The Eastern tent caterpillar (ETC) eggs will begin hatching soon after spending about nine months as eggs in masses on twigs of wild cherry and related trees, said Lee Townsend, PhD, University of Kentucky (UK) College of Agriculture, Food and Environment extension entomologist.

“The onset of the single generation that occurs each year varies with the character of the season,” he said. “Hatch was noted as early as March 14, 2012, during an unseasonably warm spring, and as late as April 2, 2014, during one that was slow to develop.”

The larvae are among the first insects to become active in the spring and are prepared to cope with Kentucky’s erratic temperature swings.

“Egg hatch is relatively random and occurs over an extended period,” Townsend said. “This increases the chance for survival in case of late freezes.

“In addition, the small but hardy caterpillars will remain clustered on egg masses to ‘wait out’ temperatures that are too low for feeding and development,” he added. “ETC grow and develop when the temperature is above 37°F.”

Townsend said that while it is possible to predict approximately when to expect tent caterpillar activity, there is no reliable information to track general population trends other than observing local activity and watching for tents to develop from mid-March through mid-April.

When mature, the 2- to 2.5-inch long hairy caterpillars wander from their developmental sites along fence lines and sometimes within horses’ reach. Inadvertent consumption of large numbers of caterpillars by pregnant mares precipitated staggering foal losses in an outbreak of the mare reproductive loss syndrome (MRLS)—which cause late-term foal losses, early- and late-term fetal losses, and weak foals—in 1999-2001.

Researchers from UK conducted studies revealing that, when horses inadvertently eat the caterpillars, hairs from the insects embed into the digestive, or alimentary, tract lining. Once that protective barrier is breached, normal alimentary tract bacteria can gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta.

If practical, farm managers should plan to move pregnant mares from areas where wild cherry trees are abundant to minimize the risk of caterpillar exposure. The threat is greatest when mature tent caterpillars leave trees and to find places to pupate and transform into moths.

Eastern tent caterpillars are also a significant nuisance to people living near heavily infested trees. The caterpillars can wander hundreds of yards in search of protected sites to spin cocoons and pupate.

To eliminate active caterpillars, Townsend recommends pruning them out and destroying nests. Farm managers can use any one of several biorational insecticides registered for use on shade trees, as needed. These types of insecticides are relatively nontoxic to humans. Apply spot treatments to tents and the foliage around them according to label directions, which vary by product.

Find more information about how to assess trees for egg masses in the UK Entomology publication Checking Eastern Tent Caterpillar Egg Masses at

Author: Lee Townsend