by Laurie Stepanek, Nebraska Forest Service
It’s not too early to look for fall webworm. The large, unsightly webbed nests constructed by fall webworm are usually noticed in August and September, but this pest of landscape trees is already at work forming small webs and feeding on leaves.
Although webworms can eat a lot of leaves, the pest is mainly an aesthetic problem, and natural enemies such as predatory and parasitic insects help keep populations in check. Older trees can usually shrug off an infestation without sustaining serious damage while young or recently transplanted trees are the most susceptible to damage.
Fall webworms are pale green, dingy yellow, or brownish caterpillars covered with white tufts of hair. Early in their development, the caterpillars spin silken threads around a few leaves and feed within the web, which protects them from inclement weather and anything looking for a meal, such as birds and predatory insects. Dozens of caterpillars may occupy one web, and trees may have several webs.
As the caterpillars grow, they encase more and more leaves within the web. The webs become unsightly as they fill with dead leaves, caterpillar droppings and molted caterpillar skins.
Fall webworm is not a picky eater. Cottonwood, walnut and ornamental fruit trees such as crabapple and herry are favorites, but its host range includes more than 100 trees and shrubs.
In September the webworms leave their silken shelter, drop to the ground and pupate in the soil or under leaf litter. The adult webworm, which is a hairy white moth about one inch long, emerges the following spring.
Fall webworms are sometimes confused with two other common tree pests: tent caterpillars and bagworms. Tent caterpillars construct their silken “tents” in the crotches of branches, rather than around leaves at the ends of the branch as fall webworm does. They also occur earlier in the year and are usually gone by the end of June.
Bagworms do not build large tents or webs, but instead each “worm” constructs an individual bag around its body. The bag is covered with bits of leaves, needles or bark—whatever the bagworm happens to be eating at the time. Like fall webworm, bagworms feed during the summer months, but the bags are more persistent, hanging on the tree through fall, winter and the following spring. Evergreen trees are commonly attacked by bagworm, while fall webworm prefers broadleaf trees.
You can begin scouting now for signs of developing webs. Remove the webs and caterpillars and dispose of them in the trash. Even just tearing open the webs will help, as it allows birds and other insect feeders to reach the caterpillars.
Many insecticides are labeled for fall webworm. The biological insecticide known as Bt is a good choice as it is very safe to use around people, pets and wildlife and doesn’t affect insects that feed on webworms. The whole tree does not need to be treated, just the leaves in and around the web. You can tear apart the web or work the sprayer wand inside the web where the insects are feeding. Treat while the caterpillars are still small for best control.
Source: Nebraska Forest Service