Forest tent caterpillar outbreak continues in parts of Michigan

Forest tent caterpillars are dark-colored with pale spots. (Michigan DNR)

Forest tent caterpillars are making a nuisance of themselves across Michigan, eating leaves from sugar maple, aspen and oak trees and leaving small strands of webbing as they go, the Michigan DNR said in a news release Wednesday, June 6.

The insects, which are native to Michigan, occur in widespread outbreaks every 10 to 15 years. The most recent outbreaks peaked in 2002 and 2010. They’ve been spotted across the Lower Peninsula and in the eastern Upper Peninsula. Outbreaks usually last two or three years; this is the second or third year for outbreaks in some areas.

An infestation of forest tent caterpillars rarely is fatal unless a tree has other stresses, said Scott Lint, forest health specialist with the Michigan DNR’s Forest Resources Division.

“The larvae begin feeding on new leaves in spring, and can strip the leaves from a tree,” Lint said.

Some also are seeing “tents” of webs in trees, but don’t confuse the forest tent caterpillar with a similar pest, the eastern tent caterpillar. That one creates tents in black cherry, apple and other fruit trees. Eastern tent caterpillars are dark-colored with a light-colored stripe, rather than dots. They create localized silk tents that encase a portion of a tree, but never enclose leaves.

“Its impact is minimal, but everybody sees the tents from alongside the road,” Lint said about eastern tent caterpillars.

Forest tent caterpillars are dark-colored with pale spots. They spin silken threads but do not form an actual tent. They will gather in large colonies on the trunk of the tree when not feeding. Large caterpillars often will wander in search of more food as they completely strip a tree.

Caterpillars will spin a yellow cocoon in mid-June, and mass flights of moths can occur in late June and early July. Adult moths do not feed, but mate and die within a few weeks, after laying eggs. Eggs overwinter until spring, when they hatch.

The forest tent caterpillar does have natural diseases, predators and parasites, including the large, slow-moving “friendly fly,” which lays its eggs on caterpillar cocoons, preventing them from developing into adult moths. These natural agents  eventually will respond and bring the outbreak under control.

Homeowners with trees that have been heavily defoliated should make sure those trees receive at least one inch of water per week during the growing season. Applying a slow-release tree fertilizer in the fall also will help trees recover quickly and prepare them for any defoliation that might occur next summer.

Learn more about caterpillars and other insects that threaten Michigan’s trees at michigan.gov/foresthealth.

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