How to Protect Pregnant Mares from Eastern Tent Caterpillars

By Blake Jackson

The Eastern Tent Caterpillar (ETC) poses a significant risk to pregnant mares, particularly due to its connection to Mare Reproductive Loss Syndrome (MRLS), which caused widespread foal losses in the early 2000s. With a delayed hatch anticipated this year due to a colder winter, horse owners have a bit more time to prepare.

“ETCs begin to hatch after enough warm days accumulate,” said Jonathan Larson, assistant extension entomology professor. “We’ve seen a slower approach to that number this winter, which gives horse owners more time to monitor and prepare.”

Typically, ETCs hatch when 90-100 Growing Degree Days (GDD) have accumulated. Horse owners should monitor wild cherry, apple, and crabapple trees, as these are common host trees where ETCs build silk "tents" for shelter.

Once hatched, the caterpillars feed primarily at dawn, dusk, or overnight, growing up to 2.5 inches. When mature, they often leave their host trees, wandering into pastures. This poses a danger to pregnant mares, as ingested ETC hairs can lead to infections affecting the fetus and placenta.

“If farm managers notice high numbers of nests, they should relocate pregnant mares from areas near wild cherry trees to reduce the risk of exposure,” Larson said. “The greatest danger occurs when mature caterpillars leave the trees to pupate and transform into moths.”

Preventive measures include monitoring host trees for egg masses and tents. Removing affected branches can control infestations.

Smaller tents can be wound onto a stick, while larger ones should be pruned. Burning tents is discouraged due to potential tree damage.

Insecticide use, such as Bacillus thuringiensis (Bt) for young caterpillars, can be effective. For larger trees or infestations, professional arborists may use trunk injections with products like Tree-age or Lepitect.

Staying informed about local GDD and consulting extension resources is crucial for anticipating ETC activity.

“ETC activity can vary annually due to climatic conditions, predators and disease,” Larson said. “However, understanding their life cycle and habitat preferences allows us to address potential risks preemptively. Awareness is our best defense. By tracking the hatching and development of these caterpillars, we can implement targeted interventions to protect our trees and, crucially, our horses.”

Photo Credit: gettyimages-patrick-jennings