Eastern tent caterpillars, the culprits behind the deaths of thousands of foals from 1999 to 2001, are back in significant numbers this year.
University of Kentucky entomologist Lee Townsend said that the numbers have gradually crept up in recent years, but he has seen a surge this spring.
Now, with warmer weather finally here, the hairy caterpillars are mature, have left the trees and are on the move.
Townsend recommends that horse farms move pregnant mares to avoid contact with crawling caterpillars.
"Mature eastern tent caterpillars leave trees in search of protected pupation sites, where they will spin cocoons and transform into adults," Townsend said. "These wandering caterpillars may move several hundred feet from the trees where they developed. The direction of travel tends to be random and directly related to air and ground temperatures. Movement will be slower when temperatures are cool and faster when they are warm."
The caterpillars often go to dark, vertical objects such as tree trunks and fence posts, he said.
He advised checking fence rails to monitor caterpillar movement. Activity is expected for the next two weeks.
Insecticides are ineffective because the caterpillars don't feed much at this stage.
The eastern tent caterpillar was responsible for outbreaks of what came to be known 15 years ago as mare reproductive loss syndrome.
MRLS, which can cause abortions as well as stillborn foals and weak foals, had a multimillion-dollar economic impact on Kentucky from the lost Thoroughbreds and other horse breeds.
Subsequent studies by UK researchers found that horses will inadvertently eat the caterpillars, and the caterpillar hairs embed in the lining of the alimentary tract. Once that protective barrier is breached, normal alimentary tract bacteria might gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta, according to a UK news release.