Tree Boring Insects: How to Identify, Prevent, and Treat Wood-Boring Pests

Boring insects kill more mature trees in the US than any other pest category. They tunnel into wood, disrupting the flow of water and nutrients between roots and canopy. By the time you notice the damage, the infestation is usually well established.

The frustrating part: borers attack stressed trees preferentially. A healthy, well-watered tree can often fight off an attack. A tree weakened by drought, construction damage, or poor pruning becomes a target. Prevention matters more than treatment with these pests.

How borers damage trees

All boring insects share one destructive habit: their larvae feed inside the tree’s wood or just beneath the bark. The tunnels (called galleries) sever the vascular tissue that moves water up from the roots and sugars down from the leaves.

A few borers in a large tree cause minor damage. A heavy infestation girdles the tree from the inside, cutting off flow so completely that entire branches or the whole tree dies. The adults do relatively little damage. It’s the larvae tunneling for months or years that kill the tree.

Healthy trees fight back with sap pressure. When a borer chews into a vigorous oak or pine, the tree floods the wound with resin or sap, physically drowning the larvae. Drought-stressed trees can’t generate that pressure, which is why borers and dry summers go hand in hand.

Signs of borer infestation

Learn these warning signs. Catching an infestation early gives you treatment options. Catching it late usually means removal.

Exit holes: Small round or D-shaped holes in the bark where adult beetles emerged. The size and shape help identify the species. Emerald ash borer leaves distinctive D-shaped holes about 1/8 inch wide. Asian longhorned beetle makes round holes nearly 3/8 inch across. Bark beetles leave tiny pin-sized holes, often clustered in groups.

Sawdust frass: Fine sawdust-like material (frass) at the base of the tree or in bark crevices. This is boring dust pushed out by larvae as they tunnel. Different species produce different textures of frass. EAB frass is fine and powdery. Longhorned beetle frass is coarser, like small wood shavings. Bark beetle frass looks like reddish-brown pepper.

Bark splits and cracks: Vertical splits in the bark, sometimes with sawdust visible in the crack. Borers tunneling just beneath the bark cause the outer bark to separate and crack. On ash trees with EAB, you’ll see vertical splits that expose the S-shaped galleries underneath.

Crown dieback: Branches dying from the top down. This happens when borers in the trunk cut off water flow to upper branches first. If the top third of your tree is dying back and there’s no obvious drought or disease explanation, check for borers. Crown dieback combined with bark holes is a strong indicator. Our guide on how to tell if a tree is dead covers the full checklist for assessing whether a tree can recover.

Woodpecker activity: Heavy woodpecker feeding on a tree trunk is a reliable sign of borers. Woodpeckers can hear larvae moving inside the wood. If woodpeckers are systematically working your tree, they’re finding something.

Epicormic shoots: Stressed trees sometimes send out clusters of small shoots from the trunk or base of main branches. These “water sprouts” appear when the upper canopy is losing its ability to photosynthesize and the tree is trying to compensate.

Staining and oozing: Some borers cause dark wet stains on the bark as sap leaks from feeding sites. Bacterial wetwood or slime flux can look similar, but borer staining usually appears near visible holes or cracks.

The worst tree borers in North America

Emerald Ash Borer (EAB)

The most destructive tree pest in US history. This metallic green beetle (Agrilus planipennis), native to Asia, has killed hundreds of millions of ash trees since it was discovered in Michigan in 2002. It’s now established in 36 states and spreading west. If you live in Zones 3-9 and have ash trees, this pest is either already in your area or on its way.

EAB larvae feed in S-shaped galleries just beneath the bark, girdling and killing ash trees within 3-5 years of infestation. The adult beetles are about 1/2 inch long and metallic green. The D-shaped exit holes are the telltale sign. Adults emerge in late May through July and feed on ash foliage for about two weeks before mating and laying eggs in bark crevices.

All North American ash species are susceptible. Green ash (Fraxinus pennsylvanica), white ash (Fraxinus americana), and black ash (Fraxinus nigra) die at roughly equal rates once infested. Blue ash shows slightly more resistance but still succumbs under heavy pressure.

What you can do: Preventive treatment with systemic insecticides (imidacloprid or emamectin benzoate) can protect healthy ash trees. Treatment costs $100-300 per tree annually. Trunk injections by a certified arborist every 2-3 years are the most effective option. Once more than 50% of the canopy is dead, treatment won’t save the tree. If you have a valuable ash, start treating it now before EAB arrives in your area.

Asian Longhorned Beetle (ALB)

A large (1-1.5 inch) black beetle (Anoplophora glabripennis) with white spots and long antennae. Native to China. ALB attacks maples, elms, willows, birches, and other hardwoods. Unlike EAB, there’s no effective chemical treatment for established infestations.

ALB has been found and eradicated in several US locations (New York, New Jersey, Massachusetts, Ohio). The USDA runs an aggressive eradication program that involves removing and chipping every infested tree plus all host trees within a quarantine zone. A single detection can mean the removal of thousands of trees in a half-mile radius.

Exit holes: Perfectly round, about 3/8 inch in diameter. Much larger than EAB holes. Look for these on the trunk and main branches from late summer through fall. You’ll also see coarse sawdust frass in branch crotches and at the base of the tree. The larvae create tunnels up to an inch in diameter deep inside the heartwood, causing structural weakening that makes branches fall without warning.

Why ALB matters so much: Maples are the primary target. Sugar maples, Norway maples, silver maples, red maples, and boxelders are all hosts. If ALB became widely established, it would change the look of every street and park in the eastern US.

Bronze Birch Borer

The bronze birch borer (Agrilus anxius) is a native flatheaded beetle that kills birch trees across much of North America. If you have a European white birch (Betula pendula) in your yard, you’ve probably already dealt with this pest or will soon. European and Asian birch species are far more susceptible than native birches.

Damage starts in the upper crown and works downward over 2-4 years. Look for thinning foliage at the top of the tree, D-shaped exit holes similar to EAB (but on birch), and raised ridges in the bark where larvae tunneled just beneath the surface. Heavily infested trees show “stagheading” where the top branches are bare and dead while lower branches still have leaves.

Best defense: Plant native river birch (Betula nigra) instead of European white birch. River birch has strong natural resistance to bronze birch borer. Keep birches well-watered during summer. These trees have shallow root systems and are among the first to stress during drought. A stressed birch is a dead birch walking when borers are active.

Bark Beetles

A large family of small beetles (1/8 to 1/4 inch) that tunnel between the bark and wood. Different species attack different tree types:

Pine bark beetles (Ips and Dendroctonus species) have killed billions of pines across the western US, driven by drought and warmer winters that let populations explode. Mountain pine beetle alone has killed trees across 27 million acres of western forest. If you’re dealing with pine problems, our pine tree diseases guide covers the full range of threats these trees face.

Elm bark beetles spread Dutch Elm Disease, the fungal infection that has devastated American elms. The beetles carry fungal spores from tree to tree as they feed.

Shothole borers (Polyphagous and Kuroshio shot hole borers) are invasive beetles devastating trees in Southern California. They attack over 60 tree species and carry Fusarium fungal pathogens that cause a disease called Fusarium Dieback. The beetles are tiny, about the size of a sesame seed, and create perfectly round entry holes. Avocado, box elder, sycamore, and coast live oak are among the hardest hit species. There’s no effective treatment once the fungus is established in the tree. For more on fungal threats to trees, check our tree fungus guide.

Signs of bark beetle attack: reddish boring dust in bark crevices, small round entry holes clustered together, pitch tubes (blobs of resin) on pines, and rapid crown fading from green to yellow to red.

Clearwing Borers

Moths (not beetles) whose larvae bore into tree trunks and branches. The adults look like wasps, which is how they got the name “clearwing.” Common targets include dogwood, lilac, ash, birch, and ornamental trees like cherry and plum.

Peachtree borer attacks stone fruits (peach, cherry, plum) near the soil line. You’ll see gummy masses of sap mixed with frass (called gummosis) at the base of the trunk from late spring through fall. Dogwood borer attacks flowering dogwoods, especially where the bark has been damaged by lawn mowers or string trimmers. This is why trunk protection matters so much for young trees.

Clearwing borers are easier to manage than beetle borers because pheromone traps can monitor adult moth flight timing. Spray the trunk with permethrin when adults are active (usually May through August depending on species and location). Timing is everything with these pests.

Which trees are most vulnerable

Not all trees face equal borer risk. Some species attract specific borers, and stressed trees of any species are more susceptible.

High risk:

• Ash (all species) — EAB is a near-guaranteed death sentence without treatment
• European white birch — bronze birch borer kills these reliably in most of the US
• Pines (especially ponderosa, lodgepole, Scotch pine) — bark beetles in western states
• Stone fruits (peach, cherry, plum) — peachtree borer is persistent
• Dogwood — dogwood borer attacks any bark wound

Moderate risk:

• Maples — ALB target, also susceptible to ambrosia beetles
• Oaks — flatheaded borers, ambrosia beetles, and the goldspotted oak borer in Southern California
• Willows and poplars — poplar borer and clearwing borers
• Elms — elm bark beetles carrying Dutch Elm Disease

Lower risk (good replacement options):

• River birch — resistant to bronze birch borer
• Ginkgo — few serious pest issues
• Bald cypress — rarely attacked by borers
• Kentucky coffeetree — pest-resistant and drought-tolerant

Prevention: the best treatment

Borers are far easier to prevent than treat. A healthy tree can resist most borer attacks through sap pressure and chemical defenses. A stressed tree is defenseless.

Water during drought. Deep water established trees during extended dry periods, especially the first two years after planting. A drought-stressed tree sends out chemical signals that attract borers. Follow proper watering techniques to keep trees hydrated without overwatering.

Don’t wound the bark. Lawn mower strikes, string trimmer damage, and careless pruning create entry points for borers. Keep a mulch ring around tree trunks to eliminate the need for mowing close to the bark.

Prune correctly and at the right time. Some borers are attracted to fresh pruning wounds. Avoid pruning oaks from April through July (oak wilt vector beetles are active). Don’t prune elms from April through August (elm bark beetle season). Prune birches in late summer or fall, never in spring when bronze birch borer adults are laying eggs near fresh cuts. Check our seasonal pruning guide for timing by species.

Fertilize appropriately. Too much nitrogen pushes soft, fast growth that’s more susceptible to attack. Use a balanced tree fertilizer and don’t over-apply.

Remove infested trees promptly. A heavily infested tree is a breeding factory that will spread borers to nearby healthy trees. Don’t leave dead or dying borer-infested wood standing. Chip or burn the wood immediately. Never move infested firewood more than 50 miles from where it was cut. Firewood transport is exactly how EAB spread so fast across the eastern US.

Choose resistant species. When planting new trees, pick species that aren’t targeted by the borers active in your region. If EAB is in your area, don’t plant ash. If pine bark beetles are a problem, consider drought-tolerant alternatives.

Monitor for early signs. Walk your property once a month during the growing season (April through September) and look at your tree trunks. Check for exit holes, sawdust frass, bark splits, and woodpecker damage. Five minutes of observation can catch an infestation months before the tree shows crown symptoms.

Treatment options

Systemic insecticides

For preventive treatment of high-value trees, systemic insecticides are the standard approach. The chemical is absorbed into the tree’s vascular system and kills larvae as they feed.

Soil drench (imidacloprid): Applied as a liquid poured around the base of the tree. The cheapest option ($20-50 DIY, $100-150 professional). Takes 4-6 weeks to reach the canopy. Effective for trees up to about 15 inches in trunk diameter. Annual application needed. BioAdvanced Tree & Shrub Protect & Feed is the widely available consumer version — mix it with water and pour it around the root zone in early spring before borer adults emerge.

Important timing note: Apply imidacloprid soil drenches in mid-April to early May in most of the US (Zones 5-7). The chemical needs 4-6 weeks to reach the canopy, and EAB adults emerge in late May. If you apply in June, you’re too late for that season. In warmer zones (8-9), apply in March.

Trunk injection (emamectin benzoate): A certified arborist drills small holes in the trunk and injects the chemical directly into the vascular system. More expensive ($150-300 per treatment) but more effective, especially for large trees. Lasts 2-3 years per treatment. This is the gold standard for EAB prevention. Find an ISA-certified arborist to handle trunk injections. The equipment and products aren’t available to homeowners, and improper injection can damage the tree.

Bark spray (permethrin, bifenthrin): Sprayed on the trunk to kill adult beetles on contact before they can lay eggs. Less effective than systemic treatments but useful as a supplement. Apply in spring before adult beetles emerge. Best suited for clearwing borers and peachtree borers where you can time the spray to adult flight using pheromone traps.

Neem oil for mild infestations

For early-stage or light borer pressure, Bonide Neem Oil Concentrate can help as part of a prevention program. Neem acts as a feeding deterrent and disrupts insect development. Spray the trunk and lower branches in early spring before adults emerge, and repeat every 14 days through the adult flight period. Neem won’t save a tree with an established infestation, but it’s a reasonable organic option for homeowners who want to reduce borer pressure on lightly stressed trees. It also works on aphids and other soft-bodied pests, so it pulls double duty in the spray rotation.

When treatment won’t work

Don’t waste money treating a tree that’s too far gone:

• More than 50% canopy dieback: treatment can’t reverse the damage
• Bark falling off in sheets: the cambium is dead
• Multiple years of heavy infestation: structural damage makes the tree hazardous
• Bark beetle attack on conifers with >50% needle loss: the tree can’t recover
• Extensive woodpecker damage covering more than half the trunk circumference

In these cases, tree removal is the only option. Remove the tree, grind the stump, chip or burn the wood (don’t store borer-infested firewood), and plant a resistant replacement species.

Professional treatment costs

Borer treatment isn’t cheap, but it’s a fraction of removal and replacement costs. Here’s what to budget:

Service	Typical cost	Frequency
Arborist inspection/diagnosis	$75-150	Once
Imidacloprid soil drench (DIY)	$20-50	Annual
Imidacloprid soil drench (professional)	$100-200	Annual
Emamectin benzoate trunk injection	$150-400	Every 2-3 years
Permethrin bark spray (professional)	$75-150	Annual (seasonal)
Tree removal (large tree)	$1,500-3,500	Once
Stump grinding	$150-350	Once

A large shade tree adds $10,000-15,000 to property value. Spending $200-400 per year on preventive injections is solid math compared to the $2,000-4,000 cost of removing a dead tree and waiting 15 years for a replacement to mature.

Identifying what’s boring your tree

Sign	Likely pest	Trees affected
D-shaped exit holes, S-shaped galleries	Emerald Ash Borer	Ash only
Large round holes (3/8”), long-antenna beetle	Asian Longhorned Beetle	Maple, elm, willow, birch
Reddish dust, pitch tubes on pines	Pine bark beetles	Pines, spruce
Small round holes clustered together	Shothole borers	Many hardwoods
Sawdust at base, damage near soil line	Peachtree/clearwing borers	Stone fruits, dogwood
Bark splits with frass, branch dieback	Flatheaded borers	Many species
D-shaped holes on birch, upper crown dying	Bronze birch borer	Birch (especially European)
Sesame-seed-sized holes, Fusarium staining	Polyphagous shot hole borer	60+ species (SoCal)

When to call an arborist

Call an ISA-certified arborist if you suspect borers. They can:

1. Identify the species from the galleries, frass, and exit holes
2. Assess the damage level to determine if treatment is worthwhile
3. Apply professional treatments (trunk injections require specialized equipment)
4. Determine if removal is necessary for safety and to protect nearby trees

The consultation typically costs $75-150. Treatment plans run $100-500 per tree per year depending on tree size and treatment method. That’s real money, but it’s a fraction of the $1,000-3,000 cost of removing a large dead tree.

Don’t wait to call. Borers work fast once established, and every month of delay means more tunneling and less chance of saving the tree. If you’re seeing symptoms of a dying tree combined with borer signs, get a professional out within a week.

Frequently asked questions

Can I save a tree with borers? It depends on how far the infestation has progressed. Trees with less than 30% canopy loss and caught early respond well to systemic treatments. Trees with more than 50% canopy dieback are past the point of recovery. The species matters too. EAB-infested ash trees respond well to trunk injections if caught early. ALB-infested trees have no chemical treatment option and must be removed.

How do borers get into my tree in the first place? Adult beetles or moths fly to your tree, often attracted by chemical signals the tree releases when stressed. They lay eggs in bark crevices or cracks. The larvae hatch and bore into the wood. Some species, like EAB, will attack even healthy trees, but most borers strongly prefer drought-stressed, wounded, or recently transplanted trees.

Is borer damage the same as termite damage? No. Termites eat dead wood and structural lumber. Borers attack living trees. Termites leave mud tubes and hollowed-out wood along the grain. Borers create tunnels across the grain and leave exit holes in bark. A termite inspector handles your house. An arborist handles your trees.

Should I spray insecticide on the exit holes? No. Exit holes mean the adult already left. The damage is done. Spraying exit holes wastes product and money. Preventive trunk sprays target adults before they lay eggs. Systemic treatments target larvae feeding inside the wood. Neither one works retroactively on holes that are already there.

The bottom line

Borers are a serious threat, but they’re predictable. They attack stressed trees. Keep your trees healthy through proper watering, mulching, and pruning, and most borers will skip your yard for easier targets. If you have ash trees and EAB is within 100 miles, start preventive injections now. And if you see D-shaped holes, sawdust frass, or unexplained crown dieback, call an arborist before the problem spreads. For more on preventing and managing tree health issues, visit mklibrary.com’s guide to protecting your landscape investment.