Keeping the bugs out

To prevent insect entry contractor wants to use pt t&g flooring in new screened porch (about 12\" off the ground)rather than 5/4 pt pine with screening underneath. Is the pt t&g dimensional stable?

I'm not sure what you mean when you write "pt", I will assume you mean "paint".  I would suggest using a cedar or redwood T & G floor.  It would be best to prime all 4 sides before you install to help minimize any future wood rot issues.  Cedar and redwood are both fairly rot and insect resistant. I am not a big fan of using pine on the floor or installing the screen below the floor joists.  Pine wood is fairly unstable under extreme weather conditions and is subject to wood rot.