Creepy-Crawlies Invade Kentucky Horse Farms: Are Pregnant Mares At Risk Again?

A sure sign of spring: Eastern tent caterpillars are hatching in Kentucky. (South Dakota forestry image)

What on earth could possess them? Researchers tell us today that horses in Kentucky pastures actually do eat eastern tent caterpillars, properly known as Malacosoma americanum (Fabricius). You know them by the damage they do; these little caterpillars spin thick webs on tree limbs…and then munch their way to metamorphosis on emerging young leaves. Once hatched, they fly away, leaving a denuded tree limb behind. But for pregnant mares, they could pose a much greater risk.

Experts at the University of Kentucky today reported that eastern tent caterpillars have begun hatching in central Kentucky and that their population numbers are trending up.

It seems like only yesterday that the horse industry in central Kentucky was devastated by an event known as Mare Reproductive Loss Syndrome (MRLS). The 2001-2002 event caused the loss of an estimated 30 percent of that year’s Thoroughbred foal crop, with serious losses suffered by mares of all breeds of horses. After several false tries, the finger of guilt was finally pointed at the caterpillars, which were especially populous that year.

University of Kentucky College of Agriculture entomologist Lee Townsend will be closely monitoring caterpillar development over the next two to three weeks. He and his colleagues anticipate full-grown larvae by the third week of April. From the end of April to the beginning of May, caterpillars will likely leave the trees where they’ve eaten the available foliage and search for additional food to complete their development.

Once the caterpillars have reached these dispersing stages, controlling them becomes much more difficult, Townsend said. If needed, control should target caterpillars while they are gathered together in the trees. Apparently they love ornamental cherry trees, the bright pink-purple blossoms of which are such an exclamation point on spring landscapes.

However, Kentucky’s Townsend cautions against spraying too early. That won’t work, either. Obviously, timing is everything.

Studies since the 2001-2002 MRLS outbreak revealed that horses inadvertently will eat the caterpillars in the grass. When they do, the caterpillar hairs embed into the protective lining of the alimentary tract. Once that barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death or weak foals from these roaming alimentary tract bacteria are hallmarks of MRLS.

UK entomologists recommend that unless horse farm managers have been aggressive in managing eastern tent caterpillars, or removing host trees, they should keep pregnant mares out of pastures bordered by cherry trees or other hosts for the next several weeks.

At many farms, steps have already been taken to cut limbs that overhang or border paddocks. But eastern tent caterpillars are found in many states and horse breeders should be aware of the danger they pose. Apparently the ingestion of the caterpillars does not have known health risks for horses other than pregnant mares.

For a fact sheet about eastern tent caterpillars, as well as periodic updates, please visit the University of Kentucky’s special web page on the caterpillar problem. Ohio State University has a helpful web page on how to wage war on eastern tent caterpillars.




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