# Borers in a young or stressed tree: the signs, the trees they pick, and what keeps them out

> What borer damage looks like on a trunk, the borers of the East and the trees each one picks, why stress invites them, and how to protect a young tree.

Published 2026-10-28. Tags: Protection & care, Decay & damage. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/borers-in-a-young-or-stressed-tree/

Borers are the larvae of beetles and clearwing moths that tunnel under the bark, and most of them pick a tree that is already weak. A tree newly planted, short of water, stripped of its leaves or nicked by a mower draws them. The signs are small holes, round or shaped like a D, sawdust-like frass in bark cracks or at the base, oozing sap, cracked bark and branches dying back. Water, mulch and a trunk kept free of wounds keep most of them out. The emerald ash borer and the redbay ambrosia beetle are the exceptions, since they also attack healthy trees.

![White birch bark with thin dark streaks and one small, dark exit hole with a flat side, shaped like a capital D](https://bluewhaletrees.com/img/articles/borers-in-a-young-or-stressed-tree-1-1000.jpg?v=7c554aaf "A bronze birch borer's exit hole in birch bark, in the United States on September 27, 2007. The emerald ash borer and the two-lined chestnut borer leave the same D shape.")
*Photo: Whitney Cranshaw, Colorado State University, [CC BY 3.0 us](https://creativecommons.org/licenses/by/3.0/us/deed.en) ([source](https://commons.wikimedia.org/wiki/File%3AAgrilus_anxius_5302066.jpg))*

## The signs of borers in a trunk

Borer damage shows on the bark, at the base and in the crown:

- **Holes.** Adults chew their way out through exit holes scattered over the bark, not in the [tidy rows a sapsucker drills](https://bluewhaletrees.com/articles/sapsucker-holes-and-wasp-damage/index.html.md). The emerald ash, bronze birch and two-lined chestnut borers leave holes shaped like a capital D, and other borers leave round or oval ones.
- **Frass.** The larvae push out a sawdust-like waste called frass, which gathers in bark cracks and at the base of the tree. Ambrosia beetles push theirs out in thin noodles that stand off the trunk.
- **Sap and gum.** Boring often starts sap flowing. A flatheaded borer leaves dark, dead patches of bark, often wet with sap, and a peachtree borer leaves gum clotted with frass and soil at the base of the trunk.
- **Cracked bark.** The bark cracks and splits over the tunnels, and on a birch the tunnels raise bumpy ridges.
- **A crown dying back.** The tunnels cut off the water, so leaves turn off-color and drop early, and twigs and branches die. The emerald ash, bronze birch and two-lined chestnut borers start near the top of the tree.
- **Woodpecker work.** Woodpeckers chip into the bark after the larvae, and their jagged holes can be the first sign of emerald ash borer.

![Split, peeling bark on a trunk opening onto a winding, S-shaped tunnel packed with pale, sawdust-like frass](https://bluewhaletrees.com/img/articles/borers-in-a-young-or-stressed-tree-2-1000.jpg?v=3454d2ff "Emerald ash borer tunnels showing through split bark in Maryland on March 5, 2016. The S-shaped tunnels cut off the water supply, and the crown dies back from the top.")
*Photo: judygva (back in town and trying to catch up), [CC BY 2.0](https://creativecommons.org/licenses/by/2.0) ([source](https://commons.wikimedia.org/wiki/File%3AEmerald_Ash_Borer_tunels.jpg))*

## Why borers pick a young or stressed tree

Most borers come after some other trouble has weakened a tree:

> "Any plant can be susceptible, but it is generally believed that wood borers are secondary pests that develop because trees and shrubs are stressed, injured, or dying from other causes." (The Morton Arboretum, *Borers*)

They come to trees stressed by a recent move, by drought, by [caterpillars that strip the leaves](https://bluewhaletrees.com/articles/caterpillars-on-a-tree/index.html.md) year after year, or by bark nicked with a mower or a string trimmer. A newly planted tree, still [settling after the move](https://bluewhaletrees.com/articles/is-my-new-tree-dying-or-just-shocked/index.html.md), and any tree in a dry summer are the likeliest targets.

A stressed or dying tree gives off ethanol, which draws ambrosia beetles. Flatheaded borers favor thin-barked young trees newly set out, though the sap of a healthy tree can kill their larvae. The dogwood borer cannot get in at all without a wound or an opening in the bark, which is why string trimmers are kept away from a flowering dogwood.

Two borers break the rule. The emerald ash borer attacks and kills healthy ashes of any size. The redbay ambrosia beetle attacks healthy trees of the laurel family and carries in the fungus of [laurel wilt](https://bluewhaletrees.com/articles/glossary/index.html.md#laurel-wilt).

## The borers of the East and the trees each picks

Each borer keeps to its own trees, so the tree is the first clue to which one is at work:

| Borer | Trees it picks | What it leaves | Goes after |
| --- | --- | --- | --- |
| Ambrosia beetles | More than 100 woody plants | Noodles of frass from tiny holes, fine sawdust at the base | Stressed or dying trees, drawn by ethanol |
| Bronze birch borer | Birches, white-barked ones most; [river birch](https://bluewhaletrees.com/plants/river-birch/index.html.md) is rarely hit | A thinning top, bumpy ridges, D-shaped holes | Stressed trees |
| Dogwood borer | [Flowering dogwood](https://bluewhaletrees.com/plants/flowering-dogwood/index.html.md), apple, [pecan](https://bluewhaletrees.com/plants/pecan/index.html.md), elm, hickory, willow | Reddish-brown frass at the holes, leaves reddening and dropping early | Bark with a wound |
| Emerald ash borer | Every ash, [blue ash](https://bluewhaletrees.com/plants/blue-ash/index.html.md) surviving best, and [white fringetree](https://bluewhaletrees.com/plants/white-fringetree/index.html.md) | D-shaped holes, S-shaped tunnels, a dying top, sprouts low on the trunk | Healthy trees too |
| Flatheaded appletree borer | Thin-barked young maples, oaks, dogwoods, hawthorns and crabapples | Dark, dead patches of bark, often wet with sap | Newly planted or weakened trees |
| Locust borer | [Black locust](https://bluewhaletrees.com/plants/black-locust/index.html.md) only | Sap oozing from small holes, knotty swellings, broken limbs | Stressed trunks |
| Peachtree borer | Peach, cherry, plum, nectarine, apricot | Gum clotted with frass and soil at the base | Young trees, through wounds |
| Redbay ambrosia beetle | The laurel family: [redbay](https://bluewhaletrees.com/plants/redbay/index.html.md), [sassafras](https://bluewhaletrees.com/plants/sassafras/index.html.md), [spicebush](https://bluewhaletrees.com/plants/spicebush/index.html.md) | Tiny round holes, toothpicks of sawdust, wilted leaves that hang on | Healthy trees too |
| Two-lined chestnut borer | Oaks of both groups, [white oak](https://bluewhaletrees.com/plants/white-oak/index.html.md) and [northern red oak](https://bluewhaletrees.com/plants/northern-red-oak/index.html.md) among them | Brown leaves in late summer, a dying top, D-shaped holes | Oaks hit by drought or stripped of leaves |

### The ashes and birches we grow

Three trees called ash are not true ashes, and the emerald ash borer leaves them alone: [American mountain ash](https://bluewhaletrees.com/plants/american-mountain-ash/index.html.md), [common prickly ash](https://bluewhaletrees.com/plants/common-prickly-ash/index.html.md) and [common hoptree](https://bluewhaletrees.com/plants/common-hoptree/index.html.md), the wafer ash. The [green](https://bluewhaletrees.com/species/green-ash/index.html.md), [white](https://bluewhaletrees.com/species/white-ash/index.html.md) and [black ash](https://bluewhaletrees.com/species/black-ash/index.html.md) of the wild are trees we do not grow.

Among the other birches we grow, [yellow birch](https://bluewhaletrees.com/plants/yellow-birch/index.html.md) resists the bronze birch borer better than most birches, though the borer can kill one weakened by heat, drought or injury. [Gray birch](https://bluewhaletrees.com/plants/gray-birch/index.html.md) resists it better than paper birch, and [sweet birch](https://bluewhaletrees.com/plants/sweet-birch/index.html.md) reportedly resists it too.

## What keeps borers out of a young tree

Keeping the tree vigorous keeps most borers out:

- **Water.** Water a young tree weekly through its first summer or two and in every dry spell after, since drought stress invites borers. The [watering calculator](https://bluewhaletrees.com/tools/watering/index.html.md) gives the amounts.
- **Mulch.** Two to four inches of mulch over the root zone keeps the soil cool and moist and keeps the mower off the trunk. Spread it as [mulch around a tree](https://bluewhaletrees.com/articles/mulch-around-a-tree/index.html.md) sets out, never heaped against the bark.
- **No wounds.** Keep mowers and string trimmers off the bark and the roots, since wounds are the way in for the dogwood and peachtree borers.
- **A white guard in winter.** Sunscald kills patches of thin young bark on the sunny side, and the wound gives pests and diseases a way in. A white wrap for the first two or three winters prevents it, as [frost cracks and sunscald](https://bluewhaletrees.com/articles/frost-cracks-and-sunscald/index.html.md) explains.
- **The right tree for the spot.** Birches want cool, moist, part-shaded ground and grow stressed in open, sunny, exposed spots, where the bronze birch borer finds them. River birch, which the borer rarely touches, is the birch for a sunnier yard.
- **Pruning in season.** Prune birches in fall, when the adult borers are not active, since fresh cuts draw them. Take dead limbs off any tree promptly.
- **Firewood that stays put.** Emerald ash borer larvae and the beetle that carries laurel wilt both travel in firewood, so buy it where it will be burned.

## Where insecticide fits

Insecticide has a narrow place against borers:

- **An ash worth keeping.** Where the emerald ash borer has arrived, a systemic insecticide protects a healthy or newly infested ash. Treatment is best begun before a big tree is infested, and it must be repeated for as long as the beetle is around.
- **Ambrosia beetles.** A spray kills the adults, but it would have to cover the whole plant again and again, so it is not practical.
- **Laurel wilt.** No treatment cures it or protects a tree from it.

Once ambrosia beetles have a tree, the damage is done:

> "There are no effective strategies to manage ambrosia beetles once they attack a tree, shrub, or vine." (Clemson Cooperative Extension, *Ambrosia Beetles*)

## What to do with a tree already hit

What to do depends on the borer and on how far it has gone:

- **An ash with D-shaped holes or bark chipped by woodpeckers.** Decide soon, while the tree can still be treated. A white or green ash usually lives 7 to 10 years once infested, but only one or two where the beetles are many or the tree was already declining.
- **A birch dying at the top.** Cut out the dead and declining branches in fall, take the wood away, and water through every dry spell.
- **A laurel-family tree wilting suddenly in summer.** Cut it down, chip it on the spot and move none of the wood, since laurel wilt has no cure.
- **Noodles of frass on a young tree.** A tree with ambrosia beetles in it will likely need replacing.
- **An oak or a dogwood with frass at the holes.** Take the stress away with water in dry spells, mulch over the roots and no new wounds.

![An ash trunk with its outer bark chipped away to pale inner bark, a cream-colored larva half out of its tunnel](https://bluewhaletrees.com/img/articles/borers-in-a-young-or-stressed-tree-3-1000.jpg?v=66e465e5 "Bark chipped off an ash by woodpeckers after emerald ash borer larvae, one still half out of its tunnel, on November 14, 2019. Woodpecker work can be the first sign.")
*Photo: USDAgov, [Public domain](https://commons.wikimedia.org/wiki/File%3A20200429-APHIS-GA-IYPH_0041_(49992690073).jpg) ([source](https://commons.wikimedia.org/wiki/File%3A20200429-APHIS-GA-IYPH_0041_(49992690073).jpg))*

Blue Whale Trees answers questions about a tree you already have through its [support page](https://bluewhaletrees.com/support/index.html.md). A photo of the whole tree, a close-up of the holes or the frass and a note on when the trouble started help us answer quickly. Photos can follow by email once we reply.

## Common questions

### How do I know if my tree has borers?

Look for small holes scattered over the bark, round or shaped like a D, with sawdust-like frass in the bark cracks or at the base. Oozing sap, cracked bark and branches dying back from the top often come with them.

### What does sawdust at the base of a tree mean?

Often borers or ambrosia beetles at work in the trunk, pushing out their frass. On a cherry or a plum, frass clotted in gum at the base points to the peachtree borer.

### Can a tree recover from borers?

Often, when the borer is one that follows stress and the stress is taken away, since the sap of a healthy tree can kill flatheaded borer larvae. A green or white ash with emerald ash borer needs treatment to live, and a tree with laurel wilt cannot be saved.

### How do you treat borers in a tree?

By taking away what weakened the tree: water in dry spells, mulch over the roots, no new wounds, and dead wood cut out in the right season. Insecticide earns its place against the emerald ash borer, on an ash still healthy enough to protect.

## Sources

- [Borers](https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/borers/): The Morton Arboretum, with its pages Bronze Birch Borer and Emerald ash borer: borers as the larvae of beetles and clearwing moths, the quotation on secondary pests, the stresses of transplanting, drought, defoliation and mower and trimmer wounds, frass, sap, cracked bark and dieback, the flatheaded borer's dark, wet bark and the sap that can kill its larvae, the locust borer on black locust alone, and prevention by vigor, site and prompt removal of dead limbs; white-barked birches most at risk, river birch highly resistant, the thinning top, bumpy ridges and D-shaped holes, birches for cool, moist, part shade, 2 to 4 inches of mulch and fall pruning; every native ash attacked, healthy ashes of any size, dieback from the top, sprouts low on the trunk, mountain-ash, prickly-ash and wafer-ash spared, and firewood.
- [Twolined Chestnut Borer](https://content.ces.ncsu.edu/twolined-chestnut-borer): NC State Extension, with its pages Flatheaded Appletree Borer, Peachtree Borer and Dogwood Borer: oaks of both groups, brown, wilted leaves in late summer and dieback from the top, D-shaped holes, stress from drought and defoliation, no wounds from landscaping equipment, and the emerald ash borer as a killer of healthy trees; thin-barked young trees newly set out; the peachtree borer's hosts, its gum clotted with frass and soil, young trees and wounds; the dogwood borer's hosts and reddish-brown frass.
- [Ambrosia Beetles](https://hgic.clemson.edu/factsheet/ambrosia-beetles/): Clemson Cooperative Extension, Home & Garden Information Center, with its page Dogwood Diseases and Insect Pests in South Carolina: more than 100 hosts, noodle-like frass tubes and fine sawdust at the base, ethanol from stressed trees, the redbay ambrosia beetle on healthy trees, sprays that are not practical, the quotation, and replacement; the dogwood borer entering only through a wound, leaves that redden and drop early, and frass near the holes.
- [Emerald Ash Borer](https://extension.psu.edu/emerald-ash-borer): Penn State Extension: woodpecker holes as an early sign, S-shaped galleries packed with frass, D-shaped holes, bark split over the galleries, and treatment begun before a large tree is infested.
- [Emerald Ash Borer (EAB) FAQs for Kentuckians](https://entomology.mgcafe.uky.edu/ef453): University of Kentucky Entomology, with the University of Kentucky's page Laurel Wilt: blue ash's resistance, white fringetree as a host, the round or oval holes of other ash borers, 7 to 10 years for an infested white or green ash and one or two under heavy attack, and treatment of healthy or newly infested ash, repeated for the foreseeable future; laurel wilt's hosts, tiny holes and toothpicks of sawdust, leaves that hang on, no cure or protection, cutting and chipping, and firewood.
- [Sunscald Injury or Southwest Winter Injury on Deciduous Trees](https://extension.usu.edu/forestry/publications/utah-forest-facts/021-sunscald-injury-or-southwest-winter-injury-on-deciduous-trees): Utah State University Extension: sunscald as an avenue for pests and diseases to enter the damaged bark.

## Related

- Next step: [Write to us](https://bluewhaletrees.com/support/index.html.md#article=borers-in-a-young-or-stressed-tree)
- [Is my new tree dying or just in shock? The first year’s signs, the scratch test and what to do](https://bluewhaletrees.com/articles/is-my-new-tree-dying-or-just-shocked/index.html.md)
- [Frost cracks and sunscald: why young bark splits in winter, and what a trunk wrap can and cannot do](https://bluewhaletrees.com/articles/frost-cracks-and-sunscald/index.html.md)
- [Sapsucker holes and the wasps that follow them](https://bluewhaletrees.com/articles/sapsucker-holes-and-wasp-damage/index.html.md)
- [Drought and an established tree: where to water, how much, leaf scorch, and the year after](https://bluewhaletrees.com/articles/drought-and-an-established-tree/index.html.md)