Hey guys --
Thanks for all the replies. Also I've been trying to find the technical term for this hole in the concrete and seems most call it a dap out or block out. FYI on that :)
As for solutions, I've thought about using a foam spray, but I've heard ants and other varmints can actually be attracted to it and over time it wouldn't hold up. I also heard some people using roofing tar, but again I'm not sure if that would last as concrete would. I just wasn't sure if there were any reasons for not doing concrete from a construction standpoint, but from reading other articles it seems it's common to fill this in for the very reason I'm wanting to do it. Unfortunately for us our tub is right next to an outside wall so the trap is literally less than a foot from the outside... and with the ground sloping up to the house at almost a 45 degree angle this means ants or any other critters only need to go horizontal in the ground for about 18 inches to two feet and they're under the trap.
So unless there's some really huge reason not to fill it in I'll probably make this a project for an upcoming weekend to put a generous layer of boric acid down then cover the hole in concrete. SHould I put some pee gravel down or something down first though or just put concrete right over the sand?
We have another tub in the second bathroom, but it's more centrally located in the house about 20 feet from an exterior wall. I don't have as easy access to that area as I do in the master (garden tub with removable side) so hopefully that tub isn't another source for ants coming in -- but we'll see over time if it continues to be a problem. If I have to rip out drywall to get to it I can in a closet. But the Master bath seems to be the biggest source of unwanted things entering the house.
Thanks for the advise...