Hi there,

The most effective thing for carpenter bees is to paint or varnish the wood, this deters most of them from using it. If that isn't a viable option, you can attempt two other methods of management. The first would be to use carpenter bee "traps" to catch segments of the population and ultimately reduce the number attempting to use the deck. It won't kill all of them in the area though. Finally, you can apply insecticides, either a dust product into each hole or a liquid across the entire deck. The dust option is more effective, you can apply a dust at night to limit stinging chances, dusts can be applied with a specialized dust applicator or a turkey baster that is never used for food again. Leave the hole open so they can get coated in the dust. This fall you can fill the hole with a dowel rod or wood putty.