Fall Webworms by Nathan Riggs, Extension Agent - IPM Texas Cooperative Extension, Bexar County

As the warmer weather approaches, the fall webworm moth is just beginning its life cycle now as a caterpillar. Late May and June signal the beginning of the fall webworm season in Bexar County. Although the fall webworm (Hyphantria cunea) begins its annual cycle in late spring, it develops its largest population and causes the heaviest damage in the fall.

The fall webworm moth emerges from it's cocoon in late spring and deposits white, "cottony" egg masses on the undersides of leaves. Fall webworms in South Texas prefer pecan trees over most other trees but will munch on mulberry, hickory, oak, willow or redbud just as readily.

The caterpillar, or webworm, is pale yellow or greenish in color, and is covered with long white hairs. The webs are made by groups of caterpillars hatched from the same egg mass. In South Texas, webworms may produce up to three generations per year. Webworm caterpillars and moths are active at night when most of their predators (wasps, birds) are inactive. The caterpillars feed on leaf material and leave the stems and veins behind. Although these caterpillars are hairy, they do not sting; however, they can cause an irritating rash if their hairs come in contact with human skin.

Various mechanical and biological control methods are available. Sprays are probably the most-utilized strategy for webworm control, with conventional and biological ingredients available for homeowner use. The use of biological sprays in the urban sector is the most prudent approach for those persons having more confidence in an environmentally-friendly product. The most successful of these "natural" products contain toxins produced by the bacteria, Bacillus thuringiensis (Bt). The product is mixed with water, as specified on the container, and is sprayed on the leaf surface of affected plants. Bt is deadly to caterpillars of all kinds while causing no ill effects for birds, fish, pets, or people. Bt products must be eaten by the caterpillars to be effective (hence-spraying the leaves). Once the caterpillars digest the treated leaf, they stop feeding, and die within three to seven days. The younger the caterpillar, the better. Apply Bt products in the late afternoon, because the bacteria-based toxins are sensitive to the sun and will degrade in bright sunlight.

Another webworm control method does not utilize chemicals of any kind; rather it depends upon the homeowner to pierce holes in the webs. Tearing open the webs with a pole will allow wasps, hornets and birds access to enter and feed on the webworms. Wasps, hornets, and birds, if given the opportunity, will reduce webworm populations quite handily with just a minimum of assistance from you. Here in the San Antonio area, webworm nests are just beginning to appear, making this the optimum time to use Bt products and remove the webs.

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Updated: October 6, 2009