Holes in trees come from a surprisingly wide range of causes, and each one leaves a different kind of mark. Insects bore tunnels through wood, birds chisel out cavities, fungi slowly dissolve the heartwood from within, fire burns openings at the base, and bacterial infections weaken tissue until it collapses. Knowing which cause you’re looking at matters, because a neat row of small holes on a birch trunk tells a completely different story than a gaping hollow at the base of an old oak.
Wood-Boring Insects
Beetles are among the most common culprits behind small, round holes in bark and wood. Dozens of beetle families contain species whose larvae develop inside living or recently dead trees. The adult female typically lays eggs in crevices or under bark, and the hatching larvae chew their way deeper into the wood, feeding as they grow. When they finally mature, they tunnel back toward the surface and emerge through small circular exit holes, often reducing the wood beneath the bark to powder in the process.1PLOS ONE. Wood-boring beetles associated with Acacia xanthophloea in Nairobi and Machakos Counties, Kenya These exit holes are what most people notice first, but by the time they appear, the damage inside the tree can be extensive.
The size of the hole gives you a rough clue about what made it. Bark beetles leave tiny holes, sometimes barely a millimetre across. Longhorn beetles and metallic wood borers tend to leave larger openings, from a few millimetres up to about a centimetre. Carpenter ants, which aren’t beetles but are frequently blamed for tree holes, don’t actually eat wood. They excavate galleries for nesting and push out frass, leaving relatively clean oval openings. If you see fine sawdust at the base of a tree, it’s worth looking up to find where the holes are and how large they are before assuming the worst.
This relationship between insects and wood is ancient. The oldest known wood-boring lineages include cockroaches, which may have started tunneling into woody plant tissue as far back as the Carboniferous period, over 300 million years ago. Termites and wood-roaches evolved as obligate wood-feeders during the Jurassic, developing tough mandibles and digestive systems specifically adapted to break down cellulose. Their boring traces have been found in plant fossils dating to the Cretaceous and beyond.2Entomologia Experimentalis et Applicata. Evidence From the Fossil Record About the Evolution of Insect Adaptations to Wood Boring Trees and boring insects have been locked in this arms race for a very long time.
Woodpeckers, Sapsuckers, and Other Birds
Woodpecker holes are easy to spot once you know the pattern. Excavating species chisel oval or round cavities into trunks and large limbs, typically choosing wood that has already begun to soften from fungal decay. The holes they create for nesting can be quite large, with entrance diameters of several centimetres, and the internal chamber is usually wider than the opening. Woodpeckers, barbets, and similar excavators are considered keystone species in forest ecosystems because their abandoned cavities become homes for a long list of secondary users, from small owls to tree squirrels.3Acta Ornithologica. Sequential Use of Tree Cavities by Birds and Nest Web in a Riparian Forest in Southwest India
Sapsuckers, a group within the woodpecker family, create a very different kind of hole. Instead of excavating nest cavities, they drill small, shallow wells through the bark to reach the sap-conducting tissue beneath. Each hole is used for a few days before the bird moves on and drills a new one slightly above the last. The result is a distinctive grid or vertical column of small holes, often clustered tightly together on a single spot on the trunk. Research on yellow-bellied sapsuckers found that each new sap hole was enlarged for an average of about three days before being abandoned for a fresh one. The tight clustering appears to be deliberate: by girdling a narrow band of the tree’s phloem with successive holes, the sapsucker causes sap to pool in the surrounding bark, effectively farming its own food source throughout the season.4The Auk. Use and Selection of Sap Trees by Yellow-Bellied Sapsuckers
If you see neat horizontal rows of shallow holes on a birch, maple, or apple tree, sapsuckers are almost certainly the cause. The holes rarely kill a healthy tree, but they can weaken branches and provide entry points for fungal infection over time. Wrapping hardware cloth around a favored trunk section during spring migration can discourage them without harming the bird.
Fungal Decay From the Inside Out
Fungi are responsible for the largest holes in trees, the kind you can stick your arm into or, in ancient specimens, stand inside. Unlike insects and birds, which create holes mechanically, decay fungi dissolve wood chemically. White-rot fungi break down both cellulose and lignin, eventually leaving behind a soft, spongy residue. Brown-rot fungi target cellulose while leaving lignin largely intact, producing crumbly, cuboid fragments. Either way, years or decades of fungal activity can hollow out a trunk’s heartwood while the living sapwood around it continues to function normally.
How fast this happens varies enormously by species. When researchers experimentally inoculated living red oaks with two different decay fungi, one species extended through the wood at roughly twice the rate of the other, with the faster fungus advancing about 29 centimetres per year vertically and the slower one about 15 centimetres per year.5Global Ecology and Conservation. Making hollow trees: Inoculating living trees with wood-decay fungi for the conservation of threatened taxa That kind of variation means two trees of the same species, infected at the same time by different fungi, could end up with dramatically different cavity sizes a decade later.
Favorable cavity sites are usually created by fungi, so they’re more common in older, dying trees that are incompatible with intensive timber production.6ISRN Forestry. Sustaining Cavity-Using Species: Patterns of Cavity Use and Implications to Forest Management That tension between forestry and wildlife conservation is one of the main reasons ecologists push to retain old-growth trees and standing dead wood in managed forests. Removing every hollow tree removes the habitat that dozens of species depend on.
Fire Damage and Basal Hollows
Fire creates holes in trees in a way that’s distinct from everything else on this list: it burns through the bark and sapwood at the base, where fuel accumulates and heat concentrates. The resulting opening, called a basal hollow or fire scar, typically faces the direction from which the fire approached. Over time, repeated burns enlarge the cavity.
A study of large eucalyptus trees found that frequent burning substantially increased the likelihood of basal hollows. Among the smallest size class examined (trees about one metre in diameter), roughly 31 percent of trees in frequently burned stands had basal hollows, compared with only 17 to 21 percent in stands that burned less often.7Forest Ecology and Management. Does high fire frequency compromise structural integrity of a large eucalypt? Frequent fire also increased the size of those hollows and accelerated their initial development. The concern is that this damage eventually causes large trees to fall, reducing the population of the very old, very large individuals that provide the most ecologically valuable habitat.
If you see a blackened, charred opening at the base of a tree, fire is the obvious explanation. But fire scars also become entry points for fungal decay, so a cavity that started as a burn wound often grows much larger through secondary infection. Separating the original fire damage from later fungal expansion usually requires looking at the char pattern on the remaining wood inside the cavity.
Bacterial Wetwood and Other Infections
Not all internal damage comes from fungi. Bacterial infections can also soften and destroy wood tissue, though they tend to work differently. Wetwood, sometimes called slime flux, is a condition where bacteria colonize the heartwood and produce gases that build up pressure inside the trunk. The result is often a dark, foul-smelling liquid that seeps through cracks or wounds. Over time, the infected wood becomes waterlogged and weakened.
In silver birch, researchers identified the bacterium Enterobacter nimipressuralis as the primary agent behind wetwood through artificial infection experiments. It produced the highest numbers of colony-forming units in bark and cambial tissue, and was even found in small quantities as part of the normal microbiome in healthy birch trees.8Folia Forestalia Polonica. Bacterial wetwood of silver birch (Betula pendula roth): symptomology, etiology and pathogenesis That last detail is interesting: the same bacterium that causes disease appears to exist harmlessly in healthy trees at low levels, becoming a problem only when conditions shift in its favor, such as after a wound or during drought stress.
Bacterial wetwood rarely creates visible external holes on its own, but the internal damage it causes can weaken wood enough that wind, ice, or secondary organisms finish the job and open up visible cavities. If you see dark staining or oozing liquid from a crack in a trunk, bacterial wetwood is a strong possibility.
How Trees Fight Back
Trees can’t run from threats, so they wall them off instead. When a tree is wounded, whether by a broken branch, an insect bore hole, or a pruning cut, it activates a defense process that researchers describe using the CODIT model, which stands for Compartmentalization of Damage/Dysfunction in Trees. The tree essentially builds a series of chemical and physical barriers around the injured area to prevent decay organisms from spreading into healthy wood. These barriers form a three-dimensional lattice of defensive compounds through the living cells of the xylem, creating “walls” that existed before the wound, walls that form during the wounding event, and walls that develop afterward.9PubMed Central. Using the CODIT model to explain secondary metabolites of xylem in defence systems of temperate trees against decay fungi
This is why pruning cuts heal over cleanly on a vigorous young tree but may become entry points for decay on an old or stressed one. The speed and effectiveness of compartmentalization vary by species, age, and health. Oaks and beeches tend to compartmentalize well; willows and poplars, less so. A tree that compartmentalizes poorly after a wound is more likely to develop the large internal cavities associated with fungal decay. This also explains why topping a tree (cutting the main trunk back to a stub) is so damaging: the massive wound overwhelms the tree’s ability to wall off the exposed wood, inviting decay deep into the trunk.
Why Cavities Matter to Wildlife
From a human perspective, holes in trees often look like damage that needs to be fixed. From an ecological perspective, they’re some of the most valuable real estate in a forest. Many bird and mammal species depend on tree cavities for nesting and roosting, and the availability of suitable cavities often limits their populations more than food does.6ISRN Forestry. Sustaining Cavity-Using Species: Patterns of Cavity Use and Implications to Forest Management
Ecologists describe this interdependence as a “nest web.” Excavating species like woodpeckers and barbets create fresh cavities each breeding season, and when they move on, a cascade of secondary users moves in. Studies in riparian forests in India documented this pattern clearly: woodpeckers and barbets served as the primary excavators, while smaller secondary cavity-nesting birds relied on those excavated holes. Larger birds, meanwhile, tended to use decay-formed cavities that no single excavator had created.3Acta Ornithologica. Sequential Use of Tree Cavities by Birds and Nest Web in a Riparian Forest in Southwest India Remove the excavators, and the smaller species lose nesting sites. Remove the old decaying trees, and the larger species lose theirs.
Even water-filled tree holes support their own miniature ecosystems. These small pools that form in branch crotches or broken stubs host insect larvae, tiny crustaceans, and other organisms that feed on decaying leaf litter. Researchers have studied water-filled tree holes for over a century as model systems in ecology, finding that both the size of the hole and the amount of organic matter it collects positively affect the abundance and diversity of species living inside.10PubMed Central. Aquatic islands in the sky: 100 years of research on water-filled tree holes A single tree with several water-filled holes can host entirely different communities in each one.
The Microclimate Inside a Tree Cavity
The shape and features of a cavity directly influence how useful it is to wildlife. Cavity characteristics like entrance-hole size, wall thickness, chamber volume, and the number of openings all affect the internal temperature. Cavities with thick walls and small entrances stay warmer in winter and cooler in summer, providing a buffered microclimate. Cavities with multiple entrance holes or very large openings tend to have less stable internal temperatures because of greater air exchange with the outside.11Ecological Modelling. A dynamic thermal model for predicting internal temperature of tree cavities and nest boxes
This is why animals are choosy about which cavities they use. A nesting owl or a hibernating bat isn’t just looking for “a hole.” It’s looking for a hole with the right entrance size (small enough to exclude predators), the right depth (deep enough for insulation), and the right wall thickness (thick enough to buffer temperature swings). Artificial nest boxes try to replicate these features, but they almost always have thinner walls than a natural cavity, which means more extreme temperature swings inside. For species in hot climates, that difference can be the margin between chick survival and heat death.
How Arborists Assess Hidden Decay
The trickiest holes are the ones you can’t see. A tree can appear perfectly healthy from the outside while its heartwood has been substantially hollowed by decay. Arborists use several non-invasive tools to detect this kind of hidden damage before a tree fails.
Sonic tomography is one of the most widely used methods. Sensors are placed around the trunk, and small taps generate sound waves that travel through the wood. Solid wood transmits sound quickly; decayed or hollow wood slows it down. The resulting tomogram shows a cross-sectional map of the trunk’s internal condition. Studies testing sonic tomography on hundreds of trees across dozens of species have found that the depth of compromised wood shown on the tomograms correlates well with physical verification from resistance micro-drilling, a method where a thin drill bit is pushed through the wood and the resistance recorded at each millimetre.12PubMed Central. Reliability of acoustic tomography and ground‐penetrating radar for tree decay detection Ground-penetrating radar provides another option, bouncing electromagnetic pulses into the trunk and reading the reflections.
These technologies provide replicable estimates of internal decay and cavities across trees of varying shapes, wood densities, and bark thicknesses.13PubMed Central. Use of sonic tomography to detect and quantify wood decay in living trees For homeowners, the practical takeaway is that if an arborist suspects internal decay in a large tree near a house or walkway, they can get a fairly reliable picture of what’s happening inside without cutting the tree down. A tree with a hollow core isn’t automatically dangerous; many hollow trees remain structurally sound for decades because the outer shell of living wood is strong enough to bear the mechanical load. But knowing how much sound wood remains, and where, is what separates an informed decision from a guess.
Telling the Causes Apart
When you’re standing in front of a tree with holes, a few quick observations narrow the field considerably:
- Tiny round exit holes: wood-boring beetles, especially if fine sawdust (frass) is visible nearby. The holes are typically under half a centimetre and scattered across the bark.
- Neat rows of shallow wells: sapsucker feeding. Look for horizontal lines of small holes, often on birch, maple, or fruit trees.
- A single large oval opening: woodpecker nest cavity, usually in a section of trunk where the wood was already softened by decay.
- A wide charred opening at the base: fire scar, often on the side that faced the fire. May be enlarged by subsequent fungal decay.
- Soft, spongy wood inside a large cavity: fungal heartwood decay, the main cause of naturally formed hollows in older trees.
- Dark oozing liquid from a crack: bacterial wetwood. The wood inside is waterlogged and may smell sour.
Multiple causes frequently overlap. A storm breaks a branch, exposing bare wood. Fungi colonize the wound. A woodpecker, attracted to the softened wood, excavates a nest cavity. Years later, the cavity has expanded through continued decay, and a screech owl moves in. Understanding holes in trees means accepting that most of them are collaborative projects between physics, biology, and time.