Eastern Tent Caterpillar

Malacosoma americanum — the eastern tent caterpillar — is one of the most familiar spring caterpillars in eastern North America and the source of the iconic large silken 'tent' nests visible in the crotches of cherry, apple, hawthorn, and other rosaceous trees from late March through May. Females lay egg masses (varnished cylindrical 'belts' encircling small twigs) the previous summer; the egg masses overwinter and hatch synchronously in early spring as host tree buds break. The hatchling caterpillars (200-300 from a single mass) immediately begin spinning silk together to construct a communal tent in the nearest branch crotch. The tent grows as the caterpillars grow, eventually reaching 30 cm or more across. Caterpillars leave the tent during the day to forage on host tree leaves (defoliating the host tree if populations are high), return at night, and rest in aggregations between feeding bouts. The species' most consequential biology is the production of hemolymph and silk containing cyanogenic compounds (acquired and concentrated from cherry leaf chemistry). When pregnant horses ingest the caterpillars or pasture contaminated with caterpillar fragments, the cyanogenic compounds cause Mare Reproductive Loss Syndrome (MRLS) — early embryonic loss, late-term abortion, and stillbirth. The 2001 Kentucky MRLS outbreak (a year of unusually high tent caterpillar populations following an unusually cold spring) caused the loss of 30%+ of the state's foal crop and over $336 million in direct economic damage to the Kentucky thoroughbred industry. The species is therefore a major concern in horse-breeding regions wherever wild cherry trees occur.