I am not a termite expert. My buddy has an exterminating service, he is pretty smart, and he does not remove termites. He refers it out because it is so easy for the termites to re-appear, so he leaves it for the specialists. Is the core of your deck wood? lf so, by the time you see the pellets, they are doing damage. Termites certainly won't eat through fiberglass to remove their waste, so it is hard to see the damage happening. I am wondering if a space heater can bring the temp of a wood core up to kill the termites?
I just bought a 1977 Lancer with repaired termite damage--replaced wood supports and such, and I still see active pellet accumulation is some of the original wood. I plan to replace all suspect wood. I am lucky because my deck has a fiberglass core, but the supports under my cockpit will need to be examined. I have cleaned everything and am monitoring for pellets.