Bagworm

What is bagworm?

Bagworm is a native moth whose caterpillar builds a bag out of silk and bits of the plant it is eating. The caterpillar lives in the bag and feeds partway out of it. Because the bag is made from the host plant, Purdue says people often mistake it for part of the tree and miss the damage until it is advanced.

Penn State says mature larvae are especially destructive to evergreens, and arborvitae and juniper are common hosts. Because the wingless females never leave their bags, the next generation hatches on the same plant, so one tree or shrub can be badly damaged in a single season (UMass). The University of Kentucky says browned areas on evergreens will likely never recover.

Bagworms are easiest to control at two times: in winter and early spring, when you can pick off bags full of eggs, and in June, right after hatch, when the caterpillars and bags are tiny.

Signs and damage

• Spindle-shaped bags of silk and bits of needles or leaves hanging from branches, often mistaken for part of the tree
• Brown spots and browning foliage from young larvae feeding
• Thin, stripped evergreens in July and August
• On broadleaf trees, leaves eaten down to the midvein

Bagworm identification: what to look for

• Adult: Male: sooty black moth with nearly clear wings, about a 1 inch wingspan, lives only a few days. Female: soft, yellow-white, wingless and legless; she never leaves her bag.
• Larva: Newly hatched larvae are tiny (about 2 mm), glossy black on top. Grown larvae are about 3/4 to 1 inch, dull dirty gray with darker blotches near the head.
• Eggs: 500 to 1,000 eggs per female, kept inside her bag over winter.
• Damage: Brown spots from young larvae, then needles and leaves eaten away. Bags 1 to 2 inches long hang from twigs.
• Timing: In PA, eggs hatch late May through early June; caterpillars feed eight to ten weeks, usually until August; males fly and mate in September and early October.

Life cycle

One generation a year. Eggs spend the winter inside the female's old bag. In late May through early June they hatch, and the tiny larvae crawl out the bottom of the bag and start building their own bags right away. They feed for eight to ten weeks, usually until August, then pupate inside the bag for about four weeks. In September and early October winged males fly to the wingless females, who lay their eggs inside the bag and die.

When to look: Pick bags from late fall through early spring, before eggs hatch. Watch for tiny new bags in early to mid-June; that is the best time to act. Damage is most visible in July and August.

How to control bagworm

Cultural control

• Hand-pick or clip off bags in late fall, winter or early spring before eggs hatch, and destroy them (drop in soapy water or seal in a plastic bag). The University of Kentucky warns not to drop live bagworms on the ground, because they will crawl back to feed.
• Check arborvitae, juniper and other evergreens in early June for tiny new bags.
• Check again two weeks after any treatment to see if live bagworms remain (Purdue).

Biological control

• At least 11 kinds of parasitic wasps attack bagworms; adding flowering plants nearby may boost them (UMass).
• Birds and white-footed mice eat bagworms.
• Penn State notes natural enemies drive the big swings in bagworm numbers from year to year.

Chemical control

Penn State says to treat from early to mid-June while larvae are small; later treatments work poorly. Purdue says Bacillus thuringiensis subsp. kurstaki (Bt) only works on bags under 1 inch long, and caterpillars must eat treated leaves to die. Other active ingredients named by Purdue and UMass include spinosad, chlorantraniliprole, tebufenozide, azadirachtin and indoxacarb (lower impact on beneficial insects), and faster-acting broad-spectrum options such as bifenthrin, carbaryl and permethrin. By late August or September the bags are too big for sprays to work. Use only products labeled for this use; the label is the law. For tall trees or big hedges, use an ISA Certified Arborist or licensed applicator. Muthler Landscaping does not spray or apply pesticides; see our pesticide applicator license guide.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Check evergreens in early June for tiny new bags; Penn State says infestations often aren't noticed until damage is well advanced.
• Remove and destroy bags each winter so they can't reinfest the landscape.

When to call a pro

Call an ISA Certified Arborist or licensed applicator if bagworms are on tall evergreens you can't reach, if a hedge or screen is badly thinned, or if you missed the early June window and damage is spreading.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about bagworm

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