Fall Webworms Are Back!

Although it has been a light infestation in the walnut tree this summer, our native fall webworms began their damage in early July.  Small silken masses appeared at the terminal part of our walnut tree branches enclosing tree foliage, the larvae’s food source. As the larvae grew, so did the webs as the need for more food to support the growing insects.

Fall webworm - Hyphantria cunea (Drury)

The female fall webworm adult is a snowy white moth that emerges from pupation in leaf litter and beneath bark of trees and lays hundreds of eggs on the underside of leaves. Eggs hatch in a couple of weeks and larvae begin to spin their webs and feed and grow for several weeks.

Female fall webworm moth visits lights at night

Over a hundred species of trees in North America fall prey to fall webworms but in my yard, it’s always our lone walnut tree. Last year I raked the leaf litter beneath the tree where webworms pupate. I may have slowed the cycle but certainly didn’t stop it entirely.

In the north, there is only one infestation during the fall season but we can have two or more in Virginia. I can only hope we don’t have the heavy infestation we had last autumn where many trees were defoliated. Unsightly as it can be, fall webworms rarely threaten the life of a large healthy tree.

Insect pests in North America often originate in Europe and Asia but this is one pest that North America or Mexico accidently shared with the world. It is now a serious pest in all parts of the globe. Like insect populations everywhere, the population of fall webworms fluctuate a great deal over time. I believe conditions must be perfect for the fall webworm in Virginia at present for those unsightly nests are showing up more and more as I travel the roads and byways of Tidewater.

Ann Hohenberger, The Garden Club of Gloucester

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