To keep carpenter bees from ruining your hard work, build your deck with painted hardwoods, fill all old bee nests on your property and provide the bees with a nesting alternative that is more attractive than your new deck. Use insecticide to deter carpenter bees only as a last resort because bees are beneficial insects. Deterring the bees will be easier if you haven't yet built the deck and can incorporate bee deterrents as you work, but existing decking still can be protected with a bit more effort.
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If you haven't built your deck, then use hardwoods when you do. Carpenter bees would much rather burrow into softer woods than hardwoods and are less attracted to hardwoods, which include poplar, oak, ash and cherry wood. Treated lumber is also less attractive to the bees than other types of wood.
Fill all cracks, gouges and other holes you find in your deck's wood because they are attractive starting points for carpenter bee burrows. The task of filling holes will be easier if you do so as you inspect each wood board as you build the deck, but filling holes can be done after the deck is complete.
Paint or Stain
Carpenter bees are most attracted to unpainted wood. So painting your deck may allow the paint to act as a bee deterrent. Bees are most likely to be deterred when wood is covered with primer and two coats of an oil- or polyurethane-based paint. Paint repels better than stain, but staining is better than leaving the wood bare.
Although paint or stain can be applied after a deck is complete, painting or staining each board before the deck is built is the best method because coating the sides and bottom of each piece of lumber is easier before the wood is installed.
Offer a Sacrifice
Build a small birdhouse-type structure out of scrap softwood, such cedar or redwood, and place it in your yard well away from your new deck. The carpenter bees may be more attracted to that structure than to your new decking, especially if the decking is hardwood or painted.
Fill Old Nests
Check your home, fencing, shed and all other wooden structures on your property for the telltale holes that signify carpenter bee activity. These holes are perfectly round, about 1/2 inch in diameter and may be stained or discolored from bee feces. When you find such holes, plug them with a ball of tinfoil. If the foil remains undisturbed for 24 to 36 hours, then apply caulk around and over the tinfoil balls and holes.
Plug a carpenter bee hole in wood only if its tinfoil ball remains undisturbed. If you plug a hole too early, bee larva will hatch and simply chew new holes in the wood to escape their nest.
Use the Last Resort
Bees, including carpenter bees, are important pollinators of the food crops humans eat. Killing them with insecticide should be only a last resort. If you must discourage carpenter bees with an insecticide, choose one that contains cypermethrin, a chemical known to repel carpenter bees. Mix a product that is 25.4 percent cypermethrin in a sprayer at a ratio of 1 ounce of the cypermethrin product per 1 gallon of water, and spray the mixture onto your deck. One gallon of the cypermethrin_-_water mixture will treat 500 to 1,000 square feet.
- Wear safety goggles, impermeable rubber gloves and a protective coverall when mixing and applying cypermethrin, and do not inhale its fumes. Cypermethrin is very toxic to fish and requires proper disposal. Always read and follow all directions on the product label when using an insecticide.
- Allow the treated deck to dry before letting people or pets onto the surface.