Carpenter bee tunnels start as a perfectly round hole about half an inch in diameter, drilled straight into the surface of bare wood. But the real damage happens inside, where the tunnel turns 90 degrees and runs with the grain of the wood, sometimes extending six inches or more in a single season. Over multiple years, returning bees expand these galleries through branching, and what began as a single small hole can become a network of tunnels that compromises the structural integrity of decks, fascia boards, pergolas, and other wooden structures.
What the Tunnels Look Like Inside
The entrance hole is drilled against the grain of the wood and goes about one inch deep before making a sharp right-angle turn. From that point, the tunnel follows the wood grain horizontally (or vertically, depending on the board’s orientation). The female bee chews through the wood using her strong mandibles, leaving behind a smooth, clean bore that looks almost machine-made.
Inside the main gallery, the bee creates six to eight individual chambers, each separated by a wall made of regurgitated wood pulp. She works from back to front, packing each chamber with “bee bread,” a mixture of pollen and nectar that will feed the developing larva. She lays a single egg on top of this food supply, seals the chamber, and moves on to the next one. The result is a series of sealed compartments stacked end to end inside the tunnel, each containing one egg and its food source.
Bees prefer wood that is at least two inches thick, which gives them enough depth to make that initial turn and build out their gallery. Thinner boards are less attractive because there simply isn’t room for the tunnel architecture they need.
How Tunnels Expand Over Time
A single season of tunneling from one bee causes only minor damage. The real problem is generational. New adults that emerge from the tunnel often reuse and expand the same galleries the following spring, adding branches off the original tunnel. After several years of this activity, a single entry hole can lead to an extensive network of interconnected passages running through the interior of a beam or board.
This cumulative damage is what eventually threatens structural integrity. A deck railing or porch joist that has been tunneled through repeatedly for three or four years can become hollow enough to weaken under load. Severely damaged wood sometimes needs full replacement, particularly if it’s a load-bearing component.
Woodpecker Damage Makes It Worse
Woodpeckers are natural predators of carpenter bees, and the sound of active boring attracts them. Once a woodpecker locates a tunnel, it will hammer into the wood to reach the adult females and larvae developing inside. This secondary damage is often more visible and destructive than the tunnels themselves, leaving ragged, splintered holes that expose even more of the wood’s interior to moisture and further decay.
If you’re seeing large, rough-edged holes alongside the clean round entry holes, woodpeckers are likely compounding the problem.
Which Wood Gets Targeted
Carpenter bees strongly prefer untreated, unpainted, bare softwood. Redwood, cedar, pine, and cypress are common targets. They don’t eat the wood (unlike termites). They simply excavate it to create nesting space. Painting or staining wood provides some deterrence because the bees prefer surfaces they can grip and bore into directly. A thick coat of paint isn’t foolproof, but it significantly reduces the odds that a bee will choose your structure over an easier option nearby.
Hardwoods are much less susceptible simply because they’re harder to chew through. If you’re building or replacing a structure in an area with heavy carpenter bee pressure, choosing a dense hardwood or a composite material eliminates the problem entirely.
How to Treat Active Tunnels
The standard approach is to apply an insecticidal dust or aerosol spray into active tunnels, then seal the entrance. Timing matters: treatments work best when applied after dark on a cool evening, when the bees are inside the tunnels and less active. Wasp and bee aerosol sprays are effective for this purpose and widely available.
The critical step most people miss is treating before sealing. If you plug a tunnel without first eliminating the bees inside, they will simply bore a new exit hole, creating additional damage. Larvae developing inside sealed, untreated tunnels will also chew their way out when they mature.
How to Seal Tunnels Properly
Once the bees inside a tunnel have been eliminated, sealing the entrance prevents reuse. The best materials for this are cork plugs or short sections of half-inch wooden dowel, not caulk or expanding foam. The reason is practical: if any larvae haven’t yet emerged, a solid plug like cork or dowel keeps the interior space open enough for them to move through residual insecticidal dust. Caulk or foam fills the interior space, which forces hatching bees to drill a new exit hole instead of crawling through the treated area.
A more aggressive approach for tunnels you’re certain are fully inactive is to push a small wad of steel wool into the hole first, followed by wood glue and a half-inch dowel. The steel wool acts as a physical barrier that bees cannot chew through if any remain. Once the glue sets, the repair is essentially permanent.
For cosmetic repairs, you can sand the dowel flush with the wood surface and paint over it. This also adds a layer of protection against future boring in the same spot.
Preventing New Tunnels
Paint is your single most effective preventive measure. A solid coat of exterior paint on all exposed wood surfaces, especially end grain and the undersides of horizontal boards, makes the wood far less attractive. Stain offers some protection but less than paint, since it doesn’t create the same surface barrier.
Carpenter bee traps, which mimic the appearance of tunnel entrance holes, can reduce populations around a structure. These are simple wooden blocks with pre-drilled holes that lead into a collection jar. They work best when placed near areas of known activity early in the spring, before nesting begins. Commercial versions cost around $20, or you can build one from scrap wood.
Almond oil applied to bare wood surfaces acts as a natural repellent. Standard sweet almond oil from a drugstore works for this purpose. One drawback: the oil can trap small insects on the wood surface, which attracts woodpeckers looking for an easy meal. In areas with active woodpecker populations, this trade-off may not be worth it.