A tree that thinned at the top last summer and looks worse this July is telling you something specific. Crown dieback that starts at the highest branches and works down is the classic shape of borer damage. It is one of the few tree problems that is well advanced before anything looks wrong from the driveway.
The insect is not eating leaves. Its larvae are feeding in a thin layer of living tissue under the bark, cutting the connections the tree needs to move food and water. Here is how borer damage develops in Draper yards, what the exit holes tell us, and why steady watering does more against borers than any spray.
What a Borer Actually Eats
Just under the bark of every branch and trunk sits the cambium, a layer only a few cells thick that produces new wood on its inner face and new bark on its outer face. Immediately outside it runs the phloem, the tissue that carries sugar made in the leaves down to the roots. Neither layer is deep. Together they are thinner than a credit card on a young ash.
Borer larvae live in exactly that zone. A female beetle or moth lays eggs in a bark crevice, the larva chews inward until it reaches the cambium, and it then tunnels sideways, feeding on the richest tissue on the tree. Each tunnel, called a gallery, kills a strip of cambium and phloem that never regrows.
How Galleries Girdle a Branch
One gallery is survivable. A heavy population is not, because galleries eventually overlap and ring the stem. Once the phloem is severed all the way around, sugar stops moving past that point, and everything above it is cut off from the root system. The leaves keep running for a while on stored water, then wilt, brown and hang.
That is why borer damage shows up as dieback of whole branches rather than spots on leaves, and why the dead sections are always above the injury, never below it. Follow a dead limb back toward the trunk and you will usually find the gallery zone at the transition, marked by sunken, cracked or loose bark.
Signs of Borer Damage You Can See From the Ground
Most of what confirms borer damage is on the bark, not in the canopy.
- Exit holes. Flatheaded borers leave a D-shaped hole; roundheaded borers and clearwing moths leave a round one. The shape tells us which group we are dealing with.
- Frass. A sawdust-and-waste mix pushed out of the bark, often collecting in bark cracks or at the base.
- Bleeding or gumming. Wet, dark streaks or amber ooze on the bark where the tree is walling off an injury.
- Bark that sounds hollow. Tap it. Loose, papery bark over a dead patch peels away in sheets.
- Woodpecker work. Rows of holes and flaked bark mean a bird found larvae before you did.
Colorado State University Extension keeps identification guides for the major borer groups in the Intermountain West.
Why Stressed Trees Get Hit First
Borers are secondary pests, which means they succeed on trees that are already struggling. A vigorous tree floods a new egg site with sap and cell growth and crushes the larva before it gets established. A drought-stressed tree cannot generate that pressure.
Stressed trees also advertise. Heat- and drought-stressed wood releases volatile compounds that egg-laying females detect from a distance, so the trees least able to defend themselves are the ones most likely to be found. That is the loop behind almost every case of borer damage we see after a dry summer in Draper: water stress first, beetles second, dieback third.
A Draper Birch, Year by Year
European white birch is the tree we lose most often to this. Picture one planted in a west-facing Draper front yard, in lawn, with afternoon sun off the driveway.
Year one, the top few feet thin out and the owner assumes it needs fertilizer. Year two, the upper branches are bare, and a close look at the trunk shows raised, wandering ridges under the bark where bronze birch borer larvae fed last season. Year three, the dieback reaches the main scaffold limbs, the large branches that form the tree’s framework, and the decision is no longer about treatment. Each of those years the birch was running short of water in August, and each year the beetles found it a little easier.
What Works Against Borers, and What Does Not
Once larvae are inside, nothing sprayed on the outside of the bark reaches them. Contact sprays only work against adults during the narrow window when they are laying eggs, which is why timing matters more than product choice. Systemic treatments move through the tree’s vascular system and can reach larvae feeding in the cambium, and they are regulated products applied strictly according to the label. Our tree and shrub insect control work begins with confirming the species and its flight period.
The part homeowners control is bigger than the spray. Deep, infrequent soaking through July and August keeps sap pressure up. A wide mulch ring keeps mower and string trimmer blades away from the trunk, and every trimmer wound is an invitation. Avoid pruning host species during peak adult flight, because fresh cuts attract egg-laying females. Utah State University’s Utah Pests program tracks local flight timing, and the USDA Forest Service publishes host and symptom guides for the major borer groups.
Safety Around a Borer-Damaged Tree
Borer-killed wood is not ordinary deadwood. Galleries and decay hollow the outer wood where the strength lives, so a limb that looks solid can fail with no warning and no sound. We treat a heavily infested tree as a structural problem, not just a pest problem.
Do not climb it, do not set a ladder against it, and do not run a chainsaw over your head to take a dead limb off. Keep clear of any branch within reach of a power line and call the utility instead. The Occupational Safety and Health Administration treats struck-by and electrical contact as the leading hazards in tree work, and both are avoidable from the ground.
Borer Damage FAQs
Can a tree recover from borer damage?
It depends on how much of the circumference is affected. A tree with scattered galleries and an intact crown often walls the injury off and keeps going. A tree that has lost the top third of its crown has already lost the phloem that supplies its roots, and recovery is unlikely.
Do borers move to my other trees?
Most borers are fussy about hosts, so a bronze birch borer will not move into your maple. The bigger risk is that the same drought conditions that weakened one tree are weakening the others.
Should I remove a dead tree right away?
Remove it before it becomes brittle. Standing dead wood dries, loses flexibility, and drops limbs unpredictably, and removal gets more hazardous every season it waits.
Is borer damage the same as woodpecker damage?
No, but they travel together. Woodpeckers flake bark and drill rows of holes to reach larvae, so concentrated woodpecker work is often the first visible sign of borer damage underneath.
Your Next Step
If branches at the top of a tree died back this year, have the trunk and scaffold limbs inspected before the tree drops anything over a driveway or fence. Call Rivendell Tree Experts to schedule tree care in Draper, or send photos of the bark and crown through our contact page.