Caterpillars capable of triggering abortions in mares hatch in Kentucky

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Newly hatched eastern tent caterpillars. Photo: Lee Townsend/University of Kentucky
Newly hatched eastern tent caterpillars. Photo: Lee Townsend/University of Kentucky

Caterpillars that can trigger abortions in mares are hatching in the early spring conditions in Kentucky, sparking a warning to horse owners.

University of Kentucky researchers said last Thursday that the annual hatching of eastern tent caterpillars was under way in the state.

The first egg hatching was confirmed that day – March 17 – in Scott County.

“This year’s first observed hatch is seven days earlier than 2015, reflecting the warm spring temperatures,” said Lee Townsend, an entomologist with the university’s College of Agriculture, Food and Environment.

The hatch is not synchronized, she explained.

She said tiny larvae would continue to emerge over the fortnight from the first hatching from eggs laid last summer on wild cherry, flowering cherry, apple and related trees.

It is, she says, a hardy insect, with low temperatures in the 30-degree Fahrenheit range unlikely to affect their survival.

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 and is well adapted to survive Kentucky’s erratic winter and early spring weather.

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 last roughly 10 years. After two or three high years, the numbers usually drop again due to diseases or natural enemies.

When mature, the large, hairy caterpillars wander from their developmental sites along fence lines. Consumption of large numbers of caterpillars by pregnant mares precipitated staggering foal losses in the outbreak of Mare Reproductive Loss Syndrome (MRLS) in 1999-2001.

MRLS can cause late-term foal losses, early and late-term fetal losses, and weak foals.

University of Kentucky 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 from around March 27. They should look for small tents produced by developing caterpillars.

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.

Eastern tent caterpillars are also a significant nuisance to people living near heavily infested trees. The caterpillars may travel 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. 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.

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