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How to identify it
- The bag is spindle-shaped silk camouflaged with bits of foliage, bark and debris. Sources give different finished sizes: 1 1/2 to 2 1/2 inches (Ohio State), close to 2 inches by mid-August (Missouri), or 1 to 3 inches (30 to 75 mm; New Mexico State).[1][2][3]
- Bags hang from branches somewhat like Christmas ornaments and are firmly attached with a band of silk once the larva matures.[2][1]
- The caterpillar pokes its head and front legs out of the bag to feed and move, and pulls back in and holds the opening shut when disturbed. Ohio State describes it as brown or tan mottled with black; Missouri says the head and thorax are yellowish spotted with black and the rest of the body dark brown. Full-grown larvae are about 1 inch long.[1][2]
- Adult males are black, furry moths with nearly transparent wings about 1 inch across. Females never leave the bag; Missouri describes them as about 0.75 inch long, soft, yellowish-white and maggot-like, with no eyes, wings or legs.[2][1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Grass bagworm | A tiny relative that makes 1-inch bags with grass pieces and may attach them to fences or walls, but does no damage to turf.[1] |
| Fall webworm | Hairy caterpillars that feed in groups inside shared silk tents at the tips of branches, rather than each carrying its own bag.[4] |
Where it lives
Ohio State gives its range as the eastern United States from New England to Nebraska and south through Texas, and Missouri Extension calls it a native moth found throughout the eastern and southern states. A study across Indiana, Kentucky and Tennessee found female mating success falling to near zero toward a northern range limit around northern Indiana that has been stable for decades. GBIF holds about 10,700 U.S. records since 1990, the most from Texas, Virginia and Oklahoma, including records from Michigan and Wisconsin north of that line and from New Mexico; a few come from Canada and Mexico.[1][2][5][6][7][3]
Biology and life cycle
Eggs spend the winter inside the female's old bag and hatch from late May to mid-June. Counts differ: Ohio State gives 300 to 1,000 eggs in a bag and Missouri 500 to 1,000.[1][2]
Newly hatched larvae spin down on silk threads and are often carried by wind to nearby plants, where each starts a tiny new bag right away. Larvae also crawl off defoliated plants with their bags, so Missouri says infestations spread slowly by walking, though eggs in bags can also float down streams.[1][2]
Larvae feed and enlarge their bags through summer; New Mexico State says the larval stage can last up to four months depending on temperature. In mid-August they fasten the bags to a twig and pupate inside.[3][2][1]
Winged males emerge from about mid-September and seek out females, which call with a sex pheromone. Males live about a day; each mated female lays her eggs inside her bag and dies, giving one generation a year.[2][1][8]
Near the northern edge of the range, late emergence leaves many females unmated, part of what holds the range boundary in place.[5][8]
Damage and signs
- Larvae defoliate arborvitae, red cedar and other junipers most often. Missouri says the bagworm reportedly feeds on 128 plant species, including black locust, maple and sycamore.[2][1]
- Crowded larvae can eat the buds of conifers, causing branch dieback, and heavy defoliation may kill a whole evergreen the following season.[1]
- The silk band holding a bag to a branch can girdle the branch as it grows, killing it years later.[1]
- Because females can't fly, local populations build rapidly on preferred hosts. Ohio State says large populations rarely build up in forests, and infestations tend to occur in discrete patches on isolated plants or clusters of plants.[1][5]
Health risk
Bagworms are a landscape plant pest; the extension sources describe harm only to plants, through defoliation, branch dieback and girdling.[1][3]
Control
Traps and physical control
On small trees and shrubs, handpick the bags and destroy them. New Mexico State says the best time is late fall to early spring, before eggs hatch, and that removed bags should be burned, sealed in trash bags or drowned in soapy water; Ohio State notes bags are easiest to spot in late fall.[3][1][2]
When removing bags, cut the silk band that attaches each one so it doesn't girdle the branch later.[1]
Biological control
Bacillus thuringiensis (Bt) works only on young larvae. Ohio State advises applying at the end of June, after all eggs have hatched and ballooning ends; New Mexico State lists Bt var. kurstaki or spinosad for small larvae.[1][3]
Birds such as woodpeckers and sapsuckers, plus insect predators and parasitoids, control small to moderate infestations but seldom large ones. New Mexico State suggests flowering plants nearby to feed adult parasitoid wasps.[2][1][3]
Pesticides
For larger plants, spray foliage thoroughly as soon as young larvae emerge; Missouri says sprays lose effectiveness as larvae and bags grow. Ohio State says pyrethroids give quick knock-down of small to large larvae but warns that mature larvae may pupate early if they detect pesticide on the foliage, and it offers degree-day timing for sprays.[2][1]
Common questions
- When is the best time to pick off bagworm bags?
- Any time from late fall to early spring, before eggs hatch in late May or June; each bag removed then can hold hundreds of eggs.[3][1][2]
- Why didn't spraying kill my bagworms in August?
- By August the bags are nearly full size and Missouri says chemical control becomes less effective as larvae and bags grow; Ohio State adds that mature larvae may pupate early when they detect pesticide. Target young larvae in early summer instead.[2][1]
- Will bagworms kill my arborvitae?
- They can. Heavy defoliation of arborvitae, cedar or juniper may kill the whole plant the following season.[1]
Sources
- Ohio State University Extension, “HYG-2149: Bagworm and Its Control.” ohioline.osu.edu/factsheet/HYG-2149-10 (accessed 2026-11-11)
- University of Missouri Extension, “Bagworms in Missouri (G7250, reviewed August 2025).” extension.missouri.edu/publications/g7250 (accessed 2026-11-11)
- New Mexico State University Cooperative Extension Service, “H-174: Integrated Pest Management (IPM) Strategies for Common Insect Pests of Trees in New Mexico.” pubs.nmsu.edu/_h/H174/index.html (accessed 2026-11-11)
- University of California Statewide IPM Program, “Fall Webworm (Pest Notes).” ipm.ucanr.edu/home-and-landscape/fall-webworm/ (accessed 2026-11-11)
- PLoS One, via PubMed, “Broad-scale latitudinal variation in female reproductive success contributes to the maintenance of a geographic range boundary in bagworms (Rhainds and Fagan 2010; full text PMC2994749).” pubmed.ncbi.nlm.nih.gov/21152445/ (accessed 2026-11-11)
- Global Biodiversity Information Facility (GBIF), “Thyridopteryx ephemeraeformis: U.S. occurrence records since 1990 by state (GBIF occurrence search).” api.gbif.org/v1/occurrence/search?taxonKey=1853312&country=US&hasGeospatialIssue=false&occurrenceStatus=PRESENT&year=1990%2C2026&facet=stateProvince&facetLimit=400&limit=0 (accessed 2026-11-11). Exact count at access: 10,658 U.S. records since 1990 (Texas 1,278; Virginia 959; Oklahoma 924).
- Global Biodiversity Information Facility (GBIF), “Thyridopteryx ephemeraeformis: occurrence records since 1990 by country (GBIF occurrence search).” api.gbif.org/v1/occurrence/search?taxonKey=1853312&hasGeospatialIssue=false&occurrenceStatus=PRESENT&year=1990%2C2026&facet=country&limit=0 (accessed 2026-11-11). Exact counts at access: 10,762 records since 1990; United States 10,658, Canada 52, Mexico 31.
- Insects, via PubMed, “Bedtime story to my mother: virgin females seek love (Rhainds 2026).” pubmed.ncbi.nlm.nih.gov/41752549/ (accessed 2026-11-11)
- iNaturalist, “Thyridopteryx ephemeraeformis taxon page (order and family).” www.inaturalist.org/taxa/121594 (accessed 2026-10-06)