Eastern Tent Caterpillars: Benign Invaders

You may have noticed them creeping along a fruit tree branch, heading home at dusk to the web-like nest they inhabit with a few dozen squirmy, striped friends.

Meet the eastern tent caterpillar, a mostly harmless caterpillar who has been moving around many Northfield yards and gardens the past couple of weeks, eating leaves and grossing out children.

“They are quite abundant this year,” said Nancy Braker, director of the Cowling Arboretum at Carleton College. “But they are not a species that concerns us.”

Eastern tent caterpillars—scientifically known as Malacosoma americanum—are the caterpillars of a nondescript moth that is native to Minnesota. The caterpillars feed in early to mid-June, mostly on fruit trees. In the Arb, they prefer wild plum and wild cherry trees, Braker said, but in home gardens or commercial orchards they will also make their homes in apple trees.

During years when they are especially abundant, the caterpillars can defoliate a tree, but they rarely kill trees, according to the University of Minnesota Extension Service. While fruit production may be less on trees where the caterpillars have nested, the caterpillar life cycle ends early enough that most trees will leaf out again. At this point, most caterpillars have finished feeding and are in their cocoons, Braker said.

TJ. Heinricy, streets and parks supervisor for the city of Northfield, attributed the higher numbers of tent caterpillars to the wet spring. City plantings along Jefferson Parkway and in an area around Hwy. 3 have had infestations this year, he said.

"They tend to prefer the sweeter trees, especially crabapples," said Heinricy. "If I see them, I just cut them out."

Braker said in most cases, the issue wouldn't be addressed.

“But I can understand they are a problem with fruit trees in an orchard or yard,” she said.

If you still have caterpillars and are concerned about the damage, Braker and the U recommend removing the caterpillars from the tree. If you are not squeamish, you can scoop them out of the tree or you can cut off the branch where the caterpillar nest is and destroy it.

The eastern tent caterpillars should not be confused the forest tent caterpillars (sometimes mistakenly called armyworms), which can also defoliate a tree but are more common in northern and central Minnesota. Forest tent caterpillars prefer aspen, birch, oak and other deciduous trees.

Braker noted one benefit of a bumper crop of eastern tent caterpillars: While most birds do not eat caterpillars because of their prickly skin, black-billed cuckoos love them. In years with many caterpillars, the number of cuckoos seen in Northfield also increases, she said.