Winter No Threat to Eastern Tent Caterpillar Eggs

Blasts of polar air across Kentucky made this winter one to remember for many, but experts say the Eastern tent caterpillars probably didn’t take notice.

This insect spends the winter as tiny, fully developed caterpillars in distinctive egg masses that encircle twigs of wild cherry and related tree species. The Eastern tent caterpillar is one of the first insects to become active in the spring and is well adapted to survive Kentucky’s often erratic winter and early spring weather.

“Hardy may be too mild a term for them,” said Lee Townsend, PhD, University of Kentucky (UK) College of Agriculture, Food, and Environment extension entomologist. “Laboratory studies have shown that caterpillars in the egg can withstand temperatures down to -31°F. It has been a cold winter, but temperatures have not been low enough to expect a significant reduction in egg hatch this spring.”

Townsend said dissections of some eggs collected in early March show a nearly 80% survival, which is typical of most years. That is also the same rate identified by UK entomology researchers during the 1999-2001 mare reproductive loss syndrome (MRLS), which resulted in staggering foal losses. MRLS can cause late-term foal losses, early- and late-term fetal losses, and weak foals.

Subsequent studies by UK researchers revealed that horses can inadvertently eat the caterpillars, and the caterpillar hairs embed into the alimentary (gastrointestinal) 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.

“The growth and development of many insects is directly dependent on temperature,” Townsend said. “Usually, this allows relatively accurate predictions of egg hatch and development. Unfortunately, the Eastern tent caterpillar is an exception. Predictive models using degree-day information can provide a general idea of when egg hatch can occur, but actual hatch in the field is variable.

“In fact, eggs from a single mass usually hatch over about a two-week period, not all at once,” he added. “This is an important survival mechanism that protects the species from high mortality. Prolonged egg hatch increases the chances of species survival, even if some early caterpillars are killed by freezes or heavy rains during early spring.”

Townsend said Kentucky arborist Larry Hanks has tracked the Eastern tent caterpillar egg hatch in Central Kentucky since 2001. His earliest observation of a hatch was March 13, 2012. The latest was April 4, 2013. Typically, the caterpillars first appear during the second or third week of March.

“It is still too early to provide a general prediction for 2014,” Townsend said. “Continued cold will slow development, but a string of 70° days can cause egg hatch to begin in a short time.”

Townsend said Eastern tent caterpillar populations seem to have increased over the past five to seven years, with numerous tents visible in wild cherry trees along fence lines in some areas of the state. Regrowth and sprouting of new trees might have resulted in increases in host trees and, subsequently, the caterpillars.

To get rid of active caterpillars, Townsend recommends pruning them out and destroying the nests as they are seen, if practical. Any one of several insecticides registered for use on shade trees can also be used to treat as needed. Spot treatments to the tents and/or the foliage around them can be applied according to label directions, which vary by product.

“This is a good time to prepare,” Townsend said. “Begin by checking pasture fence lines to see how abundant wild cherry is in them. If practical, plan to move pregnant mares from areas where these trees are abundant to minimize the chance of exposure to the caterpillars. The potential is greatest when the mature tent caterpillars leave trees and wander to find places to pupate and transform to the moth stage.”

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Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.