Well first off bugs, spiders and other critters with probably the exception of cockroaches don’t come up drains. Most critters don’t care to get wet I think, so it is my opinion that they come in from the attic. That S or P trap is designed to have water in it, if water is in the trap this will stop sewer gases and roaches from coming up. If you have a shower or bathtub that you don’t use often just remember to fill it with water occasionally. I also have recommended the use of 1 tablespoon of mineral oil almost as a cap to reduce evaporation of water in those traps.

So my recommendation is home-sealing, caulk and seal anything that goes through the ceiling drywall or wall and this will alleviate anything trying to get in.