Carpenter Bees and Other Pests: How They Damage Decks (And How to Stop Them)

Every spring we get calls about perfectly round holes showing up in deck railings and the undersides of boards. Almost always, it's carpenter bees. They're the most common pest problem we see on decks in Louisville, but they're not the only one worth watching for. Here's what actually causes damage, how to tell them apart, and what stops each one.

Carpenter Bees — The Most Common Culprit

Carpenter bees don't eat wood. They bore into it to build nesting tunnels, which is arguably worse for a deck since it's structural excavation rather than feeding damage. Look for perfectly round entrance holes about 3/8 to 1/2 inch across, usually on the underside of railings, fascia boards, or deck boards — carpenter bees prefer surfaces they can approach from below, out of direct sun and rain.

The damage compounds two ways. First, the tunnels themselves weaken the wood, especially when the same female returns to the same hole year after year and extends it deeper. Second, woodpeckers hunting for the bee larvae inside will peck the wood open to get at them, turning a small round hole into a much larger, ragged one.

Why Carpenter Bees Target Deck Wood Specifically

Carpenter bees strongly prefer bare, unpainted, or weathered softwood — that describes a lot of deck railings and trim. They're far less interested in painted wood, and composite decking isn't a wood-boring target for them at all.

Preventing Carpenter Bees

• Paint beats stain — a solid paint film is the strongest deterrent since it fully seals the wood grain carpenter bees key in on. Stain helps but doesn't block the wood surface the way paint does.
• Stay current on staining anyway — see our post on why staining matters. Freshly sealed wood is less attractive to bees than bare, weathered wood.
• Fill old holes properly — treat the hole at night when the bee is inside and less active, then fill with wood filler or a dowel and caulk, and paint or stain over it so the spot doesn't get reused next season.
• Composite trim and railing — if a particular spot gets hit every year, swapping that piece to composite removes it as a target entirely.

Termites — The Ground-Contact Risk

Termites are less of a deck-surface problem and more of a structural, ground-contact problem. Subterranean termites travel from the soil into wood that's touching or close to the ground — posts, footings, and ledger areas are the higher-risk spots, not the decking boards up top.

Preventing Termite Damage

• Keep wood off the concrete and soil — minimizing direct wood-to-ground contact on posts and footings cuts down on termite access, not just rot risk.
• Use properly treated lumber — pressure-treated wood resists termites as well as rot.
• Keep mulch and soil away from structural wood — piled mulch against a post or ledger area gives termites an easy bridge.
• Have it inspected periodically — termite damage is often invisible until a post or ledger connection is seriously compromised, since termites tunnel through the inside of the wood.

Carpenter Ants — Usually a Symptom, Not the Cause

Carpenter ants get blamed for wood damage they didn't start. Unlike termites, they don't eat wood — they excavate nests in wood that's already soft, usually from existing moisture damage or rot. Finding carpenter ants in a deck is often a sign there's already a moisture problem somewhere nearby, not a standalone pest issue.

If you see carpenter ants around your deck, the real fix is finding and correcting whatever's keeping that wood wet — a leaking ledger connection, poor drainage, or debris trapped between boards holding moisture.

Powderpost Beetles — Less Common, Worth Knowing

Powderpost beetles leave small, round exit holes and a fine, powdery frass, and they mainly target untreated hardwood sapwood. They're much less of a concern on pressure-treated decking framework, but can show up in untreated trim or railing pieces made from certain wood species.

General Prevention Checklist

• Keep all exposed wood surfaces painted or stained — including undersides of railings and fascia where carpenter bees target
• Keep structural wood off concrete and soil wherever possible
• Clear debris from between deck boards so moisture doesn't sit and invite ants
• Walk the deck each spring and check for new holes, soft spots, or frass
• Fill and refinish old bee holes promptly rather than leaving them for woodpeckers to find

When to Call a Pro

A few carpenter bee holes are a DIY fix. Widespread damage, soft or spongy structural members, or any sign of termite activity near posts or the ledger is worth a professional look — see our post on repairing vs replacing rotted posts for how we make that call on the fence side, which applies just as much to deck posts.