Are Carpenter Bees Damaging Your Nest Boxes?

By Sylvia Park, Cornell Class of 2028

While monitoring nest boxes for my summer learning experience in Sapsucker Woods, I noticed that birds weren’t the only creatures occupying our boxes. Our wooden nest boxes had attracted carpenter bees (Xylocopa spp.), which drill tunnels with almost perfectly circular entrances in soft wood to create their nests. Though these bees aren’t aggressive and are often important pollinators, their presence has caused structural damage, threatening the safety of nest box inhabitants. 

Although carpenter bee damage alone usually doesn’t deter birds from nest boxes,  the effect is intensified when their predator, woodpeckers, aggressively peck into the bees’ tunnels in search of larvae. This can leave large holes in nest box walls, making it unsafe for avian occupants.

Recognizing Carpenter Bees

Carpenter bees are often mistaken for bumble bees. However they have shiny, mostly hairless black abdomens instead of the fuzzy abdomens of bumble bees. 

During nesting season, the female carpenter bees bore a round entrance hole into wood, drill a tunnel, and lay eggs inside. Larvae develop within the tunnel and emerge when fully mature. While females are capable of stinging, they rarely do so; males, despite their territorial hovering near nest entrances, are completely harmless. 

It’s important to understand that carpenter bees are beneficial to the environment despite the structural damage they might cause. They are important pollinators for many plant species that provide essential food and habitats for birds and other wildlife.

Signs of Carpenter Bees

• Perfectly round holes—Carpenter bees drill round holes about 0.5 inches in diameter into wooden surfaces. These can appear on the roof, sides, or underside of nest boxes.
• Fresh Sawdust—Fine sawdust piles near the holes signal active excavation.
• Buzzing Sounds—You might hear loud buzzing coming from the holes, especially during warm days in late spring and early summer.
• Visible bees—Female carpenter bees will enter and exit the tunnels, while the males will patrol the area and appear territorial (though are incapable of stinging).
• Damage by Woodpeckers—If you notice irregular holes, long vertical scratches, or shredded wood around the carpenter bee holes, it’s most likely that woodpeckers have found the carpenter bees’ nesting place. This secondary damage weakens the structural integrity of nest boxes and makes it dangerous for birds.

Prevention and Maintenance 

Early detection and repair of carpenter bee damage can help prevent further harm. This can help keep nest boxes in good condition for cavity nesting birds. If you know that carpenter bees are prevalent in your area, consider painting or staining the exterior before installing the box. Carpenter bees prefer bare, untreated wood, and painting the outside of the box can discourage them (see our FAQ on how to paint nest boxes). If you prefer your nest boxes to have a natural finish, apply an annual coating of almond oil to the exterior in the spring.

Check boxes occasionally, especially during the spring for early indicators of boring. If there are signs of damage, reference the guide below on how to repair the box. Ideally, try to start repairs as soon as you notice the first signs of drilling, to minimize the loss of bee eggs or larvae which will eventually be living in the hole. This will also reduce the chance that a woodpecker will enlarge the holes while feeding. If necessary, install metal or plastic plates on the box to prevent carpenter bees from starting tunnels and woodpeckers from further excavating. 

Repair Carpenter Bee Damage to Nest Boxes 

If there are round holes in your nest box, it’s important to take action before the damage compounds or attracts predators like woodpeckers.

1. Make sure that there aren’t any adult bees inside it before sealing the hole. 
2. Take a piece of steel wool, tin foil, wooden plug, or anything large enough to fill the hole and insert it snugly into the entrance. If the tunnel isn’t physically blocked, carpenter bees can easily chew through soft fillers.
3. Apply wood filler (choose a product that is designed for exterior use) over the plugged hole to blend with the rest of the box. Once everything is dried, sand the box. Then paint the exterior to discourage any further carpenter bee activity.

Carpenter bees are ecologically important pollinators. With careful monitoring and maintenance, we can create a safe environment for birds without causing harm to these bees that support ecosystems. If you’ve experienced carpenter bee or woodpecker damage at your nest boxes, let us know your observations and solutions in the comments. 

References

• Gibb, T. 2015. Carpenter bees. In T. Gibb (Ed.), Contemporary Insect Diagnostics: The Art and Science of Practical Entomology, Chapter 5: Pest Insects, pp. 153–245. Academic Press, Oxford, UK. https://doi.org/10.1016/B978-0-12-404623-8.00005-3
• Penn State Extension. 2024. The Eastern Carpenter Bee: Beneficial Pollinator or Unwelcome Houseguest? Updated February 17, 2024. https://extension.psu.edu/the-eastern-carpenter-bee-beneficial-pollinator-or-unwelcome-houseguest