Have you noticed the large webbed sacs in the trees along the sides of the road lately? They do prompt questions to the Home & Garden Information Center’s Ask an Expert service. Often they are referred to as bagworms which causes some confusion because bagworms are a different insect.
So what are webworms and bagworms? Will they cause damage to trees? Let’s take a look at them both.
Fall webworm (Hyphantria cunea)
Fall webworms show up every year but their populations can vary in size. Often they are insignificant in number and not too noticeable, but every so often there is a large population. The sacs can be numerous and quite large, leading one to assume they will devour an entire tree’s worth of leaves.
But, thankfully, beneficial insects such as parasitoids and predators such as birds, who love to eat the caterpillars, keep populations in check and spraying is not necessary. Granted, the webs are not pleasing to look at. If they are within reach they can be pruned out and destroyed.
Fall webworm facts
- There are two generations each year. One is active in May and the second larger population is active starting in late summer into the fall. However, the sacs can be noticeable in the trees all winter.
- They feed on over 100 species of trees. Preferred hosts include walnut, oak, hickory, willow, apple, and other fruit trees.
- The adult is a ¾ inch-long moth, white or white with black spots.
- After eggs hatch on a host tree, the caterpillars produce a fine web over the ends of branches. They feed only within the web.
- Mature caterpillars are about an inch long with noticeable long, silky hairs. They come in two colors, the ones with black heads are yellowish-white and the red-headed ones are brown.
Refer to our webpage about fall webworm
Bagworms (Thyridopteryx ephemeraeformis)
By far bagworms are the more destructive of these two insects and need to be managed. They have voracious appetites and devour the needles of evergreens– particularly arborvitaes, junipers, Leyland cypresses, and cedars. We hear the cries of desperate residents wanting to know if the dead areas on their trees will regrow. Unfortunately, that answer is no.
Bagworm facts
- There is one generation per season.
- In addition to conifers, they will feed on deciduous trees (ones that shed their leaves in the fall) such as sycamore, maple, locust, boxelder, and linden, but the damage is not as significant.
- Eggs hatch in late May or early June. Caterpillars feed and create bags made from pieces of the plant they are feeding on. The bags enlarge as the caterpillars feed during the summer.
- The caterpillars pupate in late summer. Adult male moths emerge and fly to female bags that contain wingless female moths, then mating occurs. Females can lay 200-1,000 eggs which overwinter in the female bag.
Bagworm damage on Arborvitae. The brown portion will not recover. Photo: University of Maryland Extension 
Evergreen bagworm. Photo: Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org
Controlling bagworms
- Fall, winter, and spring: Remove and destroy bags containing overwintering eggs. Pinch them at the tip or use a small clipper to remove the ones you can reach from the tree. Do not drop them on the ground near the trees. Dispose of them in the trash.
- Beginning in late May into early June. Begin to look for small caterpillars moving on the trees. As they feed, small bags will form. The best time to spray is when the caterpillars have just emerged and are small. Bacillus thuringiensis (Bt), an organic insecticide, will help to control the young caterpillars.
- By mid-July the caterpillars will be too large for Bt to be effective. Look for a registered insecticide labeled for bagworms at a plant nursery or hardware store. Contact an arborist or landscaper to treat large trees or if the bags are located high up in the trees.
Young bagworms can be controlled with an organic insecticide, Bacillus thuringiensis (Bt). Photo: Chazz Hesselein, Alabama Cooperative Extension System, Bugwood.org
Other tent-making caterpillars
There are two more tent-making caterpillars that are sometimes mistakenly referred to as ‘bagworms’. They are eastern and forest tent caterpillars. Both are active in the spring and not in the summer or fall.
By Debra Ricigliano, Lead Horticulturalist, University of Maryland Extension Home and Garden Information Center