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.
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.
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.
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.
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.
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.
The worst tree borers in North America
Emerald Ash Borer (EAB)
The most destructive tree pest in US history. This metallic green beetle, native to Asia, has killed hundreds of millions of ash trees since it was discovered in Michigan in 2002. It’s now established across most of the eastern US and spreading west.
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.
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 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.
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.
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.
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 a fungal pathogen.
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. 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.
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). 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.
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.
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.
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.
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.
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
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.
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 |
When to call an arborist
Call an ISA-certified arborist if you suspect borers. They can:
- Identify the species from the galleries, frass, and exit holes
- Assess the damage level to determine if treatment is worthwhile
- Apply professional treatments (trunk injections require specialized equipment)
- 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.
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.