Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
9/9/02
Contacts:
Mark Shour, Extension Entomology, (515) 294-5963, mshour@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning Sept. 13, 2002
Is It Really a Bagworm?
By Mark Shour
Extension Program Specialist
Iowa State University Extension
Common names for plants and animals can be useful tools to help us communicate about familiar organisms. However, in describing insect pests, common names are sometimes confusing and simplistic, especially when one term refers to several different insects.
For example, Iowa has many moth caterpillars that spin silken nests that serve as a place to feed and as a refuge from predators. The common name "bagworm" has been uniformly (and incorrectly) applied to many of these caterpillars. The name has been used to describe the eastern tent caterpillar (Malacosoma americanum) that forms silken tents in the forks of branches of wild cherry, crabapple, and fruit trees early in the growing season. On honeylocust, the mimosa webworm (Homadaula anisocentra) knits together leaflets with silk and can web together several leaves on a branch, thus being called by some the "bagworm." This term is also commonly attached to fall webworm (Hyphantria cunea), which produces large, unsightly silken webs that enclose entire branches (or trees) of walnut, crabapple and fruit trees during the summer and fall in Iowa.
The true bagworm has the scientific name Thyridopteryx ephemeraeformis. It does not produce unsightly nests or webbing, but the bagworm can be an important pest in parks, windbreaks and residential landscapes.
Bagworms devour the foliage of several species of fruit, shade and ornamental trees, as well as evergreen shrubs. Some common hosts include red cedar, arborvitae, juniper, white pine, and crabapple. Feeding can be severe enough to cause complete host defoliation, resulting in weakened and unsightly deciduous plants or dead evergreen hosts.
Bagworms form characteristic spindle-shaped cases made of tough silk and small pieces of host foliage or bark. Some people have mistaken them as reproductive cones of an evergreen plant. In June, newly emerged larvae form this bag around their bodies as soon as they start feeding. The bag is enlarged as the caterpillar grows. The top of the case is open and the caterpillar head and several body segments protrude when the larva is moving, feeding or enlarging the case. The bottom end of the bag has a small opening to eliminate waste. The caterpillars carry this case with them like a turtle carries its shell. When threatened by a predator (e.g., bird), the caterpillar retreats inside the bag, pulls silken threads with its mouthparts, thus closing the opening to the case (similar to a drawstring purse), and then holds these threads until the predator gives up and drops the bag. (This behavior can account for some dispersal of this pest.)
Once the caterpillar has matured (late August or early September), it throws several silk lines around a host twig. With the case secure, the caterpillar undergoes transformation into the pupal stage. The adult male bagworm is a small moth with a black, hairy body and clear, membranous wings (no scales on the wings as you normally find on a moth). In September, the males emerge from their case and fly to females by following chemical signals (pheromones) produced by adult female bagworms. Unlike the male, adult females are vermiform (worm-like) and resemble queen termites without legs or eyes. These females remain in their protective bag and mate through the bottom opening of the case. After mating, the female deposits eggs inside her old pupal skin, alternating eggs with layers of hair from her body; this technique effectively insulates the eggs. A given female deposits 800-1000 eggs and then dies. This insect overwinters as eggs protected in the tough, silken bag.
Fortunately, there is only one generation of bagworms per year in Iowa. This pest is most often reported from counties adjacent to or south of Interstate 80, but bagworms can be found throughout Iowa. Extremely cold winters and limited natural dispersal account for this geographical limitation. Infested nursery stock is cited as being the primary long-distance movement of this pest throughout Iowa.
Control efforts include handpicking, natural enemies and insecticide sprays. If you are choosing the handpicking option, remove all bagworm cases from host plants during the winter, including the silken bands around the twig to prevent disfiguration by girdling. Be thorough because one missed bagworm case could mean hundreds of emerging caterpillars the next spring.
Several species of small wasp parasitoids use bagworms as a food source for their developing larvae. These wasps provide a moderate-to-high level of natural control of this pest but do not harm any other insects or humans. After handpicking, hold the bagworms in a fine mesh screen cage outside your home until the parasitic wasps to emerge next spring and seek out new bagworm hosts in the area.
Insecticide sprays (Bacillus thuringiensis, Malathion and Sevin) are only effective against small larvae. The best time to spray is when caterpillars have just begun feeding (mid-June). Retreatment may be required within 14 days if host foliage continues to disappear after the first insecticide treatment.
-30-
ml: isugarden
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.