What causes holes in a tree trunk?
Round, oval, or D-shaped holes in the bark, neat rows of shallow pits, sawdust collecting in bark crevices or at the base, or open cavities where limbs used to be.
Not an emergency this afternoon, but the window for a useful response is measured in weeks. Left alone, most of these get more expensive rather than less.
The shape and arrangement of the holes decide the urgency. Tidy horizontal rows are a bird and cost the tree almost nothing. Scattered round holes with sawdust mean insects are tunneling in the wood right now. And D-shaped holes in an ash mean emerald ash borer, which is a decision with a deadline rather than a problem to watch.
What this looks like
Hole shape maps to cause with unusual reliability. A perfect capital D about an eighth of an inch across is emerald ash borer and nothing else. Round holes a quarter inch or larger, with coarse sawdust below them, are longhorned or clearwing borers. Neat horizontal bands of quarter-inch pits, sometimes wrapping the whole trunk, are yellow-bellied sapsuckers. Irregular excavations with bark flaked away are woodpeckers hunting whatever is inside.
- Eighth-inch D-shaped holes on an ash trunk or limbs
- Evenly spaced horizontal rows of shallow round pits
- Fine sawdust or gritty frass packed in bark crevices or piled at the base
- Bark scaled off in patches leaving smooth blond wood exposed
What causes it in Kansas
Roughly in order of how often each turns out to be the answer here. The line under each one is what separates it from the others — open a cause for the detail.
1Emerald ash borer
How to tell: The tree is an ash, the hole is D-shaped and small, and the crown is thinning from the top.Adults leave a distinctive D-shaped exit hole when they emerge. By the time exit holes are easy to find from the ground, the tree has usually been infested for two or three years and has lost a meaningful part of its canopy.
2Roundheaded and flatheaded borers
How to tell: Round or oval holes a quarter inch or more, with coarse, fibrous frass that looks like wood shavings.Longhorned beetles and flatheaded borers attack stressed trees, particularly ones with sunscald, drought damage, or recent transplant stress. Cottonwood borer, locust borer, and flatheaded appletree borer all occur in Kansas.
3Clearwing borers
How to tell: Wet, gummy sawdust at the hole, most often low on the trunk or where a limb meets it.Ash-lilac borer, peachtree borer, and their relatives tunnel near the base and at branch unions, pushing out a mix of sawdust and gum.
4Sapsuckers
How to tell: The holes are in neat horizontal lines and consistent in size — no insect produces that pattern.A migratory woodpecker that drills orderly rows of shallow wells and returns to feed on the sap and the insects it attracts. Pine, maple, birch, and elm are frequent targets. Damage is cosmetic unless the rows are dense enough to girdle a stem.
5Woodpecker foraging
How to tell: Irregular flaked patches rather than drilled holes, concentrated on the upper trunk and larger limbs.Woodpeckers hunting borers strip bark in irregular patches, leaving pale wood exposed. On ash this 'blonding' is one of the earliest visible indications of emerald ash borer, often noticed before any exit hole.
6Carpenter ants and old cavities
How to tell: Coarse frass with insect body parts mixed in, ants trailing on the trunk, and wood that is soft where the ants are working.Carpenter ants excavate galleries in wood that is already soft and wet. They are an indicator of existing decay rather than its cause. Old limb losses and pruning stubs leave cavities that animals then enlarge.
What to do, and what to avoid
Safe to check yourself
All from the ground — no ladder, no saw.
- Identify the tree, then look at hole shape — those two facts answer most of the question
- Check whether frass is fresh: loose, pale, and accumulating means active tunneling now
- Note whether holes are scattered at random or arranged in rows
- Look up — holes plus a thinning crown is a very different situation from holes on an otherwise vigorous tree
What not to do
The responses that make this one worse or permanent.
- Do not plug holes with putty, caulk, or expanding foam
- Do not spray the trunk with a general insecticide after exit holes appear; the insects that made them have already left
- Do not attempt a do-it-yourself trunk injection, which wounds the tree repeatedly and rarely delivers a correct dose
- Do not move infested firewood, which is the main way borers travel between Kansas counties
When to contact an arborist
Whatever this narrows it down to, the assessment that decides what happens next has to happen at the tree. Why that is
Common questions
How do I know if it is emerald ash borer?
How do I know if it is emerald ash borer?
Three things together make it near-certain: the tree is an ash, the exit holes are D-shaped and about an eighth of an inch across, and the canopy is thinning from the top down. Woodpecker blonding and vertical bark splits over S-shaped tunnels round out the picture.
Should I stop the sapsucker?
Should I stop the sapsucker?
Reflective tape or burlap wrapped over the affected area during migration discourages them, and they are protected birds, so lethal control is not an option. On an otherwise healthy tree the wells are a cosmetic issue.
Do borers attack healthy trees?
Do borers attack healthy trees?
Most do not — they are drawn to trees stressed by drought, sunscald, or root damage, which is why watering and mulch are genuine borer prevention. Emerald ash borer is the exception: it attacks healthy ash of any vigor, which is what makes it so destructive.
Kansas Tree Guide is an independent educational resource. Nothing on this site replaces an on-site assessment by a qualified arborist or tree-care professional. Last reviewed August 2026.
Often seen alongside this
A second symptom narrows the cause faster than more detail about the first.
Not what you are seeing? Work the symptom guide again.