Bagworms
Bagworms are moth caterpillars that build a portable bag of silk covered in pieces of the plant they are eating, and carry it with them as they feed. They are a serious pest of arborvitae, juniper and red cedar, Leyland cypress and pine across the eastern half of the US. Unlike most deciduous trees, evergreens can be killed outright by heavy defoliation. The bags are camouflaged and easy to miss until the damage shows in late summer. Every bag left on over winter can hold hundreds of eggs, so hand-picking bags in winter and spraying Btk on the newly hatched caterpillars in early summer are the two controls that work.
Thyridopteryx ephemeraeformis bags on arborvitae after feeding has finished. Photo: pending.
Identification
The bag is what you see. It is spindle- or cone-shaped and covered in bits of needle, leaf and debris, so it looks like part of the plant, or like a small cone. Newly hatched caterpillars make a bag only 1/8 inch long, and by late summer it is 1 to 2½ inches (sources give 1–2 inches, over 2 inches, and 1½–2½ inches). Mature bags hang by a tough silk band wrapped round the twig. In summer the caterpillar pushes its head and front legs out of the top to feed and pulls back in when disturbed. It is 1/8 to almost 2 inches long, with a dark, hardened head and a paler, softer, mottled body.
The adult male is a small, brown, furry moth. Ohio State describes it as bee-like with clear wings; NC State says the wings are dark at first and clear with age. The female never grows wings or leaves her bag. She mates through its opening and lays her eggs inside it, then dies. So a bag that stays on the plant over winter can hold the whole next generation.
Easily confused with
- Pine conesHarmlessBags can look like small cones, but they are silk cases covered in the host’s own needles or leaf bits, tied to the twig with a silk band, and in summer a caterpillar head pokes out of the top.
- Eastern tent caterpillarAlso a pestMany caterpillars share one silk tent in the fork of a branch in spring. A bagworm lives alone in its own spindle-shaped bag of leaf or needle bits.
Damage symptoms
The damage is easy to miss early in the season, while the caterpillars and bags are small, and often goes unnoticed until August. On deciduous trees the damage is usually less serious. On evergreens it is permanent: Ohio State warns that excessive defoliation of a conifer can kill the whole plant the following season. The silk bands can also girdle and kill twigs years after the bag has gone.
- Brown, thin patches on arborvitae, juniper or cypress
- Needles or leaves chewed away, leaving bare twigs and dead, open areas
- Spindle-shaped bags of dead needles hanging from twigs, from 1/8 inch in June to over 2 inches by autumn
- Bags that move, with a caterpillar head poking out of the top, in summer
- Old silk bands cutting into twigs, which later die back
Lifecycle & degree days
One generation a year. The eggs spend the winter in the female's bag and hatch in May or June (late May to early June in Ohio; sometime in May in central Maryland). The tiny caterpillars spin down on silk and are blown to nearby plants, which is how an infestation spreads along a hedge. They feed and enlarge their bags all summer, finish in August or early September, pupate, and in late summer the males fly to mate with the wingless females in their bags.
- Egg hatch
- May–June (May in central MD; late May–early June in OH)
- Feeding
- From hatch to mid-August or early September
- Eggs per bag
- 200–1,000 (sources vary)
- Generations / yr
- 1
- Overwinters
- Eggs inside the female's bag
Host crops
Conifers, especially arborvitae, red cedar and other junipers, Leyland cypress and pine, are the most often damaged, and the ones that can be killed. Bagworms also feed on many deciduous trees, including sycamore, maple, black locust, boxelder, linden, willow, oak and apple, but the damage there is usually less serious.
Monitoring & action thresholds
Extension sources give no count. Because each overwintering bag can hold 200 to 1,000 eggs, and because evergreens do not recover as deciduous trees do, the working rule on a small arborvitae or juniper is to deal with every bag you find. Light feeding on a large deciduous tree is cosmetic.
Walk evergreen hedges and specimens after leaf drop in autumn and through winter, looking for bags; they stand out better once the deciduous trees are bare. In June, look closely for tiny new bags at the branch tips. They are only 1/8 inch long and look like specks of debris, and this is the moment to spray.
Biological control
The most important natural enemy is an ichneumonid wasp, Itoplectis conquisitor, which attacks the bagworm's pre-pupae and pupae. Ohio State reports parasitism of 58% to 75.9%. In one study it was above 70% close to flower beds and below 40% farther away, because the wasps feed on nectar. The same authors suggest waiting about a month in autumn before stripping bags, so the wasps can emerge. Ohio State also warns that in urban plantings natural enemies are often not enough on their own. The control you can buy is Btk, and it works only on young caterpillars.
Cultural controls
Hand-picking is the single most useful thing you can do on a garden-sized evergreen, and winter is the time for it.
- Pick bags off from late autumn to early spring, before the eggs hatch in May
- Cut the silk band with a sharp knife rather than pulling the bag off; a band left on the twig can girdle it years later
- Destroy the bags or bin them rather than dropping them under the plant (NC State suggests an open paper bag, so any parasites can still escape)
- Don't strip bags in early autumn: Ohio State suggests waiting about another month from early October, so parasitic wasps inside can emerge
- Plant nectar flowers near evergreen hedges — parasitism was highest close to flower beds
Chemical, last resort
Sources
Ohio State Ohioline HYG-2149-10, Bagworm and Its Control (Shetlar, 2011)
NC State Extension, Bagworms in Ornamental Landscapes (Frank & Baker, reviewed 2026)
University of Maryland Extension, Bagworms on Trees and Shrubs (2024)
Clemson HGIC, IPM for Evergreen Bagworms on Conifers in South Carolina (Jeffers, 2026)
Ohio State BYGL, An Important Bagworm Parasitoid (Boggs & Young, 2025)
Bonide Thuricide concentrate label, EPA Reg. 4-226 (2016)
Monterey B.t. label, EPA Reg. 70051-106-54705 (2014)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Bagworms by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.