What Eats Trees in Your Yard? Identifying Wood-Boring Pests

When homeowners ask, 'what eats trees,' they are usually reacting to sudden canopy thinning, mysterious sawdust at the root flare, or stripped bark. The organisms that consume trees fall into three distinct categories: internal wood-borers, external defoliators, and mammalian bark-strippers. Identifying the exact culprit is critical, as the treatment protocol for a beetle larva tunneling through the xylem is vastly different from the protocol for a caterpillar chewing through the foliage. This guide provides a comprehensive diagnostic framework to identify tree-destroying pests and outlines exact, modern treatment protocols.

The Primary Culprits: What Eats Trees from the Inside Out?

Wood-boring insects are the most lethal tree pests. The adult females lay eggs in bark crevices, and the hatched larvae tunnel into the cambium and sapwood, severing the tree's vascular system. By the time external symptoms appear, the internal damage is often severe.

Emerald Ash Borer (EAB)

The Emerald Ash Borer (Agrilus planipennis) exclusively targets ash species (Fraxinus). According to the multi-state Emerald Ash Borer Information Network, EAB has killed hundreds of millions of ash trees across North America. The definitive diagnostic sign is the 1/8-inch (3-4 mm) D-shaped exit hole on the trunk. Beneath the bark, larvae create serpentine, S-shaped galleries packed with fine sawdust (frass). Canopy dieback typically begins in the upper third of the tree before progressing downward.

Asian Longhorned Beetle (ALB)

The Asian Longhorned Beetle (Anoplophora glabripennis) is a polyphagous borer that attacks over 100 species of broadleaf trees, particularly maples, elms, and willows. Unlike EAB, ALB larvae tunnel deep into the heartwood. The exit holes are large, perfectly round, and measure about 3/8-inch in diameter. Heavy infestations result in structural failure, as the heartwood is hollowed out, causing branches to snap under their own weight.

Defoliators: What Eats Tree Leaves and Needles?

Defoliators consume the photosynthetic tissue of the tree. While a single season of defoliation rarely kills a mature, healthy tree, consecutive years of severe leaf loss deplete carbohydrate reserves, leading to secondary infections and eventual decline.

Spongy Moth (Formerly Gypsy Moth)

The Spongy Moth (Lymantria dispar) is a voracious defoliator that heavily targets oaks, aspens, and birches. Penn State Extension notes that during outbreak years, larvae can strip an entire oak canopy in weeks. Look for tan, spongy egg masses (about 1.5 inches long) attached to the lower trunk and branches in late winter. In spring, the hatched caterpillars spin silken threads to balloon into the canopy, leaving behind ragged leaf edges and accumulating dark, pellet-like droppings (frass) on the ground below.

Bagworms

Bagworms (Thyridopteryx ephemeraeformis) primarily attack evergreens like arborvitae, junipers, and Leyland cypress. The larvae construct protective, spindle-shaped bags out of silk and the host tree's needles, making them look like small, hanging pinecones. Because they consume the needles from the inside of the bag outward, damage is often unnoticed until large brown, dead patches appear on the evergreen.

Mammalian and Rodent Damage: Bark Strippers and Root Chewers

When investigating what eats trees, do not ignore mammalian damage, which frequently occurs in winter when alternative food sources are scarce.

Diagnostic Decision Tree: Is Your Tree Being Eaten?

Use this rapid symptom-checker to narrow down the pest category before calling a certified arborist.

• Symptom: Fine, powdery sawdust (frass) at the base of the trunk + D-shaped holes.
  Diagnosis: Emerald Ash Borer.
  Action: Immediate trunk injection if canopy dieback is under 30%.
• Symptom: Large, round holes (3/8 inch) + sawdust + branch snapping.
  Diagnosis: Asian Longhorned Beetle or native carpenterworm.
  Action: Report to local Department of Agriculture (ALB is a federally quarantined pest); removal likely required.
• Symptom: Entire canopy stripped of leaves in June/July + silken webbing on trunks.
  Diagnosis: Spongy Moth or Tent Caterpillar.
  Action: Apply Btk (Bacillus thuringiensis var. kurstaki) during early larval instar stages.
• Symptom: Bark missing at soil line exposing the cambium + small runways in mulch.
  Diagnosis: Vole girdling.
  Action: Bridge grafting for severe cases; clear mulch 3 inches away from the trunk flare.

Targeted Treatment Protocols and Costs

Treating tree pests requires precise chemical selection and application timing. According to University of Minnesota Extension, systemic insecticides are the standard for internal borers, while contact or ingestion insecticides are used for defoliators.

Wood-Borer Chemical Interventions

For Emerald Ash Borer, the gold standard treatment is Emamectin benzoate (often sold under the brand name TREE-äge). This is applied via professional trunk injection. The chemical translocates through the xylem and kills larvae feeding in the cambium. A single injection provides two years of protection.
Cost Breakdown: Professional trunk injections are priced by Diameter at Breast Height (DBH). Expect to pay between $15 and $22 per DBH inch. Treating a mature 20-inch ash tree will cost between $300 and $440 per application cycle.

For smaller trees or DIY soil drenches, Imidacloprid is an alternative. It is applied to the soil in early spring and taken up by the roots. However, it is less effective than emamectin benzoate for trees already showing moderate canopy decline and requires annual application.

Defoliator Interventions

For Spongy Moth and other caterpillars, Spinosad or Btk (Bacillus thuringiensis var. kurstaki) are highly effective biological controls. Btk is a naturally occurring soil bacterium that produces proteins toxic only to specific caterpillar species when ingested, making it safe for pollinators and mammals. Apply Btk as a foliar spray when caterpillars are small (early instars, usually mid-to-late May). A 16 oz concentrate of Btk costs approximately $35-$45 and treats multiple mature trees.

Understanding what eats trees is the first step in preserving your landscape. By matching the specific physical evidence—whether it is a 3-millimeter D-shaped hole, a 45-degree bark cut, or silken webbing—to the correct biological culprit, you can deploy targeted, cost-effective treatments that protect your property's arboreal assets for decades.