I was thinking about covering the rails of my deck with a composite. Will this deter carpenter bees?
I’m not aware of any kind of exterior siding or decking designed to deter carpenter bees. No doubt they are attracted to a wide range of wood so in general, most any home can become a target for their nests. And I’ve noted they don’t seem to like nesting in plastic wood. So I can say with certainty that any decking or railing made with recycled plastic will be ignored by carpenter bees but I can’t say the same about composite decking.
Basically composite is made of both wood and plastic. And since I’ve seen carpenter bees nest in composite, it doesn’t seem that all such decking will stop them. My guess is it depends on the wood used in the mix, the % of wood involved, the bonding agent, etc. But composites vary so much from company to company, there isn’t any standard to measure for consistency so its not possible to know for sure.
In summary, I wouldn’t install composite with the main goal being to repel carpenter bees. Especially since its so easy to stop them by spraying with Cypermethrin 1-2 in the spring. Cost wise the Cypermethrin is a lot less compared to a new composite deck and even if you did install composite, I would say to spray it with Cypermethrin anyway to prevent insect issues from developing. This would be extra important if you were to cover wood like you mentioned in your question. Basically covering wood with anything is a bad idea because underneath the outer shell the wood will decay and smell good to so many insects. And because there will be a natural void created, it will become the perfect nest location for many more pests besides just carpenter bees.