Fall Web Worms

Clint Perkins Gardening Corner     
Wood County Extension Agent

Autumn is the time of the year when we need to be on the lookout for the fall webworm. It attacks more than 88 different kinds of plants, including many fruit, nut, and ornamental trees and shrubs. It does not attack conifers, such as, pines and other needle-bearing trees.  The feeding preference of the fall webworms vary from one place to another. Oak, hickory, and pecans are the most often attacked.

The fall webworms are known for the large, unsightly webs that they produce. Heavy infestations are rarely fatal, but if they occur repeatedly over several years, they can stress the trees and make them more susceptible to drought, disease, or other insect pests.

The fall webworm moth is white and has a wing span of 1 to 1.5 inches. Sometimes there are small, dark spots on the forewings. Full-grown larvae are approximately 1 inch long, pale green to yellow, and covered with tufts of long, white and black hairs.

The fall webworm over-winters as pupae on the ground or on rough tree bark. The moths emerge from silken cocoons in the spring to disperse and mate. The female moths deposit hair-covered egg masses on the underside of the leaves of their food plants. An egg mass may be deposited in either a single or double layer and can contain up to 600 eggs. Each female moth will deposit only one egg mass. The egg masses of the walnut caterpillar, another common pest on pecans, are not covered with hairs.  Soon after the webworm eggs hatch, the larvae begin to build a silk web. As larvae consume leaves within the web, they expand the web to take in more foliage. All larvae within the web are the offspring of a single egg mass. Larvae will molt six to seven times before leaving the webbing to pupate. The life cycle from egg to adult requires approximately 50 days.

The fall webworm can often be controlled without insecticides by removing and destroying any leaves that contain egg masses. Larvae may be knocked out of low hanging webs with a stick or broom, and into a box or garbage bag for disposal. Webs can also be pruned from smaller branches.  Many beneficial insects attack the egg and larvae stages of the fall webworm. Tearing open the web will aide these beneficial predators and parasites in preying on the fall webworm.  If the webs are too numerous or too high in a tree to deal with individually, insecticides can be used to prevent damage. Hose-end sprayers or commercial high-pressure sprayers are best for reaching upper portions of tall trees. Because webworm larvae remain inside their
webbing, insecticide sprays must penetrate the web to be effective. For best control, apply insecticides
after eggs hatch and before larvae develop dense webs.

Insecticides containing acephate, Bacillus thuringiensis, carbaryl, or malathion are effective in controlling the fall webworm. Insecticides containing Bacillus thuringiensis are selective for caterpillars and do not harm beneficial insects: however, they must be applied when the caterpillars are small for effective control.  Not all insecticides are registered for fall webworm on all sites and commodities. It is up to the user to read the label to make sure the insecticide is cleared for the site and commodity.

Remember, always read and follow the product label directions.