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.
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.
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