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Bagworm on arborviteaBagworms are pests on many kinds of conifers and deciduous trees, though they’re most frequently found on arborvitae and junipers. The dark brown bagworm caterpillars are 1/8 to 1/4 inch long when they first hatch, eventually reaching one inch long. As the insect feeds, it creates a silken case covered with the leaves made from the host plant, binding the bag together and attaching it to the plant with a silken thread. The bag will look different depending on the host plant. For example, the bag on a maple will look different from a bag on an arborvitae. Since bagworms add plant materials to the top of the bag, the freshest and greenest material is on the top of the bag. When you see a bag that is entirely brown, the insect inside has stopped feeding or is dead. The bags are ultimately 1 ½ to 2 ½ inches long. Once a plant is infested, populations can grow quickly.


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.


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.


Insecticide sprays are effective on young caterpillars, but older caterpillars are more difficult to control.  Contact the Plant Clinic (630-719-2424 or plantclinic@mortonarb.org) for current recommendations.  Use pesticides safely and wisely; read and follow label directions.

