Young bagworms

By Jeff Burbrink
Extension Educator, Purdue Extension Elkhart County

Jeff Burbrink
Extension Educator, Purdue University Elkhart County

GOSHEN — There’s a little insect feeding on many trees in the county and most people are not seeing it, yet. Bagworms are a serious insect pest of many ornamental shrubs and trees in the eastern half of the United States. Conifers, especially arborvitae, cedar, juniper and pine, are the most frequently damaged host plants, but I’ve seen them on deciduous trees such as sycamore, maple, locust, boxelder, walnut, fruit trees and linden.

I found hundreds of bagworms this week feeding on arborvitae windbreaks on several Amish farms near Middlebury. They were only 1/8 inch long, but poised to do a lot of damage. 

Do not get this pest confused with the fall webworm, which many local people call bagworms. Fall webworms create large silky webs in trees in the fall of the year. True bagworm caterpillars make distinctive 1½- to 2-inch-long spindle-shaped bags that can be seen hanging from twigs of a variety of trees and shrubs. Sometimes the bags are mistaken for pine cones or other plant structures.

Female moths cannot fly, so in the fall, when bagworms mate, the male moth seeks out the female in her cocoon bag. The female dies after producing the eggs in her own cocoon. The bagworms pass the winter as eggs (300 to 1,000) inside bags that served as cocoons for last year’s females. The eggs hatch in mid- to late May and the tiny larvae crawl out to feed. Each uses silk and bits of plant material to make a small bag that protects and camouflages it as during feeding and growth. 

When the larvae leave the bag, some will release a silk thread and “balloon” the larvae through the air. Very small caterpillars can spin strands of silk and be carried by wind, an activity called “ballooning.”

Bagworm caterpillars feed for about six to eight weeks, enlarging the bag as they grow and withdrawing into it when disturbed. Older larvae strip evergreens of their needles and devour whole leaves of susceptible deciduous species leaving only the larger veins. When abundant, the caterpillars can defoliate plants. Heavy infestations over several consecutive years, especially when coupled with other stresses, can lead to plant death.

In early fall, the mature larvae attach their bags to twigs and transform into the pupa or resting stage before becoming an adult. Males emerge from their bags in early fall. They search for bags containing the immobile females. After mating, the female lays several hundred eggs, and dies. The eggs remain in the bag until they hatch the following May. 

If only a few small trees or shrubs are infested, handpicking and destroying attached bags may provide satisfactory control. This must be done effective during fall, winter or early spring before the eggs hatch.

Mature bagworm

The next two weeks are the time to carefully inspect susceptible landscape plants. Young bagworms are hard to see at first; look closely for the small, upright bags which have the appearance of tiny ice cream cones made of bits of plant material. 

When many small bagworms are present and feeding, an insecticide may be needed to prevent serious damage. The best time to apply an insecticide is while the larvae are still small (less than ½-inch long). Small larvae are more vulnerable to insecticides, and catching them early saves a lot of plant damage.

There are numerous insecticides labeled for use on bagworms. Check the label on the pesticide to be sure bagworms and the type of plant you wish to spray are listed. Carefully follow the label directions. Don’t be tempted to mix a stronger batch of product than the label says. You can only kill them once!

For larger plantings, it may be necessary to hire professionals to do the treatment. They often have equipment that can make short work off the job, and they will also have access to products that are not available to homeowners to purchase.

Interestingly, bagworms could not be found this far north until about 15 years ago. As our climate changes, they have successfully found a niche in northern Indiana.