MOUNT VERNON — Fall webworms, scientifically known as Hyphantria cunea, are a common sighting around Mount Vernon and all of central Ohio this time of year. Driving around a wooded area, one can see unsightly, light gray, silken webs in the trees in the late summer and early fall. These webs are constructed by the adult webworm, which enclose the webworm larvae while they feed on the tree’s leaves. Webs are more numerous when the weather is warm and wet for extended periods. The insect thrives on sun and moisture. They have been known to feed on over 85 species of trees in the United States, Canada and Mexico, according to Ohio State University Extension Fact Sheet HYG-2026-95.
Not to be confused with the bagworm moth and the tent caterpillars, fall webworms enclose leaves and small branches at the ends of the limbs, unlike the tent caterpillars which in the spring make a smaller nest in the crotch of branches. A totally different casing is constructed by the bagworm moth, whose larvae create cases out of silk and environmental materials such as sand, soil, lichen or plant materials. These cases are attached to rocks, trees or fences and can take on the appearance of a pine cone shape.
“More than half of my phone calls lately have been about webworms,” said Troy Cooper, OSU Knox County Extension Agent. “I’ve seen a real increase in these over the past two to three years.”
So why do we see these webs in our trees this time of year? Each summer, adult moths will lay their eggs on the underside of leaves in clusters of around 200 and encase the area of eggs in a silky web. Eggs then hatch into larvae in about a week which then feed on the leaves for about four to six weeks. The caterpillars can range in color from a pale yellow to dark gray with yellow spots, and long and short bristles. The maximum length is 35 mm, according to the OSU Extension fact sheet.
The Fact Sheet explains the pupa stage that follows where they will bury themselves in the ground, as well as tree bark and leaf litter at the base of trees over winter. They have thin brown cocoons made of silk.
The adult moth then emerges from late May until July, is mostly white in the northern portion of Ohio, but in the South may be marked with black or brown spots. It is quite hairy, and the front legs have bright yellow or orange patches. The wingspan will be about 35 mm to 42 mm.
“We can have two generations of these each year in Ohio,” said Cooper. “More generations per year can be found in the South. The adult moth then dies later that fall.”
While the larvae webs can be rather unattractive, trees can be stripped of their leaves by the caterpillars, but rarely are the trees destroyed.
There are a few different measures of controlling the webworm webs. One method Cooper recommends is removing the nests if possible. Small nests can be pruned out of small to medium trees. Small nests can be crushed. Do not attempt to burn or torch nests in trees as this may do additional damage to the tree.
Some sources may suggest the use of insecticide sprays directly onto the web. But Cooper does not suggest this practice as he believes the webs are weather resistant, and this proves to be ineffective means to destroy the webs.
Another practice that Cooper strongly recommends is the use of systemic insecticides. Extensive nests may occur in tall trees which are difficult to spray with ground equipment. These trees can be treated with translocated systemics applied to the soil for root uptake or injected. “These are applied at the base of the tree, then anything eating in the tree will be killed,” said Cooper. Orthomax products and other systemic insecticides can be found at various greenhouses and garden centers.
