Bagworms (Thyridopteryx ephemeraeformis, family Psychidae) are the larvae (caterpillars) of a native moth. The caterpillars develop within silk bags festooned with pieces of their host plants.
As the caterpillars mature, they begin weaving more host plant debris into their silk. This provides structural stability and highly effective camouflage, making them difficult to detect. This allows bagworms to wreak havoc before they are discovered.
Bagworms spend the winter as eggs inside the female’s bag. Research has shown that the eggs have a low temperature survival threshold. This explains why bagworms were historically rare above the 40th parallel north, which roughly bisects Ohio, crossing through the OSU Campus in Columbus. However, bagworms began creeping northward in the late 1990s, commonly appearing as far north as Ontario, Canada, possibly owing to “global worming.”
The low temperature winter survival threshold for bagworm eggs explains why populations crashed throughout much of Ohio after the polar vortex in 2014. Since that time, bagworms have gradually clawed their way back, with damaging populations observed in northern Ohio over the past few years.
Eggs typically hatch in late May to early June in southwest Ohio. However, the hatch is early this season. Kevin Griffin (Acme Tree and Landscape Service, Cincinnati, OH) sent pictures this past Tuesday taken in Harrison, OH, in western Hamilton County, of newly hatched bagworms.
It’s too early to assess this season’s bagworm populations. However, yesterday, I observed significant numbers on junipers (Juniperus spp.) in southern Butler County in southwest Ohio. The populations were highly localized; however, as Shakespeare wrote in Henry V, the game’s afoot!
Get Ahead of the Wave
Bagworm caterpillars are voracious feeders and can produce rapid defoliation as they develop through 7 instars before pupating. It’s important to detect the 1st instars that are hatching from the overwintered eggs to prevent significant defoliation.
First instars are very small, with their bags measuring around 1/8" in length. The bags are constructed with pieces of tan to reddish-brown sawdust-like frass (excrement) stuck to the outside of silk. They strongly resemble tiny "dunce caps."
A helpful strategy for detecting early-instar bagworms is to look for bags left from last summer. Both males and females attach their bags to their hosts with strong bands of silk at the end of the season, and the old bags remain throughout much of the next season.
However, don’t ignore other plants in the landscape. A percentage of the 1st instar caterpillars will produce a strand of silk to catch the wind and "balloon" the tiny caterpillars to establish new bagworm beachheads. This behavior is a key reason bagworms often appear out of the blue (literally!) on hosts far from trees and shrubs that were infested last season.
Also, don’t just focus inspections on evergreens. Bagworms may feed on over 125 species of evergreen and deciduous woody plants in 45 plant families.
A single bagworm female can produce 500 - 1000 eggs, meaning that populations can climb rapidly with just a few caterpillars arriving on the wind. The “founder effect” with bagworms often translates into hotspots. These can be seen both within infested plants and between infested plants.
Management
Insecticides are an effective option to prevent damage. A list of insecticides effective in suppressing bagworms and other pests is provided by Virginia Cooperative Extension’s “Horticulture & Forest Crops, 2026 Pest Management Guide (Publication 456-017)”:
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/456/456-017/ENTO-635.pdf
However, insecticide applications targeting bagworms can be problematic. Application timing and product residual activity must be considered. Also, there is a risk of killing non-target arthropods that help keep bagworms, as well as other pests such as mites and scale insects, in check.
Asynchronous Development
Although bagworm egg hatch has commenced, it doesn’t happen all at once. Overwintered eggs may hatch over an extended period, with eggs on the south side of an infested plant hatching earlier than those that are shaded on the north side.
It is common for a wide range of bagworm instars to be present at the same time, as illustrated in the image below. The bagworms were collected at the same time from the same shrub. This means that short-lived insecticides may need to be reapplied.
“Kill 'Em All” Can Exact a Price
Topically applied (sprayed) stomach poisons are highly effective and can deliver rapid results. However, it’s important to consider impacts on non-targets, including killing beneficial insects that help to keep bagworms in check.
A scientific paper published in 1976 showed that the parasitoid wasp, Itoplectis conquisitor (family Ichneumonidae) accounted for almost 76% of the parasitism observed on common bagworms in the study. This parasitoid wasp commonly visits flowers to sip nectar. It's a pollinator and a parasitoid.
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 supply parasitoid wasps with nectar by planting flowering plants.
Some groups of insecticides, such as pyrethroids (e.g., bifenthrin, cyfluthrin, permethrin, lambda- and gamma- cyhalothrin), can produce secondary pest outbreaks. Although pyrethroids can kill spider mites (Family Tetranychidae, Order Acari) and are commonly labeled as both a miticide and insecticide, they also kill predaceous mites. It’s rare for an insecticide/miticide to be 100% effective, so there will be survivors. However, spider mites have a higher reproductive potential compared to predatory mites. Consequently, the suppression of predatory mites allows spider mites to roar back and become a secondary pest problem.
Consider Using Biorational Insecticides
The risk presented by topical applications 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), chlorantraniliprole (e.g., Acelepryn), and Bacillus thuringiensis var. kurstaki (Btk) (e.g., Dipel, Thuricide, etc.).
The efficacy of these and other insecticides is subject to the general rule that the bigger they are, the harder to kill. In general, the smaller early-instar bagworm caterpillars are easier to kill compared to the larger later-instar caterpillars.
Thus, it is important to read and follow label directions regarding mixing rates and residual activity. Applying at a lower rate than recommended on the label may only kill small caterpillars. If the residual activity does not provide protection during the entire time eggs are hatching, multiple applications may be required. For example, Btk products have short residual activity, meaning that two or more applications may be required to cover the extended egg hatch.
A Final Note
Thomas DeHaas (OSU Extension, Erie County) posted a BYGL Alert on May 8 titled, Parking Lot Trees…. Killing them one Tree at a Time! He noted that confined roots, poorly drained soils, high heat, lack of irrigation, salt exposure, etc., shorten the lifespan of trees planted in parking tree islands.
I observed another threat to woody ornamentals in parking lot islands while taking pictures of 1st instar bagworms. The juniper pictured below suffered attempted vehicular herbicide. It was premeditated if the perp was applying an extreme form of bagworm control.
Selected References
Davis, D. R. (1964). Bagworm moths of the western hemisphere (Lepidoptera: Psychidae). United States National Museum, Bulletin 244.
Ellis, J. A., Walter, A. D., Tooker, J. F., Ginzel, M. D., Reagel, P. F., Lacey, E. S., ... & Hanks, L. M. (2005). Conservation biological control in urban landscapes: manipulating parasitoids of bagworm (Lepidoptera: Psychidae) with flowering forbs. Biological Control, 34(1), 99-107.
https://www.life.illinois.edu/hanks/pdfs/Ellis%20et%20al%202005.pdf
Moore, R. G., & Hanks, L. M. (2004). Aerial dispersal and host plant selection by neonate Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae). Ecological Entomology, 29(3), 327-335.
https://www.life.illinois.edu/hanks/pdfs/Moore%20and%20Hanks%202004.pdf
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.
https://go.osu.edu/bagwormeggs
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), 469-473.
https://doi.org/10.4039/Ent108469-5
