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 The Fall Webworm is Back

Round Rock Leader
Saturday, July 2, 2005
By Emsud Horozovic, City of Round Rock Forestry Manager

They are back and even hanging (literally) around Round Rock’s Main Street! Last year I got lots of calls from people asking, “What is that stuff on my trees!” That stuff is the webbing created by the pecan worm, or fall webworm.  I decided to write this article this year after noticing their uninvited presence on trees in our town.

In spite of its name, the fall webworm appears on pecans in summer as well as in the fall. My phone rings more during the summer because people are afraid they will lose their nice pecan trees. The fall webworms build larvae nests on the ends of branches. They appear in our area mostly on pecans, walnuts, mulberries, box elders, ash, and up to 90 other deciduous trees.

Damage is caused by larvae or caterpillars that eat leaves within the nest and enlarge the nest as they grow. Damage to the tree is seldom serious or deadly, but several severe infestations can defoliate and stress a tree. The grown up insect is a one inch snowy white moth, with dark spots on its wings. Larva is 1” long and covered with silky hair; color varies from pale yellow to green with a black stripe on the back and yellow stripe on each side. Usually the presence of larva is not acknowledged until large, silken white webs on pecans appear with skeletonized leaves and lots of caterpillars. Although the fall webworm is not considered a deadly pest it does gets lots complaints due to its ugly web that detracts from the esthetic value of the tree.

As for the biology of this insect; the moths emerge in the spring and after mating lay eggs on the undersides of the leaves. The moths lay a cluster of several hundred eggs. Eggs hatch in two weeks and larvae immediately begin to feed and make webs to protect themselves from predators. The larvae eat the tender portion of the leaf, often leaving the mid-rib and larger, tougher veins. As they feed and run out of leaves, they expand the webbing to enclose a fresh food supply, so that by the end of the season, large sections of the branch may be encased. There are at least two generations per year.

Your pecan tree is their preferred home, although they feed on almost a hundred other deciduous trees. Normally they do nothing more than build a home that few find visually appealing.  The reason you usually don’t need to worry about losing your tree is because the webworms are eating leaves relatively late in the growing season. Therefore the defoliation is much less costly to the plant than had it occurred earlier. So if you do not mind providing a home for the worms, do not worry about your trees health, they will be fine, and you can leave the nests be

Control of pests is rarely needed or even effective. Bad weather also takes a toll on their population. You could use stick or high water pressure to break their silk web nest and expose them to their natural predators: birds, yellow jackets, wasps, and egg parasites. Invite birds in your yard by placing birdbaths and feeders nearby. Also, you could prune infected branches and burn or other ways destroy them.

If using pesticides for the webworm, there are a few that are effective.  The best way to apply these chemicals is to spray them around the nest, on branches that the worms may spread to, and inside the nest after first breaking it with a stick.  There is an organic product called “B. T.” (Bacillus thuringiensis), sold under the product name of Biotrol, Dipel, Thurcide, and Javelin. To apply break up newly formed nests with a jet of water, then spray with B.T. in early evening during mid-summer when the caterpillar nests are small. For severe infestations, spray nests and leaves with Sevin, Diazanon, Orthene, Dursban, or Malathion, according to label direction. Always read labels carefully!  Natural control is preferred whenever possible.

So do not worry about these harmless critters, and enjoy the summer. I suggest tubing down the Guadalupe with some friends.

City of Round Rock | 221 East Main Street, Round Rock, Texas 78664 | Phone: (512) 218-5400
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