by: Karen Ripley, Forest Health Program Manager: Washington Dept. of Natural Resources
Autumn is the time to notice fall webworm Hyphantria cunea. Groups of these colonial caterpillars make sloppy silken nests that reach large sizes in the late summer. Larvae feed within the tent, so the tents are expanded to include more foliage as needed. Tents cover branches or even whole trees.
Native to North America, fall webworms feed on over 120 different deciduous trees and shrubs including birch, willow, cottonwood, alder and fruit trees.
The abundant maturing caterpillars within the nest have grey skin that’s thickly covered with yellow-orange hairs on black and orange bumps. The head is shiny black or red. They hatched from eggs laid on leaves by the adult moths back in late June or early July. By late September, most have completed feeding and have wandered away to pupate over the winter in a silken cocoon. Next spring, when the new leaves have emerged, so will the adult moths to mate, lay eggs and repeat the cycle. Adult moths have white wings, sometimes marked with small black spots, with a 1 to 1 ¾ inch wingspan. The moth’s yellowish body is covered with soft white hair and has two rows of dark spots on the back. The antennae are black and white.
Although the huge nests can be numerous and look impressive, this foliage loss does not permanently damage the plants. At the time fall webworm are consuming leaves, these deciduous hosts are usually drought stressed so they may slow photosynthesis and discard extra leaves to conserve water. The trees are also preparing to drop all the current year’s foliage at the end of the season. Having caterpillars eat the leaves at this time of year essentially means that leaf fragments and caterpillar droppings fall to the ground (pre-composted!), rather than the normal, intact leaves falling off a few weeks later.
High levels of parasitism generally keep the populations of fall webworm under control. Landowners should simply tolerate the damage.
When I see fall webworm, I continue to think about several things …
- The contrast between fall webworm and tent caterpillars. Tent caterpillars are active in early spring, just when the new leaves are emerging. They make a small tent shelter in a twig or branch crotch and leave it to feed in the open on exposed leaves. Tent caterpillars potentially do more damage because they eat the new leaves before a tree has recovered the energy it invested to grow them. Unless they are severely stressed by drought or other injury, most trees can endure defoliation by tent caterpillars without lasting effect. Fall webworm is even much less damaging than tent caterpillars.
- In recent years there has been an extensive outbreak of fall webworm feeding primarily on Pacific madrone in Southwestern Oregon. I’ve never seen fall webworm feeding on madrone in Washington. Why not madrone here? And, will there be an impact to the madrone that’s different than for other deciduous tree hosts? While a tree like alder is nearly ready to drop its leaves anyway, a madrone keeps its leaves through winter and won’t replace them until early the next summer. Does late summer defoliation hurt madrone more than it does other trees? I’m unsure about the degree of impact on the trees, but Entomologists in southern Oregon have not observed any lasting symptoms from fall webworm defoliation of madrone.
- Some years there are many, many tents of fall webworm conspicuously visible. Some years there are very few. This is likely strongly related to fluctuations in the numbers of specialized wasps and flies that parasitize the webworm caterpillars, especially if they kill young caterpillars, preventing the development of the large conspicuous nests. I think about the cycles of parasites and predators, lagging behind the populations of webworm. Some studies also indicate that the parasites are more strongly affected by cool spring temperatures and get out of synch with the most vulnerable webworm stages. Also, the use of general pesticides is often more damaging to specialized insects parasites and predators than to the target pests, another good reason not to spray fall webworm.