The fall cankerworm (Alsophila pometaria) is a common, native defoliator of hardwood trees throughout North America. Prone to outbreaks, this insect can cause severe damage on trees and be a public nuisance. Landscape-level management is rarely necessary, but control measures may be worthwhile on individual trees. (Photo by William M. Ciesla, Forest Health Management International, Bugwood.org)

By David Coyle, Ph.D.

David Coyle, Ph.D.

When I was a kid, I spent a lot of time outdoors. We lived on a farm in the country, and our home was next to Forestville State Park in southeastern Minnesota. I used to collect caterpillars and rear them in old jars I’d acquire from Mom, and she can confirm I’d have dozens of them lining the windows of the screened-in porch. I’d punch holes in the lid, and in would go a caterpillar with a bunch of leaves from whatever tree I found it munching on. Every now and again, I’d find a little caterpillar that was a bit different than the others—it had six legs just behind the head and some legs at the other end, but the middle was completely legless. It didn’t crawl with an undulating motion like most caterpillars—no, it inched along. Of course, we called them inchworms.

Now that I’m older and have a little more book-learnin’ under my belt, I now know these caterpillars to be cankerworms. Also called inchworms or loopers, these native defoliators are common throughout much of North America. In a new article published in May in the Journal of Integrated Pest Management, my colleague Molly Darr, Ph.D., and I highlight a particular variety of these curious little critters, the fall cankerworm (Alsophila pometaria).

The fall cankerworm (Alsophila pometaria) is sexually dimorphic, with males having wings and the ability to fly and females (shown here) being flightless. (Photo by William M. Ciesla, Forest Health Management International, Bugwood.org)

The fall cankerworm (Alsophila pometaria) is sexually dimorphic, with males having wings (shown here) and the ability to fly and females being flightless. The mottled, gray-brown coloring of both male and female adults allows for excellent camouflage against tree bark. (Photo by William M. Ciesla, Forest Health Management International, Bugwood.org)

Mature larvae of the fall cankerworm (Alsophila pometaria) consume entire tree leaves, leaving only the midrib and major veins. (USDA Forest Service – Forest Health Protection Intermountain Region – Ogden, UT , USDA Forest Service, Bugwood.org)

Sure, the caterpillars are cool. I mean, who doesn’t like watching them inch along? But what’s even more interesting (to me) is the fact that adult females are flightless. They crawl out of the ground after eclosing and move up a tree, where they emit a pheromone to attract a male. Males look like little gray moths, and it takes a true entomological connoisseur to really appreciate the visual beauty of a drab, gray moth. Larvae feed on several different hardwood tree species and can cause a lot of defoliation, especially in stressed stands or urban trees (which, by their very nature, are usually stressed).

Management is not all that complicated. Natural enemies do a pretty good job of keeping populations in check most of the time, and several insecticides are effective against the larvae. Tree bands can be used for scouting and population monitoring and for controlling populations on individual trees. In natural areas, there really isn’t too much that can be done other than to manage that land to encourage healthy trees. Cankerworm populations come and go—they’re known as eruptive herbivores in that their populations occasionally “erupt.” There are plenty of options to manage them whenever and wherever necessary, but, before you do, I highly recommend taking a moment to appreciate those little inchworms, just inching along. It might just take you back to your youth for a moment.

David Coyle, Ph.D., is an assistant professor in the Department of Forestry and Environmental Conservation at Clemson University. Twitter: @drdavecoyle. Email: dcoyle@clemson.edu.