Holes in a tree trunk come from three different sources, and only one of them is usually a real problem on its own. Neat rows of shallow holes are sapsucker work and rarely threaten a healthy tree. Larger, scattered excavations are woodpeckers hunting insects already living under the bark. Small round or D-shaped holes, especially with sawdust-like frass around them, point to wood-boring insects, and those can matter a lot depending on the species and the tree’s condition.
The pattern tells you almost everything. Before you assume the worst, or ignore something that needs attention, look closely at the shape, spacing, and size of the holes and whether anything is coming out of them.
Three Patterns, Three Causes
Grab a phone photo close up before you do anything else. The differences are visible from a few feet away once you know what to look for.
| What you see | Likely cause | How worried to be |
|---|---|---|
| Neat horizontal or vertical rows of shallow holes, evenly spaced | Sapsucker (a type of woodpecker) | Usually cosmetic on a healthy tree |
| Larger, ragged holes in irregular clusters or patches | Woodpecker feeding on insects under the bark | The insects are the real issue, not the bird |
| Small round holes (1/8 inch or less) or D-shaped holes, often with fine sawdust (frass) or sap oozing | Wood-boring insect (borer) | Depends on the species and the tree’s overall health |
Sapsucker Holes: Neat Rows That Rarely Kill a Healthy Tree
Yellow-bellied sapsuckers and their relatives drill tidy rows of shallow holes to feed on sap and the insects it attracts, then return to the same holes to keep them flowing. The rows are the giveaway: evenly spaced, often in horizontal or vertical bands, wrapping partway around the trunk or major limbs.
A tree in good vigor usually tolerates light sapsucker activity and seals the wounds over time. But the damage is not always trivial: university extension research on sapsucker damage notes that heavy, repeated drilling can strip large patches of bark, and girdling around a limb or the trunk from sustained feeding is capable of killing a tree. Sapsuckers favor thin-barked species such as birch, maple, and fruit trees, and they tend to return to the same tree and the same rows year after year once they start. If the rows are limited to one or two bands and the canopy looks full, watch it. If the rows are spreading, multiplying, or wrapping most of the way around a limb, that is worth a closer look.
Woodpecker Damage: A Sign of Insects, Not the Root Problem
Woodpeckers other than sapsuckers do not drill for sap. Most of the time they are hunting insects already living under the bark, though not always. Some species also excavate nest cavities or drum on sound wood for other reasons, so a single hole is not automatic proof of insects. What does point toward an infestation is heavy, concentrated feeding activity on one section of trunk: irregular, ragged holes, sometimes stripping bark away in patches, focused on a spot the bird keeps coming back to.
Treat that pattern as a signal to look closer at that section of trunk, not as the problem itself. The bird found something. Your job is to find out what.
Borer Holes: Small, Round or D-Shaped, and the Ones That Matter
Wood-boring insects spend most of their life cycle as larvae tunneling under the bark, invisible until they emerge as adults through small exit holes. Round holes usually mean roundheaded borers; oval or D-shaped holes usually mean flatheaded borers, a group that includes metallic wood-boring beetles such as the bronze birch borer. According to Utah State University Extension, flatheaded borers typically attack around pre-existing damage on a weakened tree rather than sound, healthy wood, which is why a stressed or recently transplanted tree is at more risk than one growing vigorously.
Signs to look for beyond the holes themselves, per University of Maryland Extension:
- Fine, sometimes reddish, sawdust-like frass caught in bark crevices or piled at the base of the trunk
- Sap or sticky, gummy resin (gummosis) oozing from small wounds on the bark
- Branch dieback, a thinning canopy, or bark that is cracking, lifting, or flaking away
- Lumpy or water-soaked patches of bark over active larval feeding
Which Borers Show Up in Treasure Valley Trees
Flatheaded borers are common opportunists here. They target trees under stress from Boise’s hot, dry summers, sun-scalded bark on the south and west sides of trunks, and recent transplants that have not fully rooted in. A tree in good vigor with consistent water is far less likely to draw an infestation, and keeping a tree healthy is the management advice extension entomologists give most often, since insecticide is a secondary tool at best. If your tree already shows wilting or scorched leaves in summer, that stress is exactly what draws a borer’s attention; our guide on drought stress in Boise trees covers the signs and what recovery actually looks like.
Birch trees in the valley face a more serious threat from the bronze birch borer, another flatheaded (metallic wood-boring) beetle that specifically targets stressed birches. Per Iowa State University Extension, it rarely survives in a healthy tree, but on a weakened one it can cause dieback that starts at the top of the crown and moves steadily downward, and it can kill the tree within a few seasons once established. The distinct D-shaped exit hole is the confirming sign, but by the time you can find one, the top-down thinning has usually already started. That makes crown thinning on a birch, more than the holes themselves, the earliest warning worth acting on.
If your tree is an ash, do not guess. Emerald ash borer is a separate, more serious question with its own set of facts about status and detection in Idaho. See our emerald ash borer guide rather than treating this article as the source on that pest.
What to Do When You Find Holes
- Photograph the pattern, the tree species, and the location on the trunk (north/south side, height, near the base or up in the crown).
- Check for frass, sap, or bark cracking around the holes, not just the holes themselves.
- Look up at the canopy for thinning, dead branch tips, or leaves that are smaller or paler than the rest of the tree.
- Rule out sapsucker rows first. They are the most common cause and the least urgent.
- If you suspect borers, especially on a birch, ash, or a tree already showing canopy stress, get it looked at rather than waiting a season to see what happens.
For a broader rundown of what else shows up on Treasure Valley trees, see our guide to common tree pests in Boise.
If the tree is otherwise healthy and the holes are the tidy, evenly spaced sapsucker pattern, you likely do not need to hire anyone. If you are unsure which pattern you are looking at, or the tree’s canopy is already thinning, a same-day estimate settles it faster than watching and worrying. Call (208) 247-9766 or request a free on-site estimate; our ISA-certified arborists can identify the cause on site.
When a Tree With Borer Holes Becomes a Safety Issue
Heavy, sustained borer activity can hollow out sections of a trunk or major limb over time, and hollowed wood loses structural strength even when the tree still has leaves. That is a different question from pest identification: it is about whether the tree can still hold itself up. A trunk with a visible crack in addition to borer holes changes the calculus further; see our guide on cracks in a tree trunk if that applies to your tree. Our guide on hollow trees and how arborists judge them covers how that structural decision actually gets made.
A tree health assessment (typically $75-$250, often waived if you move forward with recommended work) is the most direct way to find out whether holes in the trunk are cosmetic, a developing infestation, or already a structural concern. If you would rather have someone rule it out than keep guessing, call (208) 247-9766 or request a free on-site estimate.
Frequently Asked Questions
Will sapsucker holes kill my tree?
Usually not, if the rows stay limited and the tree is otherwise healthy. A vigorous tree tolerates light feeding and seals the wounds over time. Heavy, repeated drilling in the same spot for multiple seasons is the exception: it can strip enough bark to girdle a limb or the trunk, which is a real threat to the tree.
Do woodpeckers cause the damage, or are they helping?
Neither, really. A woodpecker working hard on one section of trunk is usually a symptom, not the cause: it found insects already living under the bark, so the hole pattern is a clue to investigate rather than the problem itself. Occasional isolated holes from nesting or drumming are a different, much less concerning thing.
How do I know if it is borers and not just a woodpecker?
Look for small round or D-shaped exit holes with fine sawdust-like frass around them, sometimes with sap oozing nearby. Woodpecker excavations are larger and more ragged, and a woodpecker’s work is often a sign that borers are already present underneath.
My tree has holes but still looks healthy. Should I worry?
Not necessarily, especially if the pattern is neat sapsucker rows. Keep an eye on the canopy over the next few months for thinning or dieback, since that is usually the first sign that an infestation is affecting the tree’s health rather than just its bark.
Can I treat borers myself?
Some preventive treatments exist, but they depend on catching an infestation early and matching the product to the specific insect and tree species, which is hard to do from a description of holes alone. A tree health assessment identifies the actual cause before you spend money on a treatment that might not fit the problem.
Is this the same thing as emerald ash borer?
Only if your tree is an ash. Emerald ash borer is a specific, serious pest with its own identification signs and status questions that go beyond generic hole patterns, so treat it as its own topic rather than assuming any hole pattern on an ash is EAB.