Spraying poisons around your house will not have much affect on your water supply although it will adversly impact the water supply of someone downstream from you, and anyone that sprays poisons upstream from you adversly affects your water supply. Given that these pollinators are dying off in large numbers and pollinators are an absolute necessity in providing the foods we eat why on earth would someone consider poisoning them? There are estimates that within 4 years of the pollinators all being killed off by the poisons we spray we too will all die, from starvation.

I would try to kill 2 birds with one stone. A once-a- year treatment of wooden structures is often recommended. I have used these products on fences and sheds. I have often thought that if I were an insect, I would not want to hang around woodwork so treated.
The insect is considered to be beneficial except in cases like you describe. Placement of the treatment is critical if you want to protect the deck, beneficial insect snd water supply. I am quite sure that it can be done but it does require a little bit of interesting investigative work. And, just in case you need it, a little bit of good luck also!

ronalawn, what kind of treatment are you referring to? A normal outdoor wood preservative kind of thing or something particular to this purpose?

legacy, the product you mention is not labeled for anything but 'crack and crevice' use, and thus, must not be used indiscriminately to 'protect' your deck. The active ingredient is very toxic, indeed.

Yes, though these insects are valuable pollinators, they can also do significant damage to wooden structures. Preventing them from using your deck as a nesting site requires painting or some sort of treatment such as ronalawn mentions.

Thanks for your responses.

I do know that bees are beneficial but this particular kind of bee can destroy wooden structures so I don't really have a choice. We are planning to only spray into the holes they have bored.

The product which I have used is "Thompson's" water seal for protection of the wood. I was merely suggesting that in so doing, you will protect the wooden structure and as a bonus, may repel the carpenter bee(s).