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 The Fall Webworm

Though an eyesore, fall webworm poses no real threat to trees
Round Rock Leader
Saturday, June 12, 2004
By Emsud Horozovic, City of Round Rock Forestry Manager

  They are back and even hanging (literally) on the Round Rock Main Street! “What is that stuff on my trees?”

Last year I got lots of calls on pecan worms so I decided to write this article this year after noticing their uninvited presence on trees in our town.

In spite the hint by its name fall webworm appears on pecans in summer as well as in fall. And my phone rings more in summer because people get frightened that they will lose their nice pecan trees. The fall webworms build larvae nests out on the end of branches. They appear in our area mostly on pecans, walnuts, mulberries, box elders, ash, and many 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. Grown up insect is 1” snowy white moth, with dark spots on wings. Larva is 1” long and covered with silky hair, color is yellow to green 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 fall webworm is not considered deadly pest it does gets lots complaints due to its ugly web that detract from esthetic value of the tree.

As for the biology of this insect; the moths emerge in the spring and after mating lay eggs on undersides of leaves. Eggs hatch in two weeks and larvae immediately begin to feed and make webs to protect themselves from predators. As they feed they enlarge the web and continue to feed for 4-8 weeks. There are at least two generations per year of this insect.

Your pecan tree is their home, and on their special menu although they feed on almost hundred other deciduous trees. Normally they do nothing more than build a home that few find appealing to the look of their yard. Their harm is merely cosmetic and will be gone within a few weeks. 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 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, 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 ones that are effective.  The best way to apply these chemicals are 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 Bt in early evening. For severe infestations, spray nests and leaves with Sevin, Diazanon, Orthene or Malathion, according to label direction.              

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

Forestry Manager Emsud Horozovic
Phone: 512-218-5540

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