Emerald Ash Borer vs Bronze Birch Borer Decision Tree

Why this matters

A senior arborist gets called to a dying tree, sees D-shaped exit holes on the trunk and dieback in the canopy, and has to call it on the spot: Agrilus planipennis (emerald ash borer, EAB) on Fraxinus, or Agrilus anxius (bronze birch borer, BBB) on Betula. The treatment paths diverge sharply (emamectin benzoate trunk injection is the gold standard for EAB; BBB management is mostly cultural plus a different injection regime), and a misdiagnosis means a USD 300 to USD 500 injection wasted plus a customer who will be back when the tree fails the next year. Both pests are buprestid metallic wood borers in the same genus, and their galleries can look similar at first glance. This decision tree drives the call.

Step 1: identify the host genus

Both borers are highly host-specific.

• EAB attacks ash, primarily Fraxinus americana, F. pennsylvanica, F. nigra, F. quadrangulata, and F. profunda; also documented on Chionanthus virginicus (white fringetree)
• BBB attacks birch, primarily Betula papyrifera, B. pendula, B. populifolia; less commonly B. nigra
• Neither attacks the other genus

Identify the host first. Ash has compound, opposite leaves and diamond-pattern bark on mature trees. Birch has simple, alternate leaves and characteristic peeling bark (white, gray, or salmon depending on species). If the tree is neither, you are looking at a different pest.

Step 2: confirm geographic plausibility

• EAB: present across most of the eastern half of the US, plus expanding pockets in the West (Oregon since 2022). Quarantine maps update annually at the USDA APHIS EAB program page.
• BBB: native across North America wherever birch grows naturally; established in every state with native birch populations.

Both are plausible across most US tree-care markets. EAB in an ash on the East Coast and BBB in a birch in the Upper Midwest are textbook. EAB cannot infest birch; BBB cannot infest ash.

Step 3: exit hole geometry

This is where novices mis-call.

• EAB exit hole: precise D-shape, approximately 1/8 in (3 to 4 mm) wide, flat side typically oriented down
• BBB exit hole: also D-shape, approximately 5/32 in (4 to 5 mm) wide, may be slightly more rounded; in practice very similar

Exit hole shape alone is not sufficient to distinguish. You need host genus plus exit hole; or you need more evidence.

Step 4: gallery pattern under the bark

Peel a section of declining bark with a draw knife or hatchet on a doomed tree (do not bark-strip a tree still worth saving).

• EAB galleries: serpentine S-curves, packed with frass that compresses to a fine sawdust, gallery width increases as larva grows, terminal pupation chamber at gallery end
• BBB galleries: irregular zig-zag, frass usually less dense, gallery in the cambium and outer sapwood, often associated with stress-driven attack on portions of the trunk exposed to heat or drought

EAB galleries cluster densely in mid-canopy and main stem; BBB galleries cluster in the upper trunk and major scaffold limbs first.

Step 5: canopy and trunk symptoms

• EAB symptoms by year:

  • Year 1: minor canopy thinning, no clear sign
  • Year 2: epicormic shoots on trunk and main scaffolds, woodpecker flecking (blonding) of bark
  • Year 3: severe canopy dieback (over 50 percent), D-shaped exit holes visible
  • Year 4: tree functionally dead

• BBB symptoms:

  • Crown dieback starting at the top, progressing downward
  • Raised ridges and oozing dark wet streaks on the trunk (callus tissue over galleries)
  • Dieback often correlated with drought, soil compaction, or heat stress
  • Tree may persist 3 to 7 years with progressive decline

If you see top-down dieback on a birch, BBB is the first hypothesis. If you see general canopy thinning with bark blonding on an ash, EAB is the first hypothesis.

Step 6: bark response

EAB: vertical bark cracks expose serpentine galleries underneath. Woodpecker flecking (blonded patches where bark is removed by woodpeckers hunting larvae) is highly diagnostic and often the earliest visible sign before exit holes are obvious. In Michigan, Ohio, and Indiana data, woodpecker flecking precedes confirmed EAB exit holes by 6 to 12 months.

BBB: raised ridges and rust-colored sap weeping from gallery sites. The bark response to BBB galleries is more visually obvious as a swelling than EAB.

Step 7: confirm with a sample if doubt remains

If after host, geography, exit hole, gallery, and symptom pattern you are still uncertain:

• Collect a 4 in to 6 in section of bark with gallery
• Mail to the state cooperative extension entomology lab; most accept samples free or low cost
• USDA APHIS PPQ also identifies regulated pest samples; EAB is a federally regulated pest

Do not skip this step if the diagnosis drives a USD 300 to USD 500 injection sale or a major removal recommendation.

Treatment decisions

Confirmed EAB on a healthy ash worth saving:

• Emamectin benzoate trunk injection (TreeAge G4 or Arborjet Tree-age R10), 2 to 3 year residual, dose by DBH per the product label
• Best when canopy decline is under 30 percent; trees over 50 percent dieback are usually not worth treatment cost
• Coordinate with municipal EAB program if street tree

Confirmed BBB on a high-value birch:

• Cultural first: irrigation during drought, mulch to 3 in depth out to dripline, hold off on summer pruning that creates fresh wounds, mitigate soil compaction
• Soil-applied imidacloprid or dinotefuran (Bayer Merit, Safari) in spring per the product label
• Trunk injection less commonly used for BBB; consult state extension recommendations

When the diagnosis is "neither"

If exit holes are visible but host genus is wrong, consider:

• Two-lined chestnut borer (Agrilus bilineatus) on oak and chestnut
• Banded ash clearwing (Podosesia aureocincta) on ash (round holes, not D-shaped)
• General secondary borers on stressed trees (Cerambycid larvae)

A clean call on the pest is the difference between a productive site visit and a customer who reads online and second-guesses you.

References

• USDA APHIS, Emerald Ash Borer Program Manual, current revision
• US Forest Service, Emerald Ash Borer Information Network (emeraldashborer dot info)
• Herms DA, McCullough DG. 2014. Emerald Ash Borer Invasion of North America. Annual Review of Entomology 59:13-30
• Katovich SA, Munson AS. Bronze Birch Borer. USDA Forest Service, Forest Insect and Disease Leaflet 111
• ISA Best Management Practices for Insect and Disease Management
• Penn State Extension, Bronze Birch Borer fact sheet, current