If the fog would ever lift this summer, the view of the eastern Middle Moors from Altar Rock might look a little strange at the moment. Large swaths of scrub oak in the Moors have been completely defoliated for the third year in a row. The culprit is an outbreak of a native species called the Fall Cankerworm, Alsophila pometaria. A walk or bike ride along the single track trails in the vicinity of Almanack Pond and north of the Milestone Cranberry Bogs may have brought you up close and personal with a face full of webs and caterpillars of this species of moth.

Silk of the larvae of Fall Cankerworms strung between the branches of scrub oak trees along trails in the eastern Middle Moors. Photo credit: Thad Jones

Adult Fall cankerworms are type of small, dull brown moth most commonly seen in the late fall and early winter, and therefore often referred to as a “winter moth”. At that time, the females emerge from their pupa as wingless moths, and crawl up the branches of a host tree. They attract nearby males, which do have wings, by emitting pheromones. After mating occurs, the female lays eggs which overwinter on the host tree. The adult stage of the cankerworm is quite short and essentially lasts just long enough to mate, lay eggs, and perhaps provide a meal for a bat just before it enters hibernation. On Nantucket, fall cankerworm eggs hatch in mid to late May, which occurs simultaneously with the spring leafing out of scrub oak. The larvae are tiny caterpillars, often called loopers or inch worms, which occur in two color variations, a back and a yellowish-green.

Both the yellow/green and black color variations of the larval caterpillars of the Fall Cankerworm can be seen here completely consuming the leaves of scrub oak.
Photo credit: Danielle O’Dell

The larval caterpillars feed en masse, and voraciously, particularly on scrub oak, but also on black cherry, black huckleberry, Shadbush, and lowbush blueberry as well. The height of caterpillar density on Nantucket falls around mid June and lasts only a couple additional weeks. They have caused extensive defoliation in areas in the eastern middle moors for the last three years. A typical outbreak of this species last two years but can persist for up to five years. While most trees can recover from short-lived outbreaks, repeated defoliation for three to five years can weaken or kill trees. Now that we are in year three of this outbreak, it will be interesting to see how long this persists and what effect this has on scrub oak stands here. We all know that scrub oak on Nantucket is incredibly hardy but we’ll be monitoring this for the next several years regardless. Over the last two years, the scrub oaks all re-leafed out and seemed none the worse for wear by mid-July. This seems to be the case again this year.

The large grey-ish areas surrounding the Milestone Cranberry bog depict the extent of the outbreak of Fall Cankerworm on Nantucket in June 2019. Photo courtesy of Massachusetts Department of Conservation and Recreation.

Unfortunately, there is not much that can be done about this kind of outbreak aside from just letting it run its natural course. Pesticides are not a viable option on this scale as the area is so large with thousands of scrub oaks, let along the impacts it would have on other desirable species of insects. The upside could be that perhaps we’ve had better than normal fledgling success of baby birds as the catbirds, towhees, robins and common yellowthroats were feasting and an endless supply of food this spring. And good crops of winter moths might help bats fatten up with extra resources in the late fall to help make it through long winter hibernation periods.

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