Those silky white webs in your trees are almost certainly the work of one of two caterpillar species: the eastern tent caterpillar or the fall webworm. Both build conspicuous silk structures that can look alarming, but they rarely kill healthy trees, and telling them apart is straightforward once you know where and when to look.
Eastern Tent Caterpillar vs. Fall Webworm
The two species responsible for nearly all caterpillar webbing in North American trees differ in three key ways: nest location, season, and feeding behavior.
Eastern tent caterpillars build thick, opaque webs in the forks and crotches of branches, typically in cherry, apple, and other fruit trees. They appear in early spring, and the caterpillars don’t actually feed inside the web. Instead, they use it as a communal shelter, gathering there at night and during rain, then venturing out along branches to eat leaves during the day.
Fall webworms, as the name suggests, show up in late summer and early fall. Their webs enclose the tips of branches, wrapping around leaves at the outer edges of the canopy. Unlike tent caterpillars, webworms feed inside the web, expanding it outward as they consume more foliage. Their nests tend to look messier and more spread out, filled with frass (caterpillar droppings) and leaf debris.
If you’re seeing a tight, cottony tent deep in the branch structure during April or May, it’s almost certainly an eastern tent caterpillar. A loose, sprawling web at the tips of branches in August or September points to fall webworms.
Why Caterpillars Build Webs
Caterpillar webbing isn’t just a nest. It’s a sophisticated temperature-control system. The silk walls are dense enough to block wind and trap solar heat, functioning like miniature greenhouses. Research at SUNY Cortland found that when tent caterpillar webs are shielded from sunlight, the caterpillars can’t raise their body temperature above the cool spring air and they fail to grow. Exposure to sun is essential.
The layered structure of the tent creates different temperature zones inside, and caterpillars move between compartments to regulate their body heat, much like you’d move between sun and shade. Caterpillars basking together on the tent surface achieve significantly higher body temperatures than a lone caterpillar on a nearby branch, thanks to the group reducing heat loss from wind. Even without sunlight, a colony’s collective body heat can raise the temperature inside the web by 2 to 7°C above the surrounding air. When caterpillars leave the structure to feed at night, researchers have measured the internal temperature dropping until they return.
The webs also provide protection from rain, birds, and some predators. For species that feed inside the web, the silk creates a physical barrier that smaller parasitic wasps and other enemies must penetrate to reach the caterpillars.
How Much Damage They Cause
A tree covered in caterpillar webs looks devastating, but healthy trees handle defoliation better than you’d expect. A single defoliation event typically isn’t fatal. Trees stripped of leaves early in the season often grow a new, smaller set of leaves by July once the caterpillars finish feeding.
Healthy trees can survive two to three consecutive years of defoliation. The real danger is cumulative stress. Repeated leaf loss reduces a tree’s energy reserves and overall resilience, making it more vulnerable to drought, flooding, other pests, and disease. According to the New York State Department of Environmental Conservation, the stress from defoliation can contribute to tree mortality eight to ten years after the event, as weakened trees succumb to additional stressors they’d normally withstand. So a single bad year won’t kill your oak or cherry tree, but several years in a row deserve attention.
How to Remove Caterpillar Webs
Early detection makes control much easier. The smaller the web, the fewer caterpillars you’re dealing with and the less defoliation has already occurred.
For mechanical removal, the simplest approach is to insert a stick into the web and twist it, which pulls the silk loose along with the caterpillars inside. You can also prune out small infested branches entirely. The best time to do this is early morning or evening when tent caterpillars are gathered inside the web rather than out feeding. For fall webworms, which stay inside their webs while eating, timing matters less.
Scouting for eastern tent caterpillar egg masses in winter, before the larvae hatch in spring, is an effective preventive step. The egg masses look like shiny, dark bands wrapped around small twigs and can be scraped off or pruned away.
One common piece of bad advice: don’t try to burn webs out of trees with a torch. This damages the bark and surrounding branches far more than the caterpillars themselves would.
Biological Treatments
If webbing is widespread or high in the canopy where you can’t reach it by hand, a naturally occurring soil bacterium sold under the name Bt (Bacillus thuringiensis) is effective against both tent caterpillars and fall webworms. It comes in liquid or powder form. The liquid version is mixed with water and sprayed onto foliage. Caterpillars must eat treated leaves for it to work, so good spray coverage matters.
Bt breaks down in sunlight and washes off in rain, so you’ll need to reapply every one to two weeks if the infestation continues. It targets caterpillars specifically and doesn’t harm birds, pets, or beneficial insects like bees, which makes it a practical option for backyard trees.
Risks to Horses
Eastern tent caterpillars pose a specific and serious threat to pregnant mares. When horses accidentally eat the caterpillars in pasture grass, the tiny barbed hairs on the caterpillar’s body can penetrate the intestinal wall and enter the bloodstream, carrying bacteria with them. These bacteria-laden fragments spread throughout the body and can reach the uterus, fetal membranes, or fetal fluids, triggering what’s known as Mare Reproductive Loss Syndrome.
The mechanism is surprisingly potent. University of Kentucky researchers found that caterpillars weighing roughly one gram can cause abortion in a 1,500-pound mare within 32 hours. Affected horses often show no obvious signs of systemic illness because the number of infectious particles circulating at any given time is small, on the order of about ten per horse per day. The damage is concentrated in the reproductive system. MRLS can also cause eye inflammation, heart inflammation, and, less commonly, brain inflammation.
If you keep horses and have cherry or apple trees near pastures, removing tent caterpillar webs and egg masses, or fencing horses away from heavily infested trees during spring, reduces the risk substantially.
Skin Reactions in People
Most web-building caterpillar species in North America cause only mild skin irritation if you handle them directly. The hairs on their bodies can trigger itchy red bumps or a rash that typically resolves within hours. Wearing gloves when removing webs by hand prevents most reactions.
Some caterpillar species (not the common tent caterpillar or fall webworm, but others you might encounter) carry hairs that can cause more significant problems. The puss caterpillar, found in the southern U.S. and Central America, delivers a painful sting, and roughly one in three people who are stung develop systemic reactions including headache, muscle spasms, and breathing difficulty. Processionary caterpillars, found in parts of Europe, shed hairs that remain irritating in the environment for at least a year and can cause eye inflammation if transferred to the eyes by wind or contact.
For the tent caterpillars and webworms you’re most likely seeing in your yard, the risk to people is minimal. The real question is usually whether the tree needs help, and for most healthy trees, the answer is that a single season of webbing is cosmetically ugly but not a long-term problem.