A previous episode of Bug of the Week visited the eastern tent caterpillar in a "Time for Tents" in March of 2006. We discussed the fact that populations of these pests would rise or fall and mused about our ability to predict what might happen this year. The caterpillar gods must have smiled on tent caterpillars. Up and down I-95, between Baltimore and Washington, there are more tents and caterpillars this spring than there have been in many years. At my favorite wild cherry tree grove, several small trees are completely enshrouded in silk with nary a leaf to be found. All of the foliage has disappeared down the gullets of thousands of hungry larvae. One particularly unfortunate tree was festooned with writhing balls of caterpillars on the exterior of their tents. At the base of the tree, caterpillars followed silken highways across the ground for several yards. I followed one of these busy trails to a nearby mountain laurel where caterpillars were happily munching leaves.
Attraction to the leafy meal
What craft or contrivance brought these insects to the leafy meal? Tent caterpillars are somewhat unique in the insect world in that the larvae have evolved sophisticated methods of chemical communication to help each other find food. Eastern tent caterpillars move from their tent along branches as they search for leaves. As they crawl along, a special gland located on their rear end lays down a chemical signal called a pheromone. The pheromone alerts other caterpillars that a nest mate is on the hunt for food. If the forager is lucky and finds a bounty of nutritious leaves, after it has eaten its fill and as it returns to the tent, it lays down a trail marked with a recruitment pheromone. This signal alerts other caterpillars to the feast at the end of the trail. It appears that once a tree has been stripped of leaves, eastern tent caterpillars use their trail-marking pheromones to locate new sources of food. Wandering on the ground is part of the tent caterpillar's normal behavior as it searches for a place to pupate after it is finished its development as a larva.
A new threat associated with the wanderings of eastern tent caterpillar surfaced in 2001 in the Ohio Valley Region of the United States when tent caterpillars reached large numbers on cherry trees on horse farms. As caterpillars migrated from trees into the grass below, they were accidentally consumed by grazing horses. Many pregnant mares that ate caterpillars spontaneously aborted their foals. The resultant losses to the horse industry were in the hundreds of millions of dollars. This phenomenon has been termed "Mare reproductive loss syndrome" or MRLS and is now suspected or confirmed in several eastern states. One hypothesis for the disease is as follows. Recall that eastern tent caterpillars are remarkably hairy. After being thoroughly munched and swallowed, hairs on the insect's body break off and hairs or their fragments puncture the cells of the mare's digestive tract carrying bacteria with them. These bacteria establish infections in the horse's reproductive system. The infection results in the abortion of the foal. MRLS has provided an added impetus to find new ways to reduce populations of eastern tent caterpillars and limit exposure of horses to nasty caterpillar hairs. If you keep horses and have cherry trees near your paddocks or pastures, you might want to inspect the trees and remove and destroy tent caterpillars before they wander too widely in the upcoming weeks.
The fascinating book "The Tent Caterpillars" by Terrence Fitzgerald was used as a reference for this episode as was the article "The 2001 Kentucky Equine Abortion Storm: The Caterpillar/Setal Hypothesis of the Mare Reproductive Loss Syndrome (MRLS)" by Thomas Tobin. For more information on Eastern Tent Caterpillars and Mare Reproductive Loss Syndrome, please visit the following web sites.