Worms: Bag or Web?

Posted: August 24, 2012

Is it a bagworm or a webworm? This summer, both of these native moths seem very prevalent. The one to worry about is the bagworm, which can cause serious damage to conifers; the fall webworm is more of a nuisance than harmful, causing little damage to trees.

Fall webworm creates large, dirty grayish silken webs enclosing foliage and the tips of branches at this time of year. These caterpillars feed on many different host plants; their preferred hosts include cherry, walnut, hickory, birch, and crabapple. A single tree may have numerous webs at the ends of branches, with hundreds of caterpillars inside each web. Although significant defoliation can occur, this late season feeding has little impact on the overall health of the tree.

These caterpillars, which have long white to yellowish hairs, feed on the leaves inside the web. As they run out of food, they expand the web down the branch to cover more foliage as needed, continuing to grow larger for about six weeks, until ready to pupate for overwintering on or in soil.

What to do about fall webworm? One option is to do nothing. But those dirty webs are unsightly, so you may want to scout for the nests while they are small, in mid to late July. Prune them out or tear them open, exposing the caterpillars to predators such as yellow jackets, wasps, and birds.

Do not burn the webs in the trees, as fire can seriously damage the tree’s bark. There are also registered insecticides which can be sprayed on the webs and nearby foliage, but it’s more effective to do so when the nests and caterpillars are still small. It is not necessary to “blow” the webs out of the trees with a heavy stream of pesticide spray.

Bagworms are much smaller and look almost like pinecones, so often go unnoticed. It is a much more significant pest that prefers conifers such as arborvitae and juniper. Its feeding, if left unchecked, can kill conifers within one or two growing seasons. Deciduous trees are less seriously affected.  As they feed on foliage, the caterpillars form 1 to 2-inch spindle-shaped silken bags, camouflaged with bits of leaves, bark, and debris, that hang from the branches like little pinecones. There can be hundreds of bagworms on one arborvitae.

By this time of year, the caterpillars are full-grown and prepare to pupate by securing the bag to a twig with a silken band. After four weeks pupation, male moths emerge to mate with females, who stay in their bags, lay hundreds of eggs, and then die. Early next summer, the eggs inside the bags hatch.

If you notice bagworms now, it is too late to spray them with any insecticide to control them. Pesticides will not penetrate the bag. The insects inside the bags are either no longer feeding, or will stop feeding if they detect pesticides on the foliage. The only control right now is to hand-pick and destroy as many bags as you can reach; be sure to cut off the silk band attaching the bag to the twig also. The best time for control with insecticides is mid to late June, when the caterpillars are newly-hatched; a registered product containing Bt is very effective on these small, vulnerable larvae.


By Annette MaCoy, Horticulture Educator