Photo by Ronald S. Kelley, Vermont Dept of Forests, Parks and Recreation
Fall Cankerworm
What’s Eating Tree Leaves in the Lakeshore This Year?
You may have noticed that something has caused noticeable defoliation in the forests of Pictured Rocks this spring. This defoliation is especially evident around Chapel Lake. While there are numerous insects feeding on tree leaves, in 2009 there are a high number of cankerworms causing visible defoliation and what is known as frass rain (falling bug poop).
Cankerworms, also called loopers or inchworms, attack deciduous trees and shrubs. The spring and fall cankerworms are the most visible pests in the hardwood forests of Pictured Rocks this year. These insects are native and unlike exotic pests have a well developed natural enemy complex that keeps them in check for long periods between outbreaks. While the apparent damage to trees looks severe it is a natural cycle and most trees will recover with no major effects to tree health.
Both fall and spring cankerworms feed on a wide variety of trees including apple, ash, beech, elm, maples and oaks. Young larvae chew small irregular holes in young leaves or skeletonize leaves. As they mature, the larvae begin eating larger irregular holes and finally entire leaves except for the major leaf veins. Low populations do not damage healthy trees but high populations can defoliate trees causing them to expend considerable resources to produce a new set of leaves.
By late June to early July the larvae of both species have matured and they then descend to the ground on silk threads. The larvae then burrow into the ground to a depth of one to four inches, spin a silken cocoon and pupate. The pupae remain in the soil until they emerge as adults in fall or early spring.
Many kinds of birds, especially the warbler, chickadees, thrushes and vireos, feed on cankerworms. In the 1850’s, invasive English sparrows were unfortunately imported from England “to destroy cankerworms.” The most important natural enemies are parasitic wasps and predaceous ground beetles.
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