I have a shower stall with a drain that is going unused. My first concern is that if the water evaporates out of the plumbing trap below, I could have sewer vapors coming up, so I pour in some water periodically. However my second concern is mosquito larvae growing down inside there since it's so infrequently disturbed i.e. flushed out. I've seen mosquitos in that bathroom, which makes me suspect this problem.
What's the best approach here to prevent both sewer vapors and mosquitos? I've thought about a one way valve for the drain, or maybe pouring oil or human-friendly insecticide down on the water in the trap.
How would you handle this?