Identifying Insect Damage in Slabs: Wood Handbook Guide

Identifying Insect Damage in Wood Slabs

Live edge slabs and reclaimed wood frequently feature insect trails, boring holes, and unusual internal markings. In an epoxy tabletop or river table build, these features often add dramatic visual character once cast in resin. However, diagnosing what caused the damage is essential for understanding whether the activity ended in the forest or if it poses an ongoing risk to seasoned wood. The USDA Forest Products Laboratory details these insect signatures in Table 14–2 of the Wood Handbook (General Technical Report FPL-GTR-282). Here is a common-mistakes breakdown for identifying insect damage in slabs.

Mistake 1: Assuming All Small Pin Holes Mean Active Beetles

When makers find small pin holes measuring 0.25 to 6.4 mm (1/100 to 1/4 in.) in diameter, they often worry about ongoing powder-post beetle infestations. In reality, the insect species and tunnel characteristics tell very different stories:

• Ambrosia beetles: These create open circular tunnels measuring 0.5 to 3 mm (1/50 to 1/8 in.) in diameter. The holes are usually centered in a dark streak or ring in the surrounding wood. Their damage begins in living trees and unseasoned logs, and it ends during seasoning. Once the slab is dry, the beetles are no longer active.
• Timber worms: These beetles make open holes of variable sizes. The surrounding wood is rarely dark-stained, and the tunnels are lined with a wood-colored substance. Their damage begins in living trees or unseasoned logs and ends before seasoning.
• Lyctinae powder-post beetles: Unlike ambrosia beetles, their exit holes measure 0.8 to 1.6 mm (1/32 to 1/16 in.) in diameter and appear exclusively in the sapwood of large-pored hardwoods. Tunnels are packed with loose, floury sawdust. Their damage begins during or after seasoning, and reinfestation continues until the sapwood is destroyed.
• Anobiid or Ptinid powder-post beetles: Exit holes range from 1.6 to 3 mm (1/16 to 1/8 in.) in diameter, primarily in sapwood and rarely in heartwood. Tunnels are loosely packed with fine sawdust and elongate pellets. Damage usually begins after wood is in use, and reinfestation continues very slowly.
• Bostrichid powder-post beetles: Exit holes measure 2.5 to 7 mm (3/32 to 9/32 in.) across, primarily in hardwood sapwood and minorly in softwoods. Sawdust ranges from fine to coarse and is tightly packed. Damage ends during seasoning or redrying.
• Wood-boring weevils: Exit holes measure 1.6 to 2 mm (1/16 to 1/12 in.) across in slightly damp or decayed wood, with very fine sawdust or pellets tightly packed. Reinfestation continues while the wood remains damp.

Mistake 2: Overlooking the Shape of Larger Grub Holes

Grub holes range from 3 to 13 mm (1/8 to 1/2 in.) in diameter, but their exit shape and sawdust packing identify the causal insect:

• Roundheaded borers: Leave circular exit holes of 3 to 13 mm (1/8 to 1/2 in.) mostly in sapwood. Tunnels contain coarse to fibrous sawdust, or sawdust may be absent. Activity begins in living trees or unseasoned logs and ends when adults emerge or the wood dries.
• Flatheaded borers: Leave mostly D-shaped exit holes of 3 to 13 mm (1/8 to 1/2 in.) in both sapwood and heartwood. Tunnels are tightly packed with sawdust, and damage ceases when adults emerge or the wood is dried.
• Old house borers: A roundheaded borer that leaves circular exit holes roughly 6 mm (~1/4 in.) in diameter in the sapwood of softwoods, primarily pine. Tunnels are packed with very fine sawdust, and reinfestation continues in seasoned wood in use.
• Woodwasps: Leave perfectly circular exit holes measuring 4 to 6 mm (1/6 to 1/4 in.) across, primarily in softwoods. Tunnels are tightly packed with coarse sawdust, often in decay-softened wood. Damage ends when adults emerge from seasoned wood or when kiln-dried.
• Carpenter bees: Drill perfectly circular nest entry holes and tunnels roughly 13 mm (~1/2 in.) in diameter in soft softwoods in structures. Nesting reoccurs annually in spring.

Mistake 3: Confusing Gallery Networks Left by Social Insects

Networks of interconnected tunnels and chambers indicate social insects with colonies. Diagnosing the causal insect relies on the interior condition of the cavities:

• Carpenter ants: Excavate damp, partly decayed, or soft-textured wood. The gallery walls look polished, and the spaces are completely clean of debris. The colony persists unless prolonged drying of the wood occurs.
• Subterranean termites: Carve galleries where walls are usually speckled with mud spots, and some chambers may be filled with clay.
• Dry-wood termites: Create chambers containing pellets, and areas within the galleries may be walled off by a dark membrane.

Mistake 4: Mistaking Growth Abnormalities for Insect Tunnels

Not all insect-related features in a slab take the form of open tunnels. According to the USDA Forest Products Laboratory, living trees respond to insects with specific internal markings that cease development while the tree is still standing:

• Pitch pockets: Openings between growth rings containing pitch, caused by various insects in living trees.
• Black checks: Small packets in the outer layer of wood created by grubs in living trees.
• Pith flecks: Narrow, brownish streaks caused by fly maggots or adult weevils in living trees.
• Gum spots: Small patches or streaks of gum-like substances produced in response to grubs in living trees.
• Ring distortion: Double growth rings or incomplete annual layers caused by larvae of defoliating insects or flatheaded cambium borers.
• Staining fungi: Stained areas more than 25.4 mm (1 in.) long introduced by insect wounds in trees or recently felled logs, which stop spreading with seasoning.

When preparing slabs with void spaces or insect galleries for casting, builders should follow the resin maker’s data sheet for sealing and deep pours to ensure complete penetration into open chambers.

Sources

• USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021)

Published October 29, 2026. Sources are linked in the text.

Header image: original graphic by BigEpoxy.com.