Potter said that, after spending about nine months in eggs masses on twigs of wild cherry and related trees, the first tiny caterpillars of the season are now leaving their eggs. The larvae are among the first insects to become active in the spring and are well-equipped to cope with Kentucky’s erratic temperature swings.
Entomology researchers at UK entomology researchers said the egg hatch occurs over several weeks in early spring, which increases the chance for survival in case of late freezes. The caterpillars grow and develop when the temperature is above 37 degrees F. Their preferred food plants are wild cherry, apple, and crabapple, but they can be found on hawthorn, maple, cherry, peach, pear, and plum trees, as well.
When mature, the 2- to 2.5-inch long hairy caterpillars wander from their host trees to seek protected areas to spin their cocoons or seek additional food if their natal tree becomes defoliated. At such times, they can crawl along fence lines and into pastures.
Consumption of large numbers of ETCs by pregnant mares caused staggering early- and late-term foal losses and weak foals during the mare reproductive loss syndrome (often referred to as MRLS) outbreak of 1999-2001. Studies by UK researchers revealed that after horses inadvertently eat the caterpillars, the caterpillar hairs embed into the lining of the horse’s alimentary (digestive) tract. 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.
If practical, farm managers should move pregnant mares from areas with abundant wild cherry trees to minimize the risk of caterpillar exposure. The threat is greatest when the mature caterpillars leave trees and wander to find places to pupate and transform to the moth stage.
Eastern tent caterpillars can also be a nuisance to people living near heavily infested trees. The nests and defoliation are unsightly, and the caterpillars can wander hundreds of yards in search of protected sites to spin cocoons and pupate.
“Managing ETCs in small ornamental trees, such as flowering crabapples, is easy,” Potter said. “Just wear a pair of grocery store plastic bags like mittens, climb a stepladder, pull out the tents, turn the bags inside out to ‘bag’ the caterpillars, and stomp them.
“Pruning out nests in ornamental trees sounds great, but in reality, by the time they are noticed, they’re often in branch crotches where pruning will compromise the symmetry of the tree,” he added.
“Spraying the flowering fruit and decorative trees preferred by the caterpillars can be a bee hazard—and with some products, a label violation—because the trees are in bloom with bees visiting them at the same time Eastern tent caterpillars are active,” Potter said. “Except for Bacillus thuringiensis, which is not all that effective once the ETC are about half-grown, the only spray product I know of that controls ETCs and is bee-compatible is Acelepryn (chlorantraniliprole). That is available in a formulation used mainly by professional grounds managers and arborists but has not yet found its way into homeowner spray products.”
Caterpillar management around horse farm paddocks comes down to keeping pregnant mares away from infested trees and either removing or not planting preferred host trees near paddocks, Potter added. In addition to those preventive measures, controlling the caterpillars with insecticides might be warranted in some settings; that might involve treating tall trees that are difficult to spray.
For the latter scenario, professional arborists treat via trunk injection. Products labeled for ETC control include Tree-äge (emamectin benzoate), Inject-A-Cide B (Bidrin), Abacide 2 (abamectin), and Lepitect (acephate). End users should read and follow all label instructions. All four of those injectable products are labeled for use on horse farms.
For more information about how to assess trees for egg masses, the UK Entomology publication, Checking Eastern Tent Caterpillar Egg Masses, is available at entomology.ca.uky.edu/ef449.
Holly Wiemers, MA, APR, is the communications and managing director of UK Ag Equine Programs.