Pests

Bagworms

Content Detail

Common name:  Bagworm

Scientific name:  Thyridopteryx ephemeraeformis

Hosts:  Bagworms can feed on many kinds of both evergreen and deciduous trees. They are most frequently found on arborvitae and junipers. A severe infestation may defoliate plants, which can kill branches or entire plants. Bagworm is found throughout much of the eastern half of the United States.

Young bagworm caterpillars feed on the upper surface of the leaves and may feed enough to create small holes in the leaf. Older caterpillars can consume entire needles or leaves. A healthy deciduous tree or shrub that has been defoliated may be able to produce a new flush of leaves and survive. However, a defoliated evergreen cannot push out an additional set of leaves and may decline or die.

There is one generation of bagworm per year. Bagworms overwinter as eggs in the bag of the mother. In northern Illinois, bagworm caterpillars hatch in early June in most years. The dark colored bagworm caterpillars are 1/8 to one ¼ inch long when they first hatch. They immediately begin feeding on host plants. They also produce long silk threads, which allows the wind to blow them to other plants. As the insect feeds, it creates a silken case covered with the leaves made from the host plant. They feed and construct their case for about three months. The appearance of the bag will vary a bit, depending on the host plant. For example, the bag on a maple will look different from a bag on an arborvitae. The bags are ultimately 1½ to 2½ inches long.

When the larvae are mature, they will be about 1 inch long.  At this time, they bind their bags to a branch with silk. In late summer, the insects pupate for seven to ten days. Females never develop wings or leave their bags. The adult males are 1 inch black moths with clear wings that fly to a female’s bag to mate. After mating, the female lays up to one thousand eggs within her bag and dies. The adult moths do not feed.

 

Cultural management:

On smaller trees and shrubs it is feasible to handpick the bags. Doing so from fall through spring eliminates the overwintering eggs. The bags should be destroyed or the eggs may hatch.

Biological management:

There are a number of natural enemies that attack bagworms. These include birds, mice and several species of parasitic wasps. In times of high populations, they do not provide effective management of the pest.

Chemical management:

Bacillus thuringiensis var. kurstaki (Btk), an insecticide derived from a bacterium, can be used on young larvae but is not as effective on older larvae. Thorough coverage of the plant is important, as Btk must be eaten to be effective. Other insecticide sprays are available as well.  They are also most effective on young caterpillars. Any of the insecticides will be most effective when the bags are less than half an inch long. Once the bag is closed and tied to the stem, insecticides will no longer be effective.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).