Controlling bagworms and tent caterpillars

By Steve Boehme

HCP columnist

Have you ever been surprised by large bare patches of dead foliage on your evergreens? If you look closely you might see pendants of dead foliage hanging from the dead twigs. Wait, they’re MOVING! You have a bagworm invasion.

Or maybe there are white “tents” made of webbing similar to spider webs, hanging in the branches of your ornamental trees. The web tents are crawling with caterpillars, and in a matter of weeks the leaves are stripped down to bare twigs.

Tent caterpillars are the culprit, and they particularly like fruit trees and ornamentals like crabapple and cherry. People often confuse tent caterpillars and bagworms, so let’s make sure we’re talking about the right one.

Tent caterpillars are easy to spot because they make large white nests in the forks and crotches of trees. They are particularly fond of wild cherry trees, but you’ll see them in other ornamental fruit trees like purple plum and crabapple, and in orchard trees. Their web-like “tents” protect the caterpillars from sprays and predators.

Bagworms are best known for attacking evergreens like arborvitae, pine, spruce and Eastern red cedar. They also eat barberry, blackberry, box elder, cherry, clematis, cotoneaster, elm, locust, maple, oak, peach, poplar, pyracantha, rose, sumac, sycamore and willow. If necessary, bagworms will even feed on clover, ragweed, parsley and nightshade.

Bagworms create camouflaged homes for themselves out of leaves and evergreen needles, which they carry around on their backs. These shelters get larger and larger as the worms grow, and they protect the worms from sprays and predators. Bagworms are tiny at first, and by the time you can see them they will be very hard to control.

Baby bagworms are about the size of the point on a sharpened pencil lead. They start feeding as soon as they hatch, and they quickly start growing and spinning silken bags studded with bits of dead foliage around their bodies for protection.

Spraying is most effective in early spring when they’re too small to see with the naked eye. The longer you wait after that, the bigger the bagworms and their damage will be, and, the more protection their bag will provide. By late summer, a mature bag actually repels pesticide sprays.?
There are many sprays and chemicals to control tent caterpillars and bagworms, and most of them work on both if your timing is right. Chemical controls include acephate, bifenthrin, carbaryl, cyfluthrin, cyhalothrin, malathion, or permethrin. Bonide’s “Rose Shield” is a great all-purpose pump spray that works on any knod of plant and kills most insect pests. All of these controls work by poisoning the plant leaves, so that bagworms or tent caterpillars are poisoned when they eat your plants.

Organic options include Bacillus thuringiensis (also called BT) and spinosad. Our favorite control for bagworms and tent caterpillars is “Captain Jack’s Deadbug Brew” from Bonide. This is a good all-purpose insecticide that works on many other garden pests, and it is an OMRI-approved organic remedy. We’ve used BT (Bacillus thuringiensis) successfully, too. Simply spray the entire plant thoroughly, wetting all the leaves, in early spring, and again when the leaves are fully open. Organic sprays work by ruining the bagworm or tent caterpillars’ appetite, causing them to starve.

Steve Boehme is a landscape designer/installer specializing in landscape “makeovers.” “Let’s Grow” is published weekly; column archives are online at www.goodseedfarm.com. For more information call GoodSeed Farm Landscapes at (937) 587-7021.