Fall webworm

Fall webworm

If you are noticing those huge webs of wiggling worms on the tips of branches on trees in your area, don’t be alarmed. The fall webworm is a native seasonal pest that doesn’t significantly harm trees even though they look pretty ugly. In Michigan we begin seeing the “tents” of fall webworm in late August. Even after a hard freeze kills the worms inside, or they have become pupae, the nests may hang in the trees until winter winds dislodge them.

Fall webworms are often confused with their spring cousins, the Eastern Tent Caterpillar. Fall webworms enclose leaves at the end of a branch with their white, web like tent. They feed inside the tent and enlarge it when all the leaves inside are eaten. Eastern tent caterpillars make their tents in the crotches of branches and they leave the tent to feed on leaves, returning to the tent for protection from weather. Eastern tent caterpillars appear in late spring and early summer, fall webworms late in the summer.

The nests of fall webworms are usually on the outside branches of a tree, where the branch extends over an open sunny area such as a road or lawn. This makes them very visible to concerned gardeners. Each nest contains a colony of small caterpillars, busily feeding on tree leaves. The caterpillars are either red headed or black headed. Black headed webworms are greenish, with two rows of black bumps on the sides. Red headed webworms are tan with orange or red bumps. Both are covered in long white hairs.

The adult fall webworm is a small white moth, occasionally marked with a few black spots. She lays her eggs on the underside of leaves, where they hatch and begin feeding. The young feed for about 6 weeks then drop to the ground to pupate and over-winter. Occasionally in Michigan’s southern counties there is enough warm weather in fall for the first generation to turn into moths and create a second generation. Levels of the pest are higher in some years too, with heavier populations every 5-7 years.

Fall webworms prefer to feed on trees such as wild cherry, walnut, hickories and fruit trees but can feed on almost any tree. They seldom feed on willows and cottonwoods. When a tent or web is disturbed all the little caterpillars move in a peculiar synchronized jiggling movement. This may be their attempt to make a predator think something much larger is lurking within.

Controlling fall webworm

Since trees are near the end of their active cycle the loss of leaves from fall webworms doesn’t harm them much. If the nests offend you, you can use your garden hose to spray them out of the tree or use a stick to knock them down, and then smash the worms. Worms won’t crawl back up the tree when knocked to the ground. You can trim the tents out of the tree if doing so doesn’t harm the looks of the tree. Valuable ornamental trees can be treated with systemic pesticides early in summer. These go through the tree and kill the worms as they start to feed on leaves later in the year.

Pesticide sprays are not recommended as the collateral damage to the environment isn’t worth it, as trees are barely affected by the feeding of the fall webworm. Most pesticides do not effectively penetrate the webs; they would have to be torn open. Spraying foliage around the nests may kill the caterpillars when they enclose more leaves into the web. And burning the tents with a blowtorch is as dangerous to the tree and to you as it is to the worms.

Fall webworms have several natural enemies including yellow jackets and paper wasps. If you tolerate these insects on your property then you may have fewer fall webworms. Birds also like to eat the worms, especially if you tear the web for them.