Inertia is a property whereby objects in motion tend to stay in motion and objects at rest tend to stay at rest. One of the finest examples on inertia is the inexorable annual disappearance of evergreens down the gullets of bagworms, objects in motion, while homeowners, objects at rest, watch their evergreens slowly disappear. Populations of bagworms seem to be on the rise again in the Washington-Metropolitan region. Whether it’s favorable weather, a dearth of natural enemies, climate change, or just bad luck, bagworms have returned in force in some places like my neighborhood. The bagworm has been reported to eat more than 100 species of plants in our area. Bagworms are most damaging to evergreens such as juniper, spruce, arborvitae, pine, and Leyland cypress; however, they will consume a wide variety of deciduous trees and shrubs with equal gusto. A complete defoliation of a pine or spruce can kill the tree in a single season. Deciduous trees such as sycamore or maple are much more tolerant of this abuse. If you have bagworms on your plants, especially evergreens, you may be in deep trouble as you read this story. But if your conifers are not yet knocking on heaven’s door, this is a good time to save them for years to come.
The bagworm is so named for its curious habit of carrying about its refuge, a bag woven of silk and plant parts. If we go back in time to May, eggs completed their development inside overwintered bags dangling on trees. By early June, tiny caterpillars hatched and emerged from the bottom of the bag. Some stayed put on their natal plant while others ballooned on silken threads to other plants nearby. The caterpillars constructed and enlarged their bags as they grew. People sometimes confuse the bagworm's bag with a plant part such as a pinecone. This makes detection a problem. However, a closer look at the bag especially on a warm summer’s day often reveals the head and legs of the caterpillar as it moves about eating leaves or needles. By late summer, larvae complete their development and pupate within the bags. After a few weeks, the male bagworm, a black moth with clear wings, emerges from the bag and flies to find his mate. The female, a repulsive, wingless sack of eggs, releases a pheromone or sex attractant to lure the male to her bag. After attracting her mate, the female bagworm does not emerge to greet him. Instead, she remains in her bag hidden from her mate during the conjugal visit. Perhaps, this is Mother Nature's way to ensure that the male bagworm does catch a peek of his grotesque partner and flee. After mating, she lays several hundred to more than 1000 eggs in the bag, then, like a tragic Greek heroine, she emerges from the bag, falls to the earth, and dies leaving eggs behind to hatch next spring. If inertia prevented you from acting earlier on your bagworm issue, by now the damage may be done. However, it is not too late to nip next year’s incipient bagworm explosion
in the bud. Removing bags by hand, can be very effective in halting an outbreak, so put on a pair of gloves, grab a garbage bag, and start pickin’. As you remove the bagworms, be sure to collect them in a container or bag, seal it tightly, and dispose of them. Do not simply toss bagworms on the ground or in the driveway. Displaced bagworms will relentlessly return, crawl up your tree, and resume their feast. If you have large numbers bagworms, too many to pick by hand, and the blighters are still feeding, you may be able to control them with a thorough application of an insecticides such as biologically based Bacillus thuringiensis (Bt) if caterpillars are small. If caterpillars are larger, an insecticide containing the organically approved, reduced risk insecticide called spinosad works well. As we approach the middle and end August in the Washington-Baltimore area, bagworms will have completed their development and sprays on foliage will not kill pupae, adults, or eggs. Physical removal is your best alternative from late summer until May of next year. Sometimes natural enemies including birds, wasps, and voracious predatory bugs kill enough bagworms to quell an outbreak, but I would not count on it. Take stock of your bagworm situation, make a plan, and snap to it!