During this week’s BYGL Inservice, participants noted that Bagworms (Thyridopteryx ephemeraeformis, family Psychidae) throughout Ohio have reached a point in their seasonal development where they are becoming noticeable. This is prompting questions to Extensioneers on managing bagworms in landscapes and Christmas tree plantations.
Overwintered eggs hatched in June (see “Be Alert to Bagworms,” posted on June 12, 2025). However, it’s amazing how long these general defoliators can continue to crawl below our radar as they chomp on evergreens and deciduous trees and shrubs before their cumulative damage and bag size finally make them apparent.
Bagworm Damage on Arborvitae with Frass
Bagworm Biological Basics
Bagworms are the caterpillars of a native moth. The caterpillars are called “bagworms” because they wrap themselves in silk bags festooned with pieces of host plant material. The arrangement provides highly effective camouflage. Presumably, the look and physical nature of the bags help to protect the soft-bodied caterpillars from becoming predator meat morsels.
Late-Instar Bagworm Caterpillar Exposed
Camouflaged Bagworm - Feeding on Arborvitae
Bagworms may feed on over 125 species of evergreen and deciduous woody plants in 45 plant families. The official Entomological Society of America common name for the species is "Bagworm." However, numerous online references refer to the caterpillars as evergreen bagworm. We believe this is one reason bagworms commonly go undetected in Ohio landscapes. Landscape managers and homeowners may focus their inspections on evergreens rather than examining all possible bagworm hosts.
Bagworm on Sweetgum – Last Year’s Bag on Left and This Year’s Bag on Right
Bagworms will continue to chomp away on their hosts until late summer, when the caterpillars tie silk to a twig or other anchorage point and then close up shop by tightly closing their bags. Male bagworms pupate and emerge as unusual-looking moths. They fly to females and mate.
Bagworm Male with Its Pupal Skin
Female bagworms do not fly. Indeed, they never develop into an insect that even vaguely resembles a moth, and they never leave their bags. Males mate with the bagged females. Once mated, the female’s body fills with eggs, and they die.
Bagworm Male Matting with Female
Dead Bagworm Female Filled with Eggs
The bagworms spend the winter as eggs inside dead females in their bags. A single female can produce 500 - 1000 eggs, meaning that populations can climb rapidly. Just a few undetected females can spawn damaging numbers of caterpillars next season.
However, bagworms have a serious temperature-dependent Achilles’ heel. Research published in 2013 showed that the lethal temperatures for 50% of egg clusters ranged from 6.8F for clusters weighing 0.1 grams, and -0.58F for clusters weighing 0.4 grams. NOTE: I de-metricalized the temperatures published in the scientific paper.
This helps to explain the historical geographical range for bagworms in Ohio. Bagworms were once rarely found in Ohio north of the 40th parallel, which is roughly the east-west track for I-70. However, bagworms now range well into Michigan as well as Ontario, Canada. We believe it’s an example of “global worming.”
Bagworms have a habit of seeming to appear out of nowhere. That's partly because of their cryptic lifestyle, but it’s also linked to the way bagworms spread. Given that the females never fly, and the caterpillars can only crawl a short distance, bagworms have evolved another highly effective way to distribute themselves.
Shortly after the overwintered eggs hatch, first instars can generate a strand of silk from modified salivary glands to catch the wind and "balloon" to new locations. This dispersal behavior is one of the reasons bagworms may appear in landscapes on hosts that were not infested last season.
Coupled with the large number of eggs that can be produced by the females, it’s also the reason we commonly find “hot spots” in landscapes with a high concentration of bagworms. This is illustrated in the two images below. The first shows a hot spot where bagworms first arrived. The second image shows a heavily infested arborvitae just across the sidewalk from an arborvitae that appears free of bagworms.
Bagworm and Arborvitae – No Damage (Left), Heavy amage (Right)
Bagworm Management
Physical Removal: If the bagworms are within easy reach, they can be effectively controlled through physical removal. However, the caterpillars must be killed by squeezing or stomping on them once they are plucked from an infested plant. If they are simply dropped to the ground, the caterpillars will crawl back upon a host plant. There's a certain satisfaction with doing the "bagworm two-step" to dispatch the troublesome caterpillars.
Digitally Extracted Bagworms from Sweetgum
Step 1 in the Bagworm Two-Step
Sept 2 in the Bagworm Two-Step
Plucking and stomping overwintering bags is also effective in eliminating the eggs inside the dead females. Indeed, it’s always a good idea to give infested trees and shrubs a thorough close inspection during the winter months to find bags that made it through the summer caterpillar dance.
Insecticides: Insecticides are an effective option; however, their use can be problematic. As shown in the image below, caterpillar development may be asynchronous, perhaps owing to whether the eggs were located on the north or south-facing side of the plant host. All of the bags were collected from the same host on the same day.
Asynchronous Bagworm Development
The efficacy of insecticides is subject to the general rule that the bigger they are, the harder they are to kill. Thus, early instar bagworm caterpillars are most susceptible to insecticide applications. Late instars may remain unaffected or survive intoxication to pupate early. Although the early pupating females produce fewer eggs, they will continue the infestation next season.
Another challenge is presented by the way the bagworm caterpillars die. They typically do not drop from their host plant because they use a few strands of silk to tie themselves to their host as they feed. You need to give them a squeeze to determine if they are dead or not.
Bagworm Tied-Off While Feeding
This Bagworm Was Not Killed But it’s now Dead
Insecticides also present a risk of killing non-target arthropods that help keep bagworms, as well as other pests such as mites and scale insects, in check. The risk can be reduced by selecting “biorational” insecticides that have a narrow target, preserving beneficial bio-allies.
Active ingredients that are considered biorational include, but are not limited to, azadirachtin (e.g., Azatrol), spinosad (e.g., Converse), and chlorantraniliprol (e.g., Acelepryn). Although Bacillus thuringiensis var. kurstaki (Btk) (e.g., Dipel, Thuricide, etc.) is also a biorational, it’s probably too late for this naturally occurring bacterium to provide relief since it is most effective on early instar caterpillars.
Connect-the-Dots: Long-Term Control with the Power of Pollinators
This management strategy has been included in just about every BYGL Alert on bagworms for the past several years because it connects pollinator gardens with a reduction in plant pests. So, how does the abundance of flowering plants translate into fewer plant pests?
It’s well documented that a wide range of enemies of insect pests are fueled by nectar. In other words, an effective long-term bagworm pest management strategy is to simply plant flowering plants that provide nectar.
Bagworms are commonly targeted by an array of predators, parasitoids, and pathogens (the 3-Ps). It’s not uncommon to find bagworm bags ripped open by baldfaced hornets (Dolichovespula maculata) to extract the caterpillar meat morsels inside. Of course, these wasps are commonly observed visiting flowers for a nectar-charge, meaning they are also pollinators.
Bag Ripped Open by a Baldfaced Hornet
Balefaced Hornet (Dolichovespula maculata)
A scientific paper published in 1976 showed that the parasitoid wasp, Itoplectis conquisitor (family Ichneumonidae) accounted for almost 76% of the parasitism observed on bagworms in the study. As with many enemies of other insects, this parasitoid wasp commonly visits flowers; it’s a pollinator.
Common Bagworm Parasitoid Wasp (Itoplectis Conquisitor) - 1
Common Bagworm Parasitoid Wasp (Itoplectis Conquisitor) - 2
A study published in 2005 showed parasitism rates of I. conquisitor exceeded 70% in bagworm-infested plants that were next to a central flower bed, but less than 40% in infested plants with flower beds further away. In other words, an effective insecticide-free long-term bagworm management strategy is to simply plant flowering plants that attract pollinators including parasitoid wasps.
A Pollinator and Pest Management Garden
Selected References:
Cox, D., and D. Potter. 1986. Aerial dispersal behavior of larval bagworms, Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae). The Canadian Entomologist, 118(6), 525-536. doi:10.4039/Ent118525-6
Ellis, J.A., Walter, A.D., Tooker, J.F., Ginzel, M.D., Reagel, P.F., Lacey, E.S., Bennett, A.B., Grossman, E.M. and Hanks, L.M., 2005. Conservation biological control in urban landscapes: manipulating parasitoids of bagworm (Lepidoptera: Psychidae) with flowering forbs. Biological Control, 34(1), pp.99-107.
https://doi.org/10.1016/j.biocontrol.2005.03.020
Kaufmann, T. 1968. Observations on the Biology and Behavior of the Evergreen Bagworm Moth, Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae), Annals of the Ent. Soc. of America, Volume 61, Issue 1, Pages 38–44.
https://doi.org/10.1093/aesa/61.1.38
Moore, R.G. and Hanks, L.M., 2004. Aerial dispersal and host plant selection by neonate Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae). Ecological Entomology, 29(3), pp.327-335.
https://doi.org/10.1111/j.0307-6946.2004.00611.x
Rhainds, M., Régniere, J., Lynch, H.J. and Fagan, W.F., 2013. Overwintering survival of bagworms, Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae): influence of temperature and egg cluster weight. The Canadian Entomologist, 145(1), pp.77-81.
Sheppard, R.F. and Stairs, G.R., 1976. Factor Affecting the Survival of Larval and Pupal Stages of the Bagworm, Thyridopteryx Ephemeraeformis (Lepidoptera: Psychidae). The Canadian Entomologist, 108(5), pp.469-473.
https://doi.org/10.4039/Ent108469-5