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Bagworm caterpillars emerge in June and immediately begin feeding on host plants. They are easily blown to other plants. They feed and construct their case for about three months. When the larvae are mature, they fix their bags to a branch, binding it to the 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 1,000 eggs within her bag and dies. Bagworms overwinter as eggs in the bag of the mother. There is one generation per year.
SYMPTOMS & DAMAGE
Bagworms usually begin feeding at the top of the tree. When small, the caterpillars feed in the layers of the leaf tissue, creating light patches on leaves. As they age, they consume entire needles or leaves. A severe infestation may defoliate plants, which can kill branches or entire plants. A healthy deciduous tree or shrub that has been defoliated usually produces a new flush of leaves and survives. However, a defoliated evergreen cannot push out an additional set of leaves and may die.
Bags can be handpicked and destroyed from fall through spring, thus removing the eggs.
Bacillus thuringiensis var. kurstaki (Btk) 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. According to research, certain species of beneficial nematodes, such as Steinernema carpocapsae, attack bagworms. The nematodes are sprayed onto the bags, infecting and killing the female bagworms inside the bags. The nematodes must be applied before females lay eggs.
Insecticides are best applied two weeks after bagworms begin to hatch (generally near the fourth of July). This allows time for all the caterpillars to blow onto the plant before being controlled.
A second spray a week after the first may be necessary. Insecticide sprays are effective on young caterpillars, but older caterpillars are more difficult to control. Refer to University of Illinois Extension’s “Pest Management for the Home Landscape” for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions.
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.
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.