If It’s Fall, It’s the Fall Webworm

We have over a month till Halloween but we if look up into the branches of black walnut trees near me, you’d think we were decorating for Trick or Treaters. The fall webworms (Hyphantria cunea) are in the business of defoliating our walnut trees.

The caterpillar also affects pecan, American elm, hickory, fruit trees, some maples, persimmon and sweet gum in the eastern U.S. but it’s the black walnut trees in our area that are taking a beating.

As with all insects, fall webworm populations fluctuate greatly over the years. The normal population is spread out in most years and webs do not draw much attention. Occasionally, populations explode when conditions are favorable and webs become numerous. Then the webbing and defoliated trees certainly attract our attention.

Although the webbing is unsightly, it rarely is cause for alarm. Since it’s so late in the season, the trees have already stored enough energy to sustain themselves for the winter.

Caterpillars will crawl down the trunk looking for a hiding place to pupate. This homeowner has applied a sticky tape around the base of their mature walnut tree to trap the caterpillar.

As a child, I watched a neighbor take a long bamboo pole with an nail in the top, pierce the web and wrap up the entire web like cotton candy. It was then burned in a barrel, caterpillars inside. I have pruned the affected branches I could reach and disposed of the web. My mother would simply rip open the nest and watch birds have a free for all.  There are insecticides that can be sprayed into the trees to kill the caterpillars but keep in mind that the poisons kill other insects as it falls to the ground. I recommend letting the insect run its course since there is no real damage done to the trees. A preventive measure would be raking and disposing of leaves beneath the trees since a good percentage of the insects overwinter in the pupal stage in leaf litter.

Fall Webworm, Hyphantria cunea

Image by GregTheBusker via Flickr

Interesting enough, it our native fall webworm that has spread to other countries as an invasive. In China it has no natural enemies as it does in the U.S. where birds and wasps feed upon the caterpillars.  We can think of foreign insects or plants that become invasive in our own country but this native U.S. insect has a negative impact in other countries.

Ann Hohenberger, The Garden Club of Gloucester

3 thoughts on “If It’s Fall, It’s the Fall Webworm

  1. I am (like many I’m sure) experiencing the complete destruction of my trees by tent worms. I live in a very wooded neighborhood in the Princess Anne area of Virginia Beach called Hunt Club Forest. Unfortunately the “Forest” is under seige! Every day I go outside to find the tiny balled waste of these dastardly defecaters all over my deck (and everything else) as they eat their way through each and every tree on my small but very vegetated lot of land that I call home. I have had huge portions of my birch trees removed which contained giant balled webs in an effort to reduce their numbers. But I am still shy of actually removing EVERY SINGLE TREE (especially the Birches) that are infested almost entirely at this point. They are hardly recognizable as trees in some cases. More like something out of a horror movie or the result of an over zealous use of Halloween cobweb decoration. My understanding is that they live in the ground at some point during their life cycle. Would it be prudent (based on that fact) to perhaps spray Borax or something like it around on the ground of the trees that are infested in hopes of breaking the life cycle of these critters at any point? Many people have told me to “torch” the webs! This seems a bit extreme AND dangerous since many of my trees are very close to my home. I would be very happy to hear of any successful methods of curtailing these outbreaks and possibly avoiding future ones if there are any known. HELP!

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    • Linda, I just found your comment you left on Aug. 30 on the Garden Club of Virginia page so I moved it to this posting. You may know by now that what you have are fall webworms.

      Borax in high concentrations will kill a number of insects on the ground, including earthworms and other good guys. Borax is tricky. Too much can kill all plant life and too little can be ineffective. Keep it away from children and pets.

      Torching the webs is very dangerous for obvious reasons. Although people do this, I know some who have had fires get out of hand.

      I would recommend removing the leaf and plant litter beneath the trees and adding sticky tape around the trunk. There is a tree wrap kit called Tanglefoot that adds a very effective barrier on the trunk.

      One last recommendation would be to contact your cooperative extension agent if you plan to use poisons.

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