It’s bad news for your porch and deck.
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Carpenter Bees
Credit: Tahreer Photography / Getty Images

Have you ever heard of the Law of Unintended Consequences? It expresses the thought that sometimes an action to improve something ends up making something worse. And that's certainly the case when it comes to the EPA, pressure-treated lumber, and a pest called the carpenter bee.

The Difference Between Carpenter and Bumblebees

You probably have carpenter bees buzzing around your house right now. They're almost dead ringers for friendly, beneficial bumblebees, but there's an easy way to distinguish them. A bumblebee has a hairy yellow abdomen. The abdomens of carpenter bees are hairless and glossy black.

Carpenter bees damage wood by boring perfectly round, half-inch wide holes into it to lay eggs and build a tunnel-like nest. They don't eat the wood as termites do—they just excavate it. I think they must have the strongest jawbones in the world because they can quickly bore through wood too hard to drive a nail. Each year the bees locate the old holes, clean them out, and enlarge the chamber to raise more young. Over time, this can seriously weaken the wood.

These insects used to be the bane of mainly weathered, untreated wood like old fence posts or unpainted siding. Carpenter bees typically avoided stained or painted wood (and still do, though they may still attack it). To prevent damage to newly built wooden decks and porches from bees, people relied on pressure-treated lumber containing CCA—chromated copper arsenate. The chemicals in CCA prevented rot and insect damage. I've dug up pieces of old pressure-treated lumber buried in the ground for 20 years and not found a single insect hole.

Why Are Carpenter Bees Still a Problem?

In 2003, however, the EPA banned the use of arsenic in pressure-treated wood used for home construction, claiming that it leached into the ground and posed a hazard. Pressure-treated lumber now contains the preservative ACQ, which stands for "alkaline copper quat." It claims the higher concentration of copper in ACQ would compensate for the insecticidal arsenic no longer there.

Unfortunately, carpenter bees found a way to persist through this new formula.

New Pressure-Treated Wood

The problem with the ACQ new pressure-treated wood is the higher concentration of copper. It's best not to use aluminum nails or flashing with this wood because the copper and aluminum generate a tiny electrical current that quickly corrodes the aluminum. Use only stainless steel or galvanized steel nails and flashing. Also, the ACQ treatment can cause skin damage, so be careful when using it and remember to protect your eyes.

How To Control Carpenter Bees

Knowing where they like to attack is essential if you plan to kill or discourage carpenter bees. They want lumber at least two inches thick, like a rail or joist, and they bore in the narrow underside where you're least likely to see them. Little piles of sawdust on or under your deck mean bees are working. I fill up a tank sprayer with Bayer Advanced Carpenter Ant & Termite Killer Plus (yes, I know, it says, "carpenter ant," but it also works on carpenter bees) according to label directions. I stick the spray nozzle into the hole and soak it. Next, I spray the narrow undersides of all rails and joists to prevent new cavities. After 24 hours, I added wood filler to fill the holes I sprayed yesterday. Unfilled holes act like magnets for bees.