What Are the Worms That Hang From Trees?

Those dangling “worms” you see swinging from tree branches on invisible threads are almost always caterpillars, the larval stage of moths or butterflies. They are not worms in any biological sense. Several species produce silk from glands near their mouths and use it to lower themselves from the canopy, sometimes by the thousands during spring outbreaks. The sight can be alarming, but the biology behind it is genuinely interesting, and knowing which species you are looking at changes what you should expect and whether you need to do anything about it.

Why People Call Them Worms

The confusion is understandable. Caterpillars are soft-bodied, legless-looking at a glance, and wriggle when disturbed. Common names reinforce the mistake: inchworms, cankerworms, armyworms, silkworms, and bagworms all carry “worm” in their names despite being moth or butterfly larvae. In everyday English, anything small and squirmy that shows up uninvited on a tree gets called a worm. Biologically, though, these creatures belong to the order Lepidoptera and will eventually pupate into winged adults. True worms, like earthworms, belong to entirely different groups and have no business in treetops.

The Species You Are Most Likely Seeing

A handful of species account for the vast majority of caterpillar-dangling encounters in North America and Europe. Which one you are dealing with depends on the time of year, the type of tree, and what the caterpillar looks like up close.

Fall cankerworms and spring cankerworms are among the most common dangling caterpillars on broadleaved urban trees. They are small, pale green or brown inchworms (family Geometridae) that move in the distinctive looping gait of caterpillars with fewer prolegs. They feed on elm, oak, maple, and other hardwoods, and when populations spike they drop on silk threads by the millions, landing on cars, sidewalks, porches, and people. Female cankerworm moths are wingless and must crawl up tree trunks to lay eggs, a detail that matters for control.

Forest tent caterpillars are larger, bluish-black with keyhole-shaped white spots down their backs, and travel in groups. Despite their name, they do not actually build tents. They feed heavily on aspen and other poplars, and during outbreak years they can strip large tracts of forest bare. Gypsy moth caterpillars (now officially called spongy moth larvae) are hairy, dark-colored, and feed on oaks and many other hardwoods. Nearly all first-instar gypsy moth larvae disperse from egg masses by “ballooning” on silk threads carried by the wind, which is why they seem to appear on trees everywhere at once during outbreak springs.¹

Bagworms are an especially odd-looking variant. Their larvae build and carry portable cases made of silk and bits of foliage, so they look like tiny decorated bags hanging from branches. Research on bagworm silk has revealed a surprisingly sophisticated structure: the larvae spin ladder-like silk footholds to move along surfaces, laying down zigzag threads and selectively applying adhesive in an on-and-off pattern as needed.² In Europe, the pine processionary caterpillar is another highly visible species. It feeds on pine needles and marches in nose-to-tail lines, leaving communal silk nests in the trees.

How the Silk Thread Works

Caterpillars produce silk from spinnerets located just below the mouth. The silk hardens on contact with air, forming a thread that the caterpillar pays out as it drops. This thread serves as a lifeline, not just a convenient way down. When a caterpillar falls off a leaf because of wind, a predator, or a clumsy step, the silk catches it in midair. More than that, caterpillars can climb back up.

A study of 13 different moth species found that lifeline-climbing behavior is widespread across the Lepidoptera, appearing in at least five superfamilies. Most of the species tested climbed their silk threads using only their three pairs of thoracic legs (the small true legs near the head), alternating left and right with side-to-side body swings. Only one species, a geometrid moth, used a different approach, pulling the thread down with its front legs while looping its body upward.³ This tells us something about how fundamental the behavior is: species that look and move nothing alike on a branch all converge on roughly the same technique when dangling from silk.

In very young, very small caterpillars, silk threads serve a different purpose entirely. First-instar larvae of many species use silk to catch the wind and disperse away from their birthplace, a behavior called ballooning. This is the same trick many spider species use, and researchers have proposed that ballooning evolved from the habit of hanging from threads in the canopy, with wind doing the rest. The behavior appears to be an evolutionary strategy for colonizing new habitat when conditions are unpredictable.⁴

What Dangling Caterpillars Do to Trees

Most healthy trees survive a single year of caterpillar feeding without lasting damage. The real trouble comes with repeated defoliation, meaning several years of heavy leaf loss in a row. Forest tent caterpillars, for instance, feed in cyclic outbreaks that can strip aspens bare for multiple consecutive seasons. Research on aspen stands shows that tree mortality increases with the intensity of defoliation, and that growth remains reduced for at least a year after caterpillars stop feeding.⁵

Repeated defoliation also disrupts the tree’s internal chemistry. Studies on poplars have found that heavy defoliation decreases growth, increases the rate of branch dieback, and strongly suppresses the tree’s ability to take up nitrogen from the soil.⁶ A tree that cannot absorb nutrients efficiently is less equipped to regrow its leaves and recover, creating a feedback loop that weakens it further if the caterpillars return the next spring.

Trees are not entirely passive in this arrangement. Oaks and maples produce tannins and other defensive chemicals in their leaves. Research on gypsy moth caterpillars eating oak leaves found that while hydrolyzable tannins did reduce the caterpillars’ digestive efficiency, they were not enough on their own to stop the larvae from growing. Maple leaves, interestingly, produced more oxidative stress in the caterpillars’ guts and interfered more with protein digestion, suggesting that some tree species are better defended than others for reasons that go beyond a single chemical.⁷ Pine trees also respond to caterpillar feeding: when processionary caterpillars ate pine needles, the tree ramped up production of defensive polyphenolics in its stem, even though needle chemistry barely changed.⁸ The defense game between trees and caterpillars is specific and complex, not a simple arms race.

Why Outbreaks Happen in Some Years and Not Others

If you have lived in one place long enough, you have probably noticed that dangling caterpillars appear in plague-like numbers some springs and are nearly absent in others. Forest tent caterpillars, one of the best-studied outbreak species, follow roughly cyclical patterns that repeat every decade or so. Winter temperatures play a role in shaping these cycles. Outbreaks tend to be most common in areas where overwintering temperatures stay above extremely cold thresholds, while severe cold snaps can kill eggs and larvae, suppressing the next generation.⁹

Climate change appears to be altering these outbreak dynamics. A tree-ring analysis from Alberta found that forest tent caterpillar outbreaks shifted from a regular, synchronized cycle between 1850 and 1910 to a more chaotic, asynchronous pattern after 1910, coinciding with warmer winters across the region. The researchers proposed that warming has weakened the ability of cold winters to synchronize caterpillar populations across large areas, making outbreaks more localized and less predictable.¹⁰ Field data from higher-elevation sites supported the idea that cold matters: winter mortality of tent caterpillar eggs was significantly higher at lower elevations (counterintuitively, because cold air pools in valleys) and in the shrub layer compared to higher on the trees.¹¹

The practical upshot: if winters in your area have been getting milder, you may be seeing more caterpillar years, or at least differently timed ones, than previous generations did. Natural enemies like parasitic wasps and viral diseases also help crash populations once they peak, which is why bad caterpillar years rarely last more than a few seasons in a row before numbers collapse on their own.

Can They Hurt You

Most dangling caterpillars are harmless to people. Cankerworms, tent caterpillars, and bagworms are nuisance pests that might land in your hair or cover your patio furniture in silk and droppings, but they do not sting or bite. The exceptions are worth knowing. Some caterpillar species have evolved defensive hairs, spines, or venom-delivery systems that cause skin reactions ranging from mild rash to serious allergic responses. A dermatology review notes that caterpillars and moths can cause localized stings, papular dermatitis, and urticaria, though these reactions are uncommonly recognized in clinical practice.¹²

The pine processionary caterpillar is a particular concern in southern Europe. Its urticating hairs break off easily and can cause severe skin irritation, eye inflammation, and respiratory reactions in both people and pets. Brown-tail moth caterpillars, found in parts of New England and across Europe, carry similar irritating hairs that persist even after the caterpillars pupate, becoming airborne and landing on laundry, outdoor furniture, and skin. If you are dealing with a hairy caterpillar species, avoid handling them bare-handed and keep pets away.

What You Can Do About Them

Whether you need to act depends on the species and the severity. A few caterpillars dangling from a backyard tree in spring are part of normal life and feed songbirds that rely on them to raise their young. An outbreak that strips your trees bare for the second year running is a different story.

For cankerworms, one non-chemical approach is banding trees with sticky material in late fall or early spring. The idea is to trap the wingless female moths as they crawl up the trunk to lay eggs. Research has confirmed that this method can reduce larval numbers, though one study of urban trees found no clear benefit when defoliation was already low.¹³ In other words, banding works best when you are already dealing with a significant population, and it works best when neighbors participate too.

Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium that is toxic to caterpillars when they ingest it. It is the most widely recommended biological insecticide for caterpillar outbreaks on shade and landscape trees. For best results, Bt must be applied while larvae are still small, typically within the first couple of weeks of feeding, because it must be eaten directly from the leaf surface to work.¹⁴ Timing matters more than anything else with Bt. Spraying after caterpillars are large and leaves are half-eaten is largely a waste. For most homeowners, a strong blast from a garden hose can knock small caterpillars off a tree, and waiting for birds, wasps, and disease to catch up with the population is a reasonable strategy if you can tolerate the mess for a season.

What Caterpillar Droppings Do to the Forest Floor

During a heavy outbreak, caterpillar frass (droppings) rains down onto the forest floor in visible quantities. You might assume this would fertilize the soil, and the nutrient cycling story is partly correct but more complicated than it first appears. Research on gypsy moth frass found that while the droppings are rich in extractable nitrogen and easily decomposed carbon, the burst of microbial activity they trigger actually locks up most of that nitrogen rather than releasing it. Soil microbes grow rapidly on the labile carbon in frass and absorb the nitrogen as they multiply, beginning within a day and continuing for months. This immobilization response acts as a mechanism for conserving nitrogen within the forest rather than letting it wash away after defoliation.¹⁵

Canopy herbivory also changes what reaches the soil in other ways. Throughfall, the water that drips through chewed-up leaves, carries dissolved nutrients including phosphate and ammonium. Experimental additions of throughfall to forest plots led to increases in soil phosphate but decreases in nitrate, and removing litter and greenfall significantly reduced soil respiration compared to undisturbed plots.¹⁶ The net effect is that caterpillar outbreaks rearrange how nutrients flow through a forest, speeding up some processes and slowing others, rather than simply enriching or depleting the soil.

When Dangling Caterpillars Caused a Public Panic

Caterpillar outbreaks have been alarming people for centuries. In the spring of 1782, London and the surrounding countryside were engulfed by webs teeming with caterpillars and eggs. The culprit was the brown-tail moth, a species native to Britain, but at the time almost nobody recognized it. Newspapers filled with warnings that the insects had brought disease upon the city, and the outbreak was remembered for decades as an illustration of the threat insects posed to society.¹⁷ The brown-tail moth does in fact carry urticating hairs that cause skin rashes, which likely fueled the disease fears. But the broader panic reflected something that has not changed much in 240 years: most people have no framework for identifying caterpillars, so an outbreak that produces millions of tiny dangling creatures feels alien and threatening in a way that, say, a bumper year for songbirds never would.

If you see those threads glinting in the sunlight this spring, you are watching one of the more remarkable small dramas in your local ecosystem. The caterpillar on the end of each thread is executing a behavior refined across hundreds of millions of years of insect evolution, using a material stronger per unit weight than steel, to solve a problem as old as life in the canopy: how to fall without dying.