I have a deck drain from my pool area which is enclosed in a screen. The
drain is a 4" PVC pipe that drains to the outside of the house into a gravel
pit.

I noticed quite a bit of no-see-ums in the screen enclosure area, and this
should not happen since it's screened in with insect screen that no-see-ums
cannot pass through. I am suspecting they are coming up from the deck drain
pipe that drains into the gravel pit. I guess the only way to stop this is
to either cover the outlet with a screen mesh material but that would
inhibit the flow when it rains and also trap debris. The other option is to
install a p-trap in the drain line, but do they make a p-trap for 4" PVC
pipes?

Thanks,

MC