Quick guide to what made the hole
Size, shape, pattern and location all help narrow it down. Treat this as a starting point, since several causes can overlap on one tree.
| What you see | Common cause |
|---|---|
| Tiny pinhead to pencil-sized round holes, scattered or in rows | Wood-boring insects, or sapsuckers if the rows are neat |
| D-shaped or round exit holes, from very small (emerald ash borer’s are only about 1/8 inch) to pencil-sized, often with sawdust | Wood-boring beetles (some borers leave D-shaped holes) |
| Round or oval hole about the size of a coin to a fist, clean edges | Woodpecker nest or foraging cavity |
| Irregular opening, ragged edges, bark missing around it | Wound, broken branch, old pruning cut or decay |
| Opening to a hollow, often high up or at a branch union | Cavity formed by decay, used by squirrels, birds or other wildlife |
Holes made by decay and wounds
Many large holes in tree trunks are not made by any animal. They form when a branch breaks or is pruned badly, or when the trunk is hit by a mower, vehicle, lightning or storm. Decay fungi then enter the wound and slowly rot the wood inside. Arborists often describe the result as a cavity.
Trees respond to wounds by walling off damaged wood, a process commonly called compartmentalization (CODIT). This helps many trees live for years or decades with some internal decay, but it does not repair the damaged wood. The more wood that is lost, the less strength the trunk has.
Borers: what the holes and insects look like
“Borer” covers many different insects, mostly the larvae of beetles or moths that tunnel under the bark or into the wood. Adults are often beetles, and larvae are usually pale, legless or short-legged grubs with a darker head. Signs of borers include:
- Small round or D-shaped exit holes in the bark
- Sawdust-like frass at the base or in bark crevices
- Sap oozing from small holes
- Thinning crown, branch dieback or bark that splits
- Rows of holes on trunks of stressed trees
Borers tend to target trees already under stress from drought, root damage, injury or disease, though some, such as emerald ash borer, attack healthy trees of particular species. Because a tree’s health is often the underlying issue, check watering, soil conditions and roots (see root rot). Wood-eating termites are another possibility in decayed trunks, covered in termites in a tree.
Woodpeckers and sapsuckers
Woodpecker holes are usually round and clean-edged. Nest cavities are fairly deep, with an entrance sized to the bird, from a little over an inch across for small species to larger ovals for big ones like pileated woodpeckers. Woodpeckers also peck at bark to feed on insects, which can leave irregular patches or many shallow holes. Woodpeckers feeding heavily in a tree sometimes means insects are present in or under the bark. To judge whether the tree is still alive, see how to tell if a tree is dead or dying.
Sapsuckers drill neat horizontal or vertical rows of small, shallow holes to feed on sap. This usually does not kill a healthy tree, though repeated girdling of a small tree can harm it. Woodpeckers and many other birds are legally protected in many regions, so do not disturb active nests.
Do squirrels dig holes in trees?
Squirrels generally do not excavate large holes in solid wood themselves. They more commonly use existing cavities, woodpecker holes and decayed hollows, and they may enlarge an opening by gnawing around the edge. Squirrels also strip bark from branches, which is a different type of damage. If a hole in your tree has chewed edges and nesting material nearby, a squirrel or other animal is probably using it.
Should you fill the hole?
Older advice often recommended filling cavities with concrete, foam or cement. Modern arborist guidance generally advises against this. Filling can trap moisture, interfere with the tree’s own sealing response, and add rigid material to a trunk that needs to flex in wind.
In most cases:
- Leave the cavity open unless an arborist advises otherwise
- Do not paint wound dressings or sealants on the tree, which are usually not recommended
- Remove loose, broken wood cleanly only if an arborist says to
- If water collects, an arborist can advise on options, since drilling drain holes can cause more harm
Is the tree safe? Warning signs
A hollow trunk is not automatically dangerous, and many trees with cavities remain stable. The risk depends on how much sound wood is left around the hole. Our healthy vs unhealthy tree guide shows other warning signs to check alongside it. Get a certified arborist to assess the tree if you notice:
- The hole is large compared with the width of the trunk, or there are several
- Mushrooms or brackets growing from the trunk or base
- Cracks running up or down from the hole
- A leaning trunk or soil lifting at the base
- Dead branches, a thin crown or sudden crown dieback
- The tree is near a house, play area, driveway or power line
What to do next
- Photograph the hole, the surrounding bark and the whole tree.
- Look for insects, frass, droppings, nesting materials or sap, which point to the culprit.
- Check watering, soil compaction and recent disturbance, as stress often lies behind borers and decay.
- Avoid DIY filling or sealing.
- If the tree is large, near structures, or you notice any warning signs above, have a certified arborist inspect it. Sticky drips from the bark are not always from the hole itself, so see sticky stuff falling from trees for other possibilities.