The rise and fall of the fall webworm

It’s August and time for the annual visit of this generally harmless showstopper known as the fall webworm.

Fall webworm. Photo by G. Keith Douce, University of Georgia,
Fall webworm. Photo by G. Keith Douce, University of Georgia,

As the end of summer looms, one of the most noticeable insects hits its stride. These insects are not as eye-catching as the home that they build for themselves. The fall webworm, or Hyphantria cunea, will be webbing the ends of branches near you soon, if it has not already. This insect could easily be grouped in the “much ado about nothing” category. A big, showy bag of webbing appears over the end of a branch on a shade, fruit or ornamental tree. Inside, the hungry fall webworms are devouring all the leaves. It is highly visible and leaves are being consumed. This has frightened tree-owners around the state to try risky and sometimes dangerous methods to get rid of the webbing. Michigan State University Extension Master Gardener hotlines and educators are receiving calls asking about how dangerous this pest might be.

Branches have been set on fire or cut off, and pesticides have been sprayed that have rained down on the sprayer to stop this perceived pest threat. This is a case of the human doing much more damage to the tree than the fall webworms ever could. This native insect has more than 50 natural predators and 36 parasites that help control it. Best of all, fall webworms do not eat the buds of next year’s leaves. They are feeding on leaves that are nearing the end of their photosynthesis careers and only have a bit more to give. Next year, the leaves will appear on the currently affected branches with no sign of last year’s damage.

Fall webworms have a wide variety of trees for potential food. Over 90 species of deciduous trees make the menu with fruit and nut trees, like walnuts and elms, and some maples as regular targets. It is unusual to see fall webworms in the same tree year after year and especially on the same branch. They are “one and done” feeders.

Eggs are laid by the female moth on the bottoms of leaves about a month or so before the larvae or caterpillars hatch. The caterpillars construct a web over the end of the branch, enclosing leaves. They feed inside the web, enlarging it as they feed. Their lives as larvae are usually about six weeks, but long after they have left, the webs remain. If the web is white, it is new. If it is tan or brown, there are no larvae there. Webs can last into the winter before falling out of the tree during a wet snow or a windstorm.

On large trees, fall webworms are just as annoying, but not a reason to do anything. If a small tree is engulfed, that’s another matter. If fall webworms are on a small tree and you choose to remove it, the easiest way could be called “10-year- old boy biological control.” Push a stick into the webbing and pull everything and everybody out of the tree and into a bucket of soapy water to soak for the day. Or the webby mass can be burned or buried. It is possible to just do the 10-year-old boy happy dance on top of them.

Since fall webworms construct such good webbing, pesticides that are sprayed are worthless and just repel off. It would be necessary to tear a hole in the webbing to access the inside. If you are close enough for this, the trusty stick is a safer solution. Smart gardeners always use the path of least pesticides used, especially if it is not necessary.

Consider fall webworms as a showy display to herald summer’s passing. A big, white filmy bag on the end of branches should not bother the smart gardener.

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