Garden Insects: Bagworms

Identification

Bagworms are small, dirt-brown colored caterpillars, which are sometimes difficult to see. They disguise themselves with parts of the shrubs and foliage they eat by forming a spindle-shaped bag that measures 1 1/2 to 2 1/2 inches in length. The bag is formed by gluing parts of leaves together with silk, and carried along with the caterpillar as it feeds. As the larvae grow, the bag grows with it and when disturbed, the larvae will hide inside.

In autumn, the bags reach their maximum size of 1-1/2 to 2 inches. At this time the larvae permanently suspend their bags downward from twigs, and transform into the pupa or resting stage before becoming an adult. The females stay in a larva-like stage, never emerging from the bag. The males transform into bee-like adults with clear wings and fur-covered bodies.

Favorite Plants

Bagworms feed on many conifers and deciduous trees. They are especially fond of arborvitae, juniper and cedar.

Damage

Female bagworm larvae cannot fly, so populations may build up on trees. When this occurs, crowded larvae may eat the buds, causing branch dieback and large open dead areas. Older larvae strip evergreens of their needles and consume whole leaves of susceptible deciduous species, leaving only the larger veins. Moderate amounts of defoliation can be unsightly, but excessive amounts may result in severe Damage (even death) to the tree the following season.

Mature larvae attach their bags to a branch by wrapping it with extra silk. Be sure to cut off this silk band when removing bags from a plants or it may girdle the branch as the tree grows.

Organic Methods of Control:

Cultural or Mechanical

• Hand picking the bags is the most effective method for small infestations. This can be done easily in the late fall when deciduous foliage has dropped, or when bits of plant material on the bags turn brown and can be easily located on evergreens.

Biological

• Spray Bt (Bacillus thuringiensis) on foliage for heavier infestations. Apply in June after all the eggs have hatched.