The silk webs you’re seeing in trees are almost certainly made by one of two common caterpillar species: eastern tent caterpillars or fall webworms. Both spin conspicuous silk structures in deciduous trees, but they show up at different times of year and build their webs in different spots. A third group, ermine moth caterpillars, can produce even more dramatic webbing in parts of Europe and occasionally North America. In most cases, these webs look worse than they are.
Eastern Tent Caterpillars vs. Fall Webworms
These two species account for the vast majority of webbed nests in North American trees, and telling them apart is straightforward once you know two things: when the web appeared and where it sits on the tree.
Eastern tent caterpillars are the spring species. They hatch from egg masses in early spring and immediately begin building a dense silk tent in a fork between branches, usually in the tree’s interior. The tent grows larger as the caterpillars grow. They leave the tent during the day to feed on leaves, then return to it to rest and molt. Their preferred hosts include cherry, apple, and crabapple trees.
Fall webworms show up in late summer and early fall. Instead of building a central tent, they spin loose, messy webs over the tips of branches, enclosing the leaves they’re feeding on. As they eat through one section, they expand the web outward to cover more foliage. You’ll find them on a wide range of hardwoods, including walnut, hickory, birch, and elm. Their webs tend to look more ragged and sprawling than the tighter tents of the spring species.
If you’re seeing webs in May or June in branch forks: tent caterpillars. If you’re seeing webs at branch tips in August or September: fall webworms.
Ermine Moth Caterpillars
If the webbing is extraordinarily extensive, covering entire branches or even the whole tree in ghostly white silk, you may be looking at ermine moth caterpillars. Eight species of small ermine moths produce silk webbing, though three are responsible for the most dramatic displays. These caterpillars primarily feed on blackthorn, hawthorn, spindle, and bird cherry trees, and their webs typically appear from May through June.
Ermine moth colonies can contain tens of thousands of caterpillars. The bird cherry ermine, which has a more northern range, occasionally covers entire trees so completely that the stripped, silk-wrapped branches give the tree an eerie, haunted look. Sometimes the webs even spread onto nearby objects like benches, fences, and gravestones. Despite the alarming appearance, these webs and caterpillars are harmless. The webbing is a defensive strategy: living in huge groups provides safety in numbers, and the silk adds a physical barrier against predators.
Why Caterpillars Build Silk Webs
Silk webs serve two main purposes. First, they provide physical protection from predators. Birds, parasitic wasps, assassin bugs, lacewings, and spiders all prey on caterpillars, so a layer of silk creates a barrier that many of these enemies can’t easily penetrate. Second, the web acts as a home base. Tent caterpillars, for instance, leave their tent to feed on surrounding foliage and then return to it to rest, digest, and molt. The silk is produced by glands in the caterpillar’s head.
In some species, communal webs also help with thermoregulation. A silk tent in a sunny branch fork traps warmth in the cool mornings of early spring, letting the caterpillars raise their body temperature faster so they can begin feeding earlier in the day.
Will They Kill Your Tree?
In most cases, no. Healthy trees can survive two to three consecutive years of defoliation. Trees that lose their leaves early in the season often grow a new, smaller set of leaves by July once the caterpillars have finished feeding and moved on to their pupal stage.
That said, repeated defoliation does take a toll. It reduces the tree’s overall energy reserves and resilience, making it more vulnerable to drought, flooding, other pests, and disease. According to the New York State Department of Environmental Conservation, stress caused by defoliation can contribute to tree mortality eight to ten years after the event, particularly if the tree was already weakened by other factors. A single defoliation event on a healthy tree is rarely a serious concern. A tree that’s already stressed by poor soil, drought, or disease is more vulnerable and may benefit from intervention.
How to Remove Caterpillar Webs
Physical Removal
The simplest approach is pulling or pruning the webs out by hand. For larger webs, you can insert a stick into the nest and twist it to wind up the silk and caterpillars, then drop the mass into a bucket of soapy water. Pruning out smaller infested branches works well for tent caterpillars, whose nests sit in branch forks that are easy to reach. For fall webworms at branch tips, you can often just tear open the web to expose the caterpillars to birds and other predators.
With tent caterpillars specifically, you can reduce next year’s population by looking for egg masses on branches during winter. These are dark, shiny bands that wrap around small twigs. Scraping or pruning them off before spring eliminates the colony before it starts.
Biological Control
A naturally occurring soil bacterium called Bt (specifically the kurstaki strain) is the most targeted biological option. When caterpillars eat leaves sprayed with Bt, it disrupts their digestive system, causing them to stop feeding within hours and die of starvation over the next few days. It’s selective, killing caterpillars while remaining safe for birds, mammals, and beneficial insects like bees.
Timing matters. Bt works best when the caterpillars are small and in early larval stages. Once they’ve grown large or sealed themselves inside dense webs, the spray can’t reach the foliage they’re eating. For tent caterpillars, apply it in early spring as soon as you see small tents forming. For fall webworms, spray when the first loose webs appear in late summer. Bt also breaks down quickly in sunlight, degrading within three to seven days, so you may need a second application if caterpillars are hatching in waves.
What Not to Do
Burning webs with a torch is an old folk remedy that causes far more damage to the tree than the caterpillars ever would. The flames scorch bark and living tissue, creating wounds that invite disease and decay. Stick with physical removal or Bt.
Natural Predators That Help
Web-making caterpillars have a long list of natural enemies that keep their populations in check most years. Birds are major predators, especially during nesting season when adults need protein-rich food for their young. Cuckoos are particularly effective at eating hairy caterpillars that other birds avoid.
Several types of parasitic wasps lay their eggs inside or on caterpillars. The wasp larvae feed on the caterpillar from within, eventually killing it. Tachinid flies do the same. Assassin bugs, lacewing larvae, and spiders round out the predator guild. These natural controls are the main reason most caterpillar outbreaks collapse on their own after a year or two. Avoiding broad-spectrum insecticide sprays helps keep these beneficial predators around to do their work.