Bagworm

What it does

Each caterpillar builds a tough, portable bag of silk camouflaged with fragments of the plant it eats, carrying it everywhere and pulling inside when disturbed. It feeds through an opening at the top, enlarging the bag as it grows. On evergreens, feeding strips needles branch by branch; because evergreens regrow slowly, sustained feeding can kill limbs or whole plants.

Where it lives

Bagworms attack more than 120 plant species but are most damaging on arborvitae, juniper, cedar, and spruce, and are also common on deciduous trees like maple and honeylocust. They are found on landscape and foundation plantings, hedges, and ornamentals, mainly in the eastern, southern, and midwestern United States.

How it breeds

There is one generation a year. Eggs overwinter inside the mother's old bag and hatch in late spring. Tiny larvae 'balloon' on silk threads to new plants, then feed and enlarge their bags all summer. In fall males emerge as small moths and mate with the wingless, larva-like females, which lay 500-1,000 eggs inside their bag and die.

What it eats

Bagworm caterpillars eat the foliage of their host plants, chewing needles of evergreens such as arborvitae and juniper or the leaves of deciduous trees. The wingless adult females do not feed. The bag itself is made of silk and incorporated plant material, not eaten.

Identification

The obvious sign is the bag: a tough, 1.5 to 2.5 inch spindle- or cone-shaped case covered in bits of needles or leaves, hanging from branches and often mistaken for a cone. The caterpillar inside is dark and rarely seen. Adult males are small, clear-winged, sooty moths; females never leave the bag and look grub-like.

Signs of an infestation

• Spindle-shaped bags hanging from branches, resembling small cones
• Brown, chewed, or bare patches of foliage on evergreens
• Bags moving slowly as caterpillars feed and drag them
• Thinning, dieback, or dead branches on arborvitae and juniper
• Clusters of old bags left on branches through winter

Prevention tips

• Handpick and destroy bags in fall and winter to remove overwintering eggs
• Inspect susceptible evergreens each spring for new, small bags
• Prune out and dispose of heavily infested branches
• Avoid moving infested nursery stock or plants to clean areas
• Encourage birds and parasitic wasps that feed on bagworms

Professional solutions

For small plantings, picking off and destroying bags is often enough. When larvae are young in early summer, Bacillus thuringiensis (Bt) or other labeled insecticides can be sprayed, but timing is critical and older larvae in tough bags resist treatment. For tall trees or severe infestations threatening valuable evergreens, a certified arborist or licensed applicator can treat effectively and safely.