Tree borer damage: ID by gallery pattern

Wood-boring insects -- beetles and moths whose larvae feed in the cambium and wood of trees -- are responsible for some of the most serious tree losses in North American landscapes. Their damage is often hidden under bark until it is severe, and by the time crown symptoms are visible, the.

Wood-boring insects — beetles and moths whose larvae feed in the cambium and wood of trees — are responsible for some of the most serious tree losses in North American landscapes. Their damage is often hidden under bark until it is severe, and by the time crown symptoms are visible, the infestation may be too advanced to manage. Knowing what exit holes look like, what gallery patterns indicate, and which borer species attack which tree hosts narrows the ID quickly.

Exit hole shapes

Exit holes are left by emerging adults. Their shape is determined by the cross-sectional shape of the emerging beetle or moth.

D-shaped exit holes (Buprestidae, flatheaded borers): The flat-headed woodborers (Buprestidae) are oval in cross-section, and their exit holes are D-shaped — flat on one side. Per Penn State Extension, this D-shaped hole shape is the most reliable marker for Buprestid beetles. Emerald ash borer (EAB) exit holes are D-shaped, 1/8 inch wide.

Round exit holes (Cerambycidae, roundheaded borers; Scolytinae, bark beetles): Most Cerambycid (longhorned beetle) larvae are cylindrical and leave round exit holes. Bark beetle galleries also produce round entry and exit holes but much smaller (1–3 mm). Per Penn State Extension, round holes of 0.25–0.5 inch diameter suggest a Cerambycid; smaller round holes suggest bark beetles.

Oval exit holes (Cossidae, carpenterworms; Lepidoptera): Carpenterworm moths and related species leave oval, slightly irregular exit holes.