I do think some people build up an immunity to sand flea bites. My husband used to have those huge red/purple welts for a couple weeks whenever he was bit. Now, either he doesn't get bit (unlikely) or he doesn't react.

I've been on Ambergris for about a month and have only seen a couple of mosquitos and that was early on, when there was a bit of rain. If the sand fleas are biting, I haven't noticed. The bug spray remains unused on the counter, haven't touched it at all. I do live south of town, not north, if that makes a difference, and I'm on the beach, so it's windy.