It’s easy to see why small bagworms may be overlooked early in the season.
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 of inertia is the inexorable annual disappearance of evergreens down the gullets of bagworms. Bagworms are objects in motion. Sometimes, as I witnessed recently, property owners are too often objects at rest. Here’s why. Last week I stopped by my favorite coffee shop and as I sipped, I noticed a peculiar aberration in a lovely planting of arborvitaes lining the parking lot. Smack in the middle of the row of emerald beauties was one decidedly ugly brown rogue. Victim of too much mulch, too little water, or girdling plastic around the root ball, perhaps? Nah, a closer examination revealed hundreds of tiny bagworm caterpillars whose busy jaws were shredding foliage, transforming the evergreen to ever brown.
Hundreds of small bagworm caterpillars hatched in spring and now have shredded foliage on this pretty arborvitae.
Populations of bagworms seem to be on the rise again in the DMV. Whether it’s favorable weather, a dearth of natural enemies, climate change, or just bad luck, bagworms have returned in force to many places in our region. Bagworms have been reported to feed on more than 100 species of plants in our area. They 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. Complete defoliation of an evergreen such as pine, arborvitae, or spruce can kill the tree in a single season. Deciduous trees such as sycamore or maple are much more tolerant of this abuse, as they may be able to produce new leaves if defoliation has not occurred too late in the season. 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.
Oh no, what’s up with this brown arborvitae? Hundreds of tiny bagworm caterpillars, that’s what. Some are dining on foliage, others are adding silk to their bag from silk glands on their mouth, others are relocating their shelter, and some are just hiding from the camera. In another month or so, tiny caterpillars will be more than an inch long, performing aerial acrobatics as they move about. Left unchecked, this is what bagworms will do to an evergreen tree, sorrow and death may await these evergreens.
The bagworm is named for its curious habit of carrying about its refuge, a bag woven of silk and plant parts. Back in May, eggs of bagworms completed their development inside overwintered bags dangling on trees. 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. Caterpillars construct and enlarge their bags as they grow. 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, securely attach their bags to twigs with silken threads, and pupate within the bags.
By late summer, fully developed bagworms like this one can be easily removed from plants to reduce problems next year.
After a few weeks, the male bagworm, a black moth with clear wings, emerges from his bag and flies to find a mate. The female, a maggot-like wingless sack of eggs, releases a pheromone, a 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. After mating, she lays from several hundred to more than 1,000 eggs in the bag, and then, like a tragic Greek heroine, she dies, leaving her eggs behind to hatch next spring.
Grotesque adult female bagworms are wingless. No wings translate to more eggs and females can lay hundreds. Image: John Davidson, PhD
If bagworms are no longer actively feeding or moving, the damage is done and applying insect sprays is useless. So, don’t do that. However, if large bagworms are still active, 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 are relentless and they will return, crawl up your tree, and resume their feast. If offending bagworms have completed development in late summer or early autumn, you can dramatically reduce the population next year by removing the egg-laden bags now.
Unchecked, bagworms can completely defoliate evergreens, which may result in tree death.
Returning to the present, what can be done now to reduce the carnage? Large numbers of tiny bagworms may be too numerous to be picked by hand. If the caterpillars are small, you may be able to control them with a thorough application of an insecticide such as biologically based, Bacillus thuringiensis (Bt). Some formulations of Bt are listed by the Organic Material Research Institute (OMRI) as safe to use in the production of organic foods you purchase in the market. Bt works well on small caterpillars. If caterpillars are larger, an insecticide containing the OMRI listed, reduced risk active ingredient called spinosad works well to control bagworms.
Remember, as we approach the middle through the end of August in the DMV, 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!
Acknowledgements
Information for this Bug of the Week came from “Managing Insects and Mites on Woody Landscape Plants: An IPM Approach” by J. A. Davidson and M. J. Raupp. For more information on bagworms check out the following website: https://extension.umd.edu/resource/bagworms-trees-and-shrubs/
