The “worms” you’re seeing on or in your trees are almost certainly caterpillars, beetle larvae, or bagworms, and identifying them comes down to three things: where on the tree they are, what kind of webbing or damage they leave behind, and what the larvae themselves look like. Most tree worms fall into a handful of common species, and each one has telltale signs that make identification straightforward once you know what to look for.
Start With the Webbing
If your tree has visible silk webbing, its location instantly narrows the possibilities. Eastern tent caterpillars build webs in the center of a tree’s canopy, typically in the crooks where branches meet the trunk. You’ll spot these in late spring through early summer, most often on wild cherry, crabapple, and apple trees. The caterpillars themselves are black with a gold or white stripe running down the back.
Fall webworms look similar at first glance but behave differently. Their webs appear at the tips of branches rather than in the center of the tree, and they show up later in the year, from late summer into fall. Inside those branch-tip webs you’ll find masses of tiny, hairy caterpillars. Fall webworms are generalists: they feed on over 600 species of trees and shrubs, though pecan, sourwood, and persimmon are favorites.
If there’s no webbing at all but you’re finding diamond-shaped silk bags hanging from twigs, you’re looking at bagworms. These are easy to miss because the larvae camouflage their bags with bits of the host tree. The bags range from an eighth of an inch to two inches long and hang from branches of conifers like arborvitae, Leyland cypress, and juniper. Each bag contains a single worm that lives and feeds from inside its portable shelter.
Forest Tent Caterpillars
Despite the name, forest tent caterpillars don’t actually build tents. They’re one of the most common defoliators of hardwood trees, particularly oaks and gums. Mature larvae are about two and a quarter inches long with dark bluish bodies, yellow lines running along each side, and a distinctive row of white keyhole- or footprint-shaped spots down the middle of the back. Those white markings are the easiest way to tell them apart from eastern tent caterpillars, which have a continuous stripe instead of individual spots.
Outbreaks tend to be periodic and localized. In a bad year, these caterpillars can strip a tree’s canopy almost completely, but healthy trees usually recover if defoliation doesn’t happen several years running. Repeated defoliation over consecutive seasons is what creates real danger: trees rely on stored starches and sugars to produce new leaves each spring, and when caterpillars strip them year after year, those reserves can become depleted to a point the tree can’t bounce back.
Spongy Moth Caterpillars
Formerly called gypsy moths, spongy moth caterpillars are an invasive species and one of the most destructive leaf-eaters in North America. They feed on oaks, crabapple, linden, willow, birch, aspen, and more than 300 other tree species. The caterpillars hatch in April or May and feed through July.
Identification is reliable once the larvae are a few weeks old. Look for a fuzzy, hairy caterpillar with five pairs of blue bumps followed by six pairs of red bumps running down its back. That blue-then-red bump pattern is unique to spongy moths and the single best field marker. Very young caterpillars, just a week or two after hatching, appear mostly black and don’t yet show the colored bumps clearly. During outbreaks, spongy moths can strip entire trees seemingly overnight. They do not build silk tents, so if you’re seeing bare branches with no webbing, this species is a strong possibility.
Wood-Boring Larvae
Not all tree worms feed on leaves. Some tunnel inside the wood itself, and you’ll never see the larvae directly. Instead, you identify them by the evidence they leave behind: exit holes and frass (the sawdust-like mixture of wood fragments and waste they push out of their tunnels).
Powderpost beetle larvae leave tiny, perfectly round exit holes so small that only the tip of a ballpoint pen fits inside. The frass they produce is extremely fine, with the consistency of baking flour or talcum powder. You might notice small piles of this dust on surfaces below infested wood.
Deathwatch beetle larvae create exit holes of varying sizes, which helps distinguish them from powderpost beetles, whose holes are all uniform. Their frass has a slightly grittier texture, made up of very small pellets, but still finer than what false powderpost beetles produce. These pellets are packed tightly inside the feeding tunnels and only become visible when larvae or adults push them out.
False powderpost beetle larvae make the largest exit holes of the three groups. The entire point of a pen typically fits inside the hole, and the frass is difficult to dislodge from it. Their waste has the coarsest texture, similar to powdered borax soap, and is packed extremely tightly into the tunnels. If you’re finding gritty, coarse sawdust and relatively large holes, false powderpost beetles are the likely culprit.
Quick Identification Guide
- Silk webs in branch crotches (spring): Eastern tent caterpillar. Black body, white or gold stripe down the back.
- Silk webs at branch tips (late summer/fall): Fall webworm. Small, hairy caterpillars in large communal webs.
- Diamond-shaped bags on conifer twigs: Bagworm. Camouflaged with plant material, one larva per bag.
- No webbing, keyhole-shaped white spots on back: Forest tent caterpillar. Bluish body, yellow side stripes.
- No webbing, blue then red bumps down the back: Spongy moth caterpillar. Hairy, feeds on 300+ tree species.
- Exit holes and sawdust in wood: Wood-boring beetle larvae. Hole size and frass texture identify the species.
Managing Leaf-Eating Species
For the caterpillar species that feed on foliage, timing matters far more than the product you use. A naturally occurring soil bacterium (sold as Bt) is one of the most effective biological controls, but it only works when caterpillars eat it. That means applying it while larvae are still small and actively feeding. Once caterpillars are large and nearing maturity, Bt is unlikely to have much effect. Targeting early-instar larvae, within the first couple weeks after hatching, gives the best results.
For bagworms, hand-picking the bags off branches during fall and winter (before eggs hatch in spring) is surprisingly effective on smaller trees. Each bag can contain hundreds of eggs, so removing even a few makes a real difference. On larger trees or during heavy infestations, spraying while larvae are young and still emerging from their bags gives the best chance of control.
Healthy, well-watered trees can tolerate a surprising amount of leaf loss in a single season. The real risk comes from repeated defoliation year after year, which drains the stored energy reserves a tree depends on to leaf out each spring. If your tree has been heavily stripped for two or more consecutive years, it’s under serious stress and worth having evaluated by an arborist.