Bagworm on arborvitae and evergreens
Bagworms are one of those pests that are easy to overlook until the damage is severe -- the bags blend into arborvitae foliage so effectively that heavily infested plants can appear normal from a distance while dozens of caterpillars are stripping interior foliage. By the time the bags are obvious.
—- title: "Bagworm on arborvitae and evergreens" slug: bagworm-on-arborvitae hub: problems category: "Disease-by-host" description: "Bagworms camouflage as plant debris but kill arborvitae branches and entire plants. Identify the spindle-shaped silk bags, understand the window for Bt treatment, and remove bags by hand before eggs hatch in spring." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-
Bagworms are one of those pests that are easy to overlook until the damage is severe — the bags blend into arborvitae foliage so effectively that heavily infested plants can appear normal from a distance while dozens of caterpillars are stripping interior foliage. By the time the bags are obvious and visible, many of the caterpillars inside have already completed their feeding for the season. The difference between a plant that recovers and one that dies is often catching the infestation at the right time.
I don't grow arborvitae at my Long Island property, so this guide draws on Penn State Extension, Clemson HGIC, and NC State Extension research.
The pest
The bagworm (Thyridopteryx ephemeraeformis) is a moth in the family Psychidae. Per Penn State Extension, it infests a wide range of trees and shrubs:
- Preferred hosts: arborvitae (Thuja spp.), Eastern red cedar (Juniperus virginiana), junipers, spruce, pine
- Also infests: oak, maple, locust, sycamore, willow, rose, and many other deciduous trees and shrubs
The bags are constructed by the larvae from silk and plant debris specific to the host plant, making them effectively camouflaged. On arborvitae, the bags look like small, spindle-shaped bunches of arborvitae foliage.
Biology
Per Clemson HGIC:
- Overwintering: eggs overwinter inside the bag made by the female moth; each bag may contain 500–1,000 eggs
- Egg hatch: late May to mid-June in most of the mid-Atlantic and Northeast
- Larval development: caterpillars feed from June through August–September; bags enlarge as the caterpillar grows inside; bags are 1–2 inches at maturity
- Pupation: September; males emerge as adult moths; females remain in the bag, mate, lay eggs, and die inside the bag over winter
- One generation per year
Identification
The bags
Per Penn State Extension:
- Young bags (June–July): tiny (0.5 inch), spindle-shaped, pointed at both ends; may appear as small debris clusters at shoot tips
- Mature bags (August–September): 1–2 inches, spindle-shaped, covered with arborvitae foliage (or whatever host material the larva is using); bags are attached by silk to twigs
- Winter/spring empty bags: dull brown, still attached to twigs; gently squeeze or open the bag to check for eggs (small, pale blue-green eggs inside in winter indicate the pest will emerge next spring)
Damage to arborvitae
Per Clemson HGIC, bagworm feeding:
- Removes foliage from inside the plant outward; arborvitae foliage near bags turns brown and dies
- Heavy infestations over 2–3 years can kill arborvitae entirely — arborvitae does not reliably regenerate from dead, bare branches
- Brown interior foliage with visible bags at the brown-to-green interface is characteristic
Management
Hand-removal of bags in fall, winter, or spring
Per Penn State Extension, physically removing and destroying bags between September and late May eliminates next season's population. Each bag removed = up to 1,000 eggs removed. This is labor-intensive but chemical-free and highly effective for small to medium plantings.
Remove bags by hand (wear gloves — the silk is tough); place in a trash bag for disposal. Do not leave removed bags on the soil — eggs will hatch in the bag if conditions are suitable.
Per NC State Extension, this approach requires checking the plant thoroughly, including the interior branches where bags are most concentrated.
Bt at egg hatch — the critical window
Per Clemson HGIC, Bacillus thuringiensis (BT spray) var. kurstaki (Btk) spray is highly effective against newly hatched bagworm larvae in late May–June. Critical points:
- Apply when larvae are small (bags less than 0.5 inch) — young larvae are susceptible; larvae inside 1-inch or larger bags are protected from spray
- Apply to the entire plant with thorough coverage; larvae are mobile at this stage and must contact or ingest the Bt
- Reapply every 5–7 days for 2–3 applications during the egg-hatch window
Per Penn State Extension, the 2–3 week window after egg hatch is the only time Bt is effective against bagworms.
Spinosad
Per NC State Extension, spinosad is effective against young bagworm larvae and provides a longer residual period than Bt. Apply on the same timing — late May through June when larvae are small.
Synthetic insecticides against older larvae
Per Penn State Extension, for older larvae (bags 0.5–1 inch) that have not yet sealed themselves, bifenthrin, permethrin, or cyfluthrin can provide control with thorough application. These are more effective on older larvae than Bt or spinosad. Once larvae are sealed in mature bags (1+ inch), no insecticide provides reliable control.
Inspecting the following year
Per Clemson HGIC, inspect arborvitae and other susceptible evergreens in late May and early June each year. Finding bags in this early stage allows Bt application while larvae are still susceptible.
Common problems table
| Symptom | Likely cause | Action |
|---|---|---|
| Spindle-shaped debris clusters on arborvitae in June | Young bagworm bags | Apply Bt immediately; this is the treatment window |
| Brown interior foliage with 1–2 inch bags in August | Mature bagworms — past Bt window | Hand-remove bags; plan fall cleanup |
| Brown bags attached to twigs in winter | Overwintering egg masses | Remove and destroy each bag by April |
| Arborvitae branches brown and dead after September | Bagworm-caused branch death | Prune dead wood; assess plant health; remove remaining bags |
Frequently Asked Questions
Do arborvitae recover from bagworm damage?
Per Penn State Extension, arborvitae does not produce new growth from branches that have been fully stripped of foliage. Once a branch is dead from bagworm defoliation, it will not recover. Lightly damaged plants can recover by growing new foliage from surviving branch tips. Heavily defoliated plants (more than 50% branch die-off) often do not recover to acceptable appearance.
How do I know if the bags in winter still have eggs?
Per Clemson HGIC, open a bag at the wider end (the female's end, which is sealed with silk) and look inside. Live eggs are pale blue-green and fill the interior; if the bag contains only debris or a hollow cavity, the eggs have already hatched or the bag is from a male that emerged. When in doubt, remove all bags regardless.
Does bagworm spread to deciduous trees nearby?
Per NC State Extension, yes. Newly hatched larvae are blown by wind (a process called "ballooning") from infested to uninfested plants. They can travel considerable distances and infest deciduous trees, roses, and other hosts. Heavy bagworm populations on arborvitae can seed nearby plantings.
Is one season of bagworm treatment enough to eliminate the problem?
Per Penn State Extension, no. New larvae blow in from surrounding areas each spring. Annual hand-removal of overwintering bags and Bt application during egg hatch are ongoing practices, not one-time treatments.
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Sources
- Penn State Extension — Bagworm
- Clemson HGIC — Bagworms
- NC State Extension — Bagworm Management