vary dramatically from year to year and by location.

Lately in Louisiana, we have been encountering the black-headed, or northern, morph of fall webworms. Once they become adult moths, they are primarily white and may have small black dots on their wings.

Fall webworms make webbed areas at the ends of tree branches and form aggregations there, where they feed on the foliage. The leaves inside the webbing will be consumed, skeletonizing the leaf tissue. As the larvae grow, they will create more web tents to protect themselves while they feed on the leaves inside.

The fall webworm has a wide host range and can consume more than 400 species of trees. The webbing can combine to cover larger sections of the tree where defoliation is occurring. In heavily forested areas, they are not as big of a problem, as the damage can be well tolerated if their populations are dispersed among trees. In urban settings where people are relying on tree leaves to provide shade, their damage can be more noticeable and severe.

Additionally, webworms attack fruit- and nut-bearing trees, which can greatly impact production. Therefore, in some cases, control may be warranted. Many natural enemies exist for fall webworms, including general predators, parasitoid wasps and different species of flies. Therefore, we recommend using reduced-risk insecticides such as those that contain Bacillus thuringiensis (Bt) or chlorantraniliprole. Other broad-spectrum products such as carbaryl and pyrethroids can be used but will affect off-target insects.

As always, be sure to follow the label instructions and use products that are approved for fall webworms. When using insecticidal products that are not Bt based, we recommend the addition of dish soap to help with penetration of the insecticides.

Caterpillars also are susceptible to heavy rains and other weather events. Removing the tree branches that have caterpillars on them is not recommended, as it causes more harm to the trees than the pests themselves.

As fall webworm larvae grow, they will create more web tents to protect themselves while they feed on the leaves inside. Photo by Heather Kirk-Ballard/LSU AgCenter

Fall webworms make webbed areas at the ends of tree branches and form an aggregation there, where they feed on the foliage. Photo by Heather Kirk-Ballard/LSU AgCenter

Adult moth of the fall webworm. Photo by Aaron Ashbrook/LSU AgCenter