Small and large holes in the leaves of trees, caterpillars hanging from trees on silken threads, caterpillars falling on clothing while mowing under trees, caterpillars crawling on picnic tables and decks and other nuisance behaviors all indicate the activity of cankerworms. Homeowners have been reporting an unusually large incidence of cankerworm activity this year. In the June 1, 2007, edition of Yard and Garden News, Jeff Hahn, University of Minnesota entomologist, wrote an article in which he lists cankerworms as one of “Caterpillars Gone Wild”. Following are some facts and observations about cankerworms taken from Hahn’s article. Mature spring cankerworm larva A type of inchworm, these caterpillars are smooth, slender insects about one inch long when fully grown. There are two very similar looking species, the spring and fall cankerworm. You can distinguish between them by examining the fleshy prolegs on the abdomen; spring cankerworms have two pairs while fall cankerworms possess three pairs. They are variable in color ranging from a light or yellow green to brownish to black in color. Spring cankerworms have a whitish or yellowish stripe running along the side of their body. Although they have differences in their physical characteristics and life cycles, spring and fall cankerworm larvae feed at the same time. Cankerworms are found on many deciduous trees, especially elm, linden, basswood, oak, ash, apple, maple, walnut, hackberry, and boxelder. It is not unusual to see cankerworms hanging suspended from a silken thread. When the wind catches them, they can be transported to adjacent trees. When they first start feeding, they chew small holes between the major veins, making it look lacey. As they continue to feed, they enlarge the damaged areas until all that remains are the big veins. Healthy, well-established shade trees can easily tolerate cankerworm feeding, even for two or three consecutive years without causing undue harm to the trees. Any chemical control of the cankerworms should take place when the caterpillars are less than ½ inch which requires careful monitoring of the trees for early damage and the small caterpillars. Application of chemical products is not effective when the damage has become extensive and very noticeable. Several insecticides are effective for treating cankerworms, including Bacillus thuringiensis (Dipel, Thuricide), carbaryl (Sevin), cyfluthrin, esfenvalerate, malathion, and permethrin (Eight). Because it does not harm beneficial insects, wildlife or humans, Bacillus thuringiensis should be used whenever possible. When American elms were very common (before the devastation of Dutch elm disease), cankerworms would go through cycles of very high populations lasting for 2-7 years followed by periods of very low populations for 13-17 years. We have not seen high numbers of cankerworms since the 1980s. Is this the beginning of a new cycle that has just taken some time to gather steam or is this just a small blip that will even out shortly? Time will tell.
