Eastern tent caterpillar egg hatch begins; horse owners should be wary

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Eastern Tent Caterpillar
(Photo: Joseph O'Brien, USDA Forest Service, Bugwood.com)

LEXINGTON, Ky.    Eastern tent caterpillar egg hatch was reported March 17 in Scott County in central Kentucky.

“This year’s first observed hatch is seven days earlier that 2015, reflecting the warm spring temperatures,” said Lee Townsend, University of Kentucky College of Agriculture, Food and Environment extension entomologist. “The hatch is not synchronized; tiny larvae will continue to emerge over the next two weeks from eggs laid last summer on wild cherry, flowering cherry, apple and related trees. This is a hardy insect so predicted low temperatures in the 30-degree Fahrenheit range late this week should not affect their survival.”

Increase in numbers

Populations of eastern tent caterpillars have been increasing steadily over the past four to five years. This trend is likely to continue, producing locally high numbers in some areas, Townsend said. The rise in numbers is normal and mirrors the cyclical aspects of insect populations in general.

Eastern tent caterpillar cycles are roughly 10 years in length. After two or three high years, the numbers usually drop again due to diseases or natural enemies.

Active in spring

Eastern tent caterpillars spend the winter as tiny, fully developed insects in distinctive egg masses that encircle twigs of wild cherry and related trees. It is one of the first insect species to become active in the spring.

When mature, the large, hairy caterpillars wander from their developmental sites along fence lines.

Foal losses

Consumption of large numbers of caterpillars by pregnant mares precipitated staggering foal losses in the Mare Reproductive Loss Syndrome outbreak of 1999-2001. An unprecedented number of mares – more than 3,000 – aborted their foals in Kentucky in 2001. MRLS can cause late-term foal losses, early- and late-term fetal losses and weak foals.

Embedding caterpillars

UK researchers conducted studies that revealed horses will inadvertently eat the caterpillars, and the caterpillar hairs embed into the lining of the horse’s alimentary tract.

Once that protective barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Townsend said horse owners and farm managers with pregnant mares should begin to monitor fence lines containing wild cherry and other host trees in about 10 days. They should look for small tents produced by developing caterpillars.

Minimize the risk

If practical, farm managers should plan to move pregnant mares from areas where these trees are abundant to minimize the chance of caterpillar exposure. The threat is greatest when the mature tent caterpillars leave trees and wander to find places to pupate and transform to the moth stage.

Nuisance

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

To get rid of active caterpillars, Townsend recommends pruning them out and destroying the nests if practical.

Insecticides

Farm managers can use any one of several biorational insecticides registered for use on shade trees as needed. Spot treatments to the tents and the foliage around them can be applied according to label directions, which vary by product.

For more information about how to assess trees for egg masses, check out the UK Entomology publication, Checking Eastern Tent Caterpillar Egg Masses, which is available at https://entomology.ca.uky.edu/ef449.

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