Tent caterpillars are the insects responsible for those conspicuous silk webs you see in trees each spring. Found across much of North America, they belong to the genus Malacosoma and feed on the leaves of hardwood trees, sometimes stripping branches bare. While they rarely kill healthy trees, large populations can cause significant stress, and the caterpillars pose a unique danger to horses. Here’s what you need to know to identify them, understand their impact, and deal with an infestation.
How to Identify Tent Caterpillars
Two species account for most sightings in the eastern half of North America: the eastern tent caterpillar and the forest tent caterpillar. They look similar at a glance but have distinct markings once you know what to check.
The eastern tent caterpillar has a dark head and a prominent white or yellow stripe running down the center of its back, with small blue spots along its sides. The forest tent caterpillar, by contrast, has a blue head and a row of white or yellow markings shaped like keyholes or footprints down its back, with bluish coloring along its sides. That center marking is the quickest way to tell them apart: a continuous stripe means eastern, a row of separate keyhole shapes means forest.
Both species are hairy, grow to roughly two inches long at maturity, and are most visible in spring.
Tent Caterpillars vs. Fall Webworms
Many people confuse tent caterpillars with fall webworms because both spin silk structures in trees. The differences are straightforward once you look at timing and web placement.
Tent caterpillars appear in spring. Their webs are thick, tightly constructed structures built in the forks and crotches of branches. The caterpillars don’t actually feed inside the web. They use it as a shelter at night and during rain, then crawl out onto branches to eat leaves.
Fall webworms show up in summer and fall. Their webs are loose, messy, and enclose the tips of branches rather than sitting in branch forks. The larvae feed inside the web, expanding it outward as they consume more foliage. If you’re seeing webs in July or later, you’re almost certainly looking at fall webworms, not tent caterpillars.
Life Cycle and Seasonal Timeline
Tent caterpillars produce one generation per year. The cycle starts with egg masses that overwinter on twigs. Each mass contains 150 to 350 eggs, wrapped around small twigs about a quarter inch in diameter, typically within 15 to 18 inches of the twig tip. The eggs are dark and shiny, easy to spot on bare branches in winter if you know where to look.
Caterpillars hatch in early spring, usually April in most regions. Within two days of hatching, larvae begin weaving their tent. They add successive layers as they grow, making the tent larger and more noticeable over the following weeks. Caterpillars of all sizes return to the tent for protection from predators but feed on leaves away from it.
By early summer, mature caterpillars leave the tent entirely. They wander away from the host tree, sometimes crossing roads and sidewalks in large numbers, searching for a sheltered spot to spin a cocoon and pupate. Adults emerge as small, reddish-brown moths in late June and July. The moths lay new egg masses on twigs, and those eggs sit dormant through winter until the cycle repeats the following spring.
Which Trees They Target
Tent caterpillars feed on a wide range of hardwoods. Sugar maple is the preferred food source for forest tent caterpillars, along with aspen, cherry, and apple trees. Other common hosts include oaks, birch, ash, alder, elm, basswood, and willow. They do not feed on red maple, sycamore, or conifers.
Cherry and apple trees are especially favored by eastern tent caterpillars, and ornamental crabapples in yards are frequent targets. If you have wild cherry trees on your property, they’re likely to be the first trees affected.
How Defoliation Affects Trees
A heavy infestation can strip a tree’s canopy down to bare branches, which looks alarming. In most cases, though, healthy trees survive. Trees defoliated early in the season often grow a new, smaller set of leaves in July once the caterpillars finish feeding.
The real risk comes from repeated defoliation over multiple years. Healthy trees can survive two to three consecutive years of being stripped, but the cumulative stress reduces their resilience. Growth slows, and the tree becomes more vulnerable to drought, flooding, other pests, and disease. In some cases, stress from defoliation contributes to tree mortality eight to ten years after the defoliation event, as weakened trees fail to withstand additional pressures.
Natural Population Control
Tent caterpillar populations naturally rise and fall in cycles, often peaking for a few years before crashing. Several natural enemies help keep numbers in check. Young, exposed caterpillars are eaten by birds. Older caterpillars are less vulnerable to bird predation because their body hairs deter feeding, but small mammals and predatory insects also target them.
Parasitic wasps are among the most effective natural controls. Several species of tiny wasps lay their eggs inside tent caterpillars, and the wasp larvae consume the host from within. Tent caterpillars are also susceptible to a nuclear polyhedrosis virus and a microsporidian pathogen, both of which can sweep through dense populations and cause dramatic die-offs. These natural boom-and-bust dynamics mean that a bad year or two is often followed by a sharp population decline without any human intervention.
Managing an Infestation
For homeowners with a few affected trees, manual removal is the most practical approach. In winter or very early spring, inspect twigs for egg masses. They’re most commonly found within 15 to 18 inches of twig tips, at any height on the tree. Pruning off these small twigs and discarding them eliminates hundreds of caterpillars before they hatch.
If you miss the egg stage, watch for tents in late March and early April. Small tents can be pulled out of branch forks by hand (wear gloves) or wound around a stick early in the morning or on cool days when caterpillars are gathered inside. Dropping the web into a bucket of soapy water kills the larvae. Avoid burning tents in the tree, a common suggestion that risks damaging the bark far more than the caterpillars themselves would.
For larger trees or heavier infestations, a biological spray containing Btk (a naturally occurring soil bacterium toxic to caterpillars but harmless to humans, birds, and pets) can be applied to foliage. Timing matters: it works best when caterpillars are small and actively feeding, typically in spring. Because not all eggs hatch at the same time, two applications spaced five to ten days apart are more effective than a single treatment. Spraying works best in early morning when humidity is high and wind is low, so the product doesn’t evaporate or drift.
Risks to Horses
Eastern tent caterpillars pose a serious and specific threat to pregnant mares. Horses that accidentally ingest the caterpillars while grazing can develop Mare Reproductive Loss Syndrome (MRLS), which caused widespread foal losses in Kentucky in 2001 and 2002.
The mechanism is unusual. The caterpillars’ tiny barbed hairs act like microscopic harpoons. Once swallowed, the hair fragments penetrate the intestinal lining and enter the bloodstream, carrying bacteria with them. These bacteria-laden fragments spread throughout the body and can reach the uterus, fetal membranes, and fetal fluids. Pregnant mares are highly sensitive to this bacterial contamination. The process is remarkably fast: caterpillars weighing roughly one gram can trigger abortion in a 1,500-pound mare within 32 hours of ingestion.
Beyond pregnancy loss, MRLS can cause eye inflammation, heart sac infection, and brain inflammation in affected horses. If you keep horses and have cherry or apple trees near pastures, managing tent caterpillar populations and keeping horses away from heavily infested areas during spring is important.
Skin Irritation in Humans
Tent caterpillar hairs can cause skin reactions in people, though the risk is lower than with some other hairy caterpillar species. The tiny barbed setae detach easily and can lodge in skin on contact, causing an itchy rash or hives through simple mechanical irritation. In some individuals, repeated exposure triggers an allergic response that can become progressively more severe with each encounter. Wearing gloves and long sleeves when handling nests or caterpillars is a simple precaution that prevents most problems.