Bagworm Update May 2009

Cliff Sadof and Marc Rhainds, Department of Entomology, Purdue University

As the summer approaches, homeowners who had their trees defoliated by bagworms last year are getting prepared for this year’s round of plant destruction.  Bagworms are caterpillars that strip the leaves from a wide variety of trees and shrubs.  Most commonly they devastate dense plantings of evergreen spruces, pines and arborvitae used as visual screens or windbreaks.

Bagworms get their name from their unique habit of hiding from birds by gluing bits of leaf on to the silken bags in which they live. At this time of year, they are in the egg stage concealed in last year’s brown bags that are hanging from trees (Figure 1). In mid May in Evansville, early June in Indianapolis and mid June in Fort Wayne, the eggs will hatch into small caterpillars that will make new silken bags to be covered by freshly cut leaves (Figure 2).

For the past 10 years, the relatively mild winters have caused bagworm problems to increase in severity and move northward. People in these communities have been learning the importance of inspecting their plants in the month of June for these small bags, and spraying them with a foliar insecticide to kill the caterpillars before too much defoliation occurs.

Common insecticides to use include carbaryl (sevin), or spinosad (Fertilome Borer and Bagworm Killer). For a complete listing of pesticides to use see Purdue’s bulletin  http://extension.entm.purdue.edu/publications/E-27.pdf.  Currently, there are no soil applied pesticides registered for controlling this pest that are effective. Although research conducted in 2008 indicate there are some new pesticides can work when applied to the soil, they are not likely to be registered soon enough to help homeowners in the summer of 2009.

This year, some communities north of Lafayette, IN may be lucky enough to have had their bagworms freeze to death when they experienced one of the coldest winters in 10 years.  These people won’t have to hand pick or spray their young bagworms this June to protect their plants.

There are two ways to determine whether or not a bagworm infestation has survived. The easiest way is to wait until the eggs have hatched and inspect host plants for small bagworms feeding on leaves (Figure 2). The second way is to cut open the bags to reveal the body of the female (Figures 3). Then, break apart the female and examine the eggs. If the eggs are creamy white (Figure 4), the eggs are viable and will hatch into caterpillars that will feed on the overwintering host plants.  In this case treatment is neaded.  If eggs are brown (Figure 5) the eggs are dead and no bagworms will be expected to hatch and no treatment will be needed.

Click image to enlarge

Figure 1. Overwintering bagworm on arborvitae

Figure 2. Young bagworm covering itself with leaves

Figure 3. Cut bagworm to reveal overwintering female

Figure 4. Crushed female bagworm and live creamy white eggs

Figure 5. Dead eggs (brown) and live eggs (cream colored)