Signs of Insect Damage in Trees: Holes, Sawdust & More
Not all tree damage is obvious. While a broken branch or missing leaves are easy to spot, the most dangerous pest infestations often happen beneath the bark where they remain hidden until significant damage is done. Learning to read the subtle clues that insects leave behind can help you catch problems early when treatment is still effective.
Quick Answer
The six key signs of insect damage in trees are: exit holes in bark (D-shaped from emerald ash borer, round from longhorned beetles, tiny from bark beetles), sawdust-like frass at the base, canopy dieback starting at the top, bark falling off to reveal tunneling galleries, leaf damage patterns (skeletonizing, mining, curling), and webbing or silk masses in branches. Exit holes and frass are the most urgent signs because they indicate boring insects that can kill trees. Use our diagnosis tool to identify the specific pest, or contact a certified arborist for professional assessment.
Exit Holes: What Each Shape Tells You
Exit holes in tree bark are created by adult insects emerging after completing their larval development inside the wood. The shape and size of these holes are diagnostic, helping you identify which pest has infested your tree and determine the appropriate response.
D-Shaped Holes (1/8 inch) — Emerald Ash Borer
Distinctly D-shaped exit holes about 1/8 inch (3mm) wide are the hallmark of the emerald ash borer. Found exclusively on ash trees (Fraxinus species). By the time you see exit holes, the tree has been infested for at least one year and damage to the cambium layer is already extensive. Immediate professional treatment with systemic insecticides may save the tree if canopy loss is under 50 percent. Bronze birch borers create similar D-shaped holes on birch trees.
Round Holes (3/8 inch or larger) — Longhorned Beetles
Perfectly round exit holes roughly the diameter of a pencil (3/8 inch or 10mm) are created by the Asian longhorned beetle. These holes penetrate deep into the heartwood and are often found on maple, birch, elm, willow, and other hardwoods. ALB is a federally quarantined pest; if you suspect an infestation, report it to your state department of agriculture or USDA APHIS immediately. Smaller round holes (1/4 inch) may indicate native longhorned beetles or clearwing moth borers.
Tiny Shot-Holes (1/16 inch) — Bark Beetles
Very small, round holes about the size of a pin head scattered across the bark surface indicate bark beetles (Ips, Dendroctonus, or Scolytus species). These holes are entry points as well as exits. You may also see boring dust in bark crevices and pitch tubes where the tree tried to push beetles out with resin. On pines, look for fading green-to-yellow-to-red needle color progressing rapidly.
Oval Holes (various sizes) — Other Borers
Oval or irregularly shaped exit holes may indicate various native wood-boring beetles, horntail wasps, or carpenter ants. While these are generally less destructive than the invasive borers listed above, they often attack trees already stressed by drought, disease, or poor growing conditions. The presence of these holes should prompt an overall assessment of tree health.
Frass and Sawdust at the Base
Frass is the technical term for the waste material produced by wood-boring insects. It accumulates at the base of infested trees, in bark crevices, or on branches below boring sites. The appearance of frass provides important diagnostic clues about which insect is responsible.
- Fine, powdery sawdust resembling talcum powder typically comes from bark beetles boring through the outer bark layers.
- Coarse, stringy frass mixed with dark insect waste (excrement) indicates large beetle larvae such as longhorned beetle grubs tunneling through sapwood and heartwood.
- Pellet-like frass ejected in small piles from clean holes suggests clearwing moth borers or certain weevil species.
- Carpenter ant frass looks like finely shredded wood shavings and accumulates in cone-shaped piles at the base. Carpenter ants excavate galleries for nesting but do not eat wood; their frass is very clean and contains no insect waste.
Any fresh frass accumulation at the base of a tree warrants closer inspection. Brush away the frass and check back in a week; if new frass appears, the infestation is active and professional evaluation is needed.
Canopy Dieback Patterns
When boring insects damage the vascular system beneath the bark, the canopy responds with predictable dieback patterns that serve as early warning signs. Understanding these patterns helps differentiate pest damage from other causes of decline.
Emerald ash borer causes dieback starting at the top of the crown and progressing downward over two to four years as successive generations of larvae destroy increasing amounts of cambium. This top-down pattern is characteristic of vascular disruption. The tree may produce epicormic shoots (dense clusters of sprouts) from the trunk and lower branches as a stress response. By the time half the canopy has died back, the tree typically cannot be saved.
Bark beetles on pines cause rapid, whole-crown fading as the mass attack overwhelms the tree's defenses all at once. Needles turn from green to yellow to red over a period of weeks to months. Branch-level dieback (individual branches dying while others remain green) is more typical of native borers or fungal diseases. Learn more about overall tree decline in our why is my tree dying guide.
Bark Damage: Galleries and Tunneling
When bark loosens or falls away from an infested tree, it often reveals the distinctive feeding galleries that larvae carved in the cambium layer. These gallery patterns are among the most reliable tools for identifying the specific pest responsible for the damage.
Serpentine Galleries
S-shaped, winding galleries that increase in width from start to finish are characteristic of emerald ash borer larvae. The gallery starts narrow where the tiny larva began feeding and widens as the larva grows. Galleries are packed with frass.
Radiating Galleries
A central egg gallery with smaller larval tunnels branching off perpendicularly in both directions creates a centipede-like pattern characteristic of bark beetles. The central tunnel was made by the adult female as she laid eggs; the branches were made by hatching larvae.
Large Irregular Cavities
Broad, irregular chambers in the sapwood and heartwood indicate longhorned beetle larvae. Asian longhorned beetle larvae create tunnels up to one inch in diameter that penetrate deep into the structural wood of the tree.
Clean Smooth Galleries
Clean, smooth-walled galleries without frass inside indicate carpenter ants rather than boring beetles. Carpenter ants excavate galleries for nesting, creating smooth tunnels that follow the grain of the wood, often in areas already softened by decay.
Leaf Damage Patterns
Leaf-feeding insects leave distinctive damage patterns that help identify the culprit. While leaf damage is generally less threatening than trunk boring, severe or repeated defoliation weakens trees and makes them vulnerable to secondary attackers.
- Skeletonizing: Tissue between leaf veins consumed, leaving a lace-like skeleton of veins. Caused by Japanese beetles, elm leaf beetle, and viburnum leaf beetle. Severely skeletonized leaves cannot photosynthesize and often drop early.
- Leaf mining: Winding trails or blotch-like areas of dead tissue within the leaf, created by tiny larvae feeding between the upper and lower leaf surfaces. Common on birch, holly, and many ornamental trees. Usually cosmetic damage only.
- Leaf curling: Leaves curl, pucker, or form galls in response to sap-feeding insects like aphids, psyllids, or gall-making mites feeding on developing foliage. Most curling occurs in spring when leaves are young and soft.
- Edge feeding: Scalloped or notched leaf margins from caterpillars or weevils chewing from the outside in. Usually cosmetic unless defoliation is severe.
- Window-paning: Feeding from one leaf surface only, creating translucent windows where tissue on one side has been consumed. Common with sawfly larvae on oak and rose slugs on roses.
Webbing and Silk Masses
Silk webbing in trees is alarming to homeowners but is usually caused by caterpillar species that are more unsightly than they are dangerous. Identification depends on the web's location and the time of year it appears.
Eastern Tent Caterpillar (Spring)
Dense silk tents in branch crotches of cherry, apple, and crabapple. Caterpillars leave the tent to feed on nearby leaves during the day. Remove tents manually while small.
Fall Webworm (Late Summer)
Loose, messy webs enclosing branch tips on many deciduous tree species. Caterpillars feed within the expanding web. Damage occurs late enough in the season that it rarely affects tree health.
Bagworms (Year-round)
Spindle-shaped silk bags covered with bits of foliage, hanging from branches of arborvitae, juniper, spruce, and other evergreens. Can cause serious damage to evergreens if populations are high. Hand-pick bags in winter before eggs hatch in spring.
Spongy Moth Egg Masses
Tan, fuzzy egg masses about the size of a quarter on tree trunks, branches, and outdoor surfaces. Each mass contains 500 to 1,000 eggs. Scrape off and destroy during winter to reduce spring caterpillar populations.
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Frequently Asked Questions
What do D-shaped holes in tree bark mean?
D-shaped exit holes approximately 1/8 inch wide are the signature of the emerald ash borer (Agrilus planipennis). These holes are created by adult beetles emerging from beneath the bark after completing larval development. If you see D-shaped holes on an ash tree, the infestation is already at least one year old and the tree has sustained internal damage. Contact a certified arborist immediately for treatment options. Other Agrilus species borers also create D-shaped holes on birch and oak, so correct tree identification is important.
Why is there sawdust at the base of my tree?
Sawdust-like material (called frass) at the base of a tree indicates that a boring insect is actively tunneling into the wood. Different pests produce different types of frass: fine, powdery frass suggests bark beetles; coarse, stringy frass mixed with insect waste points to longhorned beetle larvae; and pellet-like frass ejected from small holes indicates clearwing moth borers. The color, texture, and location of frass are diagnostic clues. Frass accumulation is a serious sign that requires professional evaluation.
What causes tiny holes in tree leaves?
Tiny holes in tree leaves are usually caused by flea beetles (very small jumping beetles), shothole disease (a fungal condition that creates round holes after infected spots fall out), or early-stage caterpillar feeding. Leaf miners create winding trails between leaf surfaces rather than holes. Skeletonized leaves, where the tissue between veins is consumed leaving a lace-like pattern, are caused by Japanese beetles, elm leaf beetles, or viburnum leaf beetles. While unsightly, leaf damage is rarely fatal to established trees unless it occurs repeatedly over several years.
Are webbing and silk masses in trees dangerous?
Silk masses and webbing in trees are created by several caterpillar species, most commonly eastern tent caterpillars (webs in branch crotches in spring), fall webworms (webs at branch tips in late summer), and bagworms (spindle-shaped bags hanging from branches). While they look alarming, these pests rarely kill healthy trees. Eastern tent caterpillars and fall webworms defoliate branches within their webs but trees typically releaf. Bagworms on evergreens can be more serious because evergreens cannot replace consumed needles as readily as deciduous trees replace leaves.
How can I tell if bark damage is from insects or disease?
Insect damage typically shows distinct entry or exit holes, tunneling galleries beneath loose bark, frass or boring dust, and often follows predictable patterns based on the pest species. Disease damage tends to appear as discolored, sunken, or swollen areas (cankers) without bore holes, often with oozing sap, cracked bark margins, or fungal fruiting bodies. Sometimes insects and disease work together; for example, bark beetles introduce fungal pathogens as they bore into wood. If you are unsure, a certified arborist can examine the damage pattern and identify the cause accurately.
Need Help Identifying Pest Damage?
Not sure what is damaging your tree? Use our tree diagnosis tool for step-by-step identification, or connect with a certified arborist in your area through our arborist directory. Professional pest identification and treatment plans typically cost $75 to $200 for an initial consultation.