Bagworms Identification Guide

Bagworm Identification Guide

Physical Characteristics

Bagworms are moth larvae in the family Psychidae. You rarely see the caterpillars, but their protective cases are easy to spot. These bags look like small spindles, ranging from a quarter inch when new to over two inches when full grown. The bags are made of silk mixed with bits of leaves, twigs, and bark from the host plant. This gives them excellent camouflage.

The larvae inside are grayish caterpillars with dark brown to black markings on the head and first three body segments. Full grown larvae measure about 1 inch in length. Newly hatched larvae are tiny at about 2 mm long with glossy black backs and amber undersides.

Adult male moths are hairy, black insects with clear wings about 1 inch across. Female bagworms never develop into typical moths. They stay worm-like their whole lives without wings, working legs, eyes, or mouthparts. Females never leave their cases.

Common Species

The Evergreen Bagworm (Thyridopteryx ephemeraeformis) is also called the Eastern Bagworm or Common Bagworm. This is the main species that damages landscapes across eastern North America. Most homeowners who find bagworms on their trees and shrubs are dealing with this species. Other bagworm species exist in North America, but T. ephemeraeformis causes the most damage and is the most common.

Bagworm Behavior and Biology

Life Cycle

Bagworms complete one generation per year. They spend winter as eggs inside the female’s bag. Each female can lay 500 to 1,000 eggs before dying in the fall. The eggs stay protected inside the old bag through winter.

In late May to early June, tiny larvae hatch and start a behavior called “ballooning.” The young caterpillars spin silk threads that catch the wind. The wind carries them to new host plants. This explains why bagworm infestations can show up suddenly on plants that were clear before.

Once on a host plant, larvae start building their protective bags right away. As the caterpillars feed and grow through four or more molts, they keep making their cases bigger. By late summer, full grown larvae pupate inside their bags.

Adult males come out in late summer or early fall. They fly around looking for females, which release scents to attract mates. After mating through an opening at the bottom of the female’s bag, the female lays her eggs inside her pupal case and dies. Neither adult eats, and males live only one to two days.

Feeding Habits and Host Plants

Bagworm caterpillars are heavy feeders. They strip the leaves from over 50 types of evergreen and deciduous plants. Their favorite hosts include:

• Evergreens: Arborvitae, juniper, red cedar, spruce, pine, and cypress
• Deciduous trees: Maple, elm, willow, oak, sycamore, and apple

Feeding damage usually starts at the top of plants and moves down. On evergreens, this damage is extra harmful because these plants cannot grow new leaves as fast as deciduous trees. Even light infestations can make ornamental plants look bad. Severe defoliation often kills branches or entire plants.

Why Infestations Persist

Bagworm populations often stay in the same area over multiple seasons. Female bagworms cannot fly. Larvae only travel short distances on the wind. So heavy infestations tend to build up near where the previous generation lived. Without treatment, populations grow each season. Hundreds or thousands of eggs hatch from each surviving female’s bag.

Treatment Methods for Bagworms

Good bagworm control requires proper timing and a mix of methods. The following sections describe control strategies that work well against bagworms.

Manual Removal

For small infestations, picking bagworm cases by hand during late fall through early spring works well. This removes eggs before they can hatch and breaks the cycle. Put collected bags in soapy water or sealed bags. Do not just drop them on the ground where larvae can still come out.

This method works best when:

• Infestations are caught early
• Plants are small enough to check fully
• Bags are removed before late May when eggs hatch

Chemical Control Timing

The best window for insecticide sprays is late May through mid-June when larvae are small. Young bagworms are easy to kill with various insecticides. But once the protective bags grow larger, treatments work much less well. By mid to late summer, the bags block most contact insecticides.

Products with Bacillus thuringiensis var. kurstaki (Bt) work well when applied to small, actively feeding larvae. Synthetic pyrethroids and other longer lasting insecticides also provide control during the early larval stage.

Other Control Options

Several natural enemies attack bagworms:

• At least 11 species of parasitic wasps target bagworm larvae
• White-footed mice and various birds feed on bagworms
• Certain nematodes can kill bagworms

These factors cause bagworm numbers to change from season to season. But when populations are already high, they usually cannot stop the damage on their own.

Prevention Strategies

• Regular inspection: Check plants prone to damage, especially evergreens, in early summer for small bags
• Prompt removal: Remove any bags found during winter pruning
• Monitor at-risk plants: Arborvitae and juniper get hit hard by bagworms
• Act early: Small populations are much easier to handle than large infestations

References

• Bagworm - University of Florida IFAS Extension
• Bagworm - Penn State Extension
• Bagworm - Texas A&M AgriLife Extension
• Evergreen Bagworm Moth - Butterflies and Moths of North America
• Bagworm Forecast - USA National Phenology Network