This is the source most people never think to check, and it’s often the actual root of the problem. Bathroom drains typically contain a P-trap or floor trap designed to hold a small amount of water and block sewer gas from coming back up, and if organic debris builds up in that trapped water, it becomes a genuinely ideal spot for mosquitoes to lay eggs undisturbed. Shine a torch directly into the drain to check for thin, wriggling larvae near the water’s surface, then pour boiling water down slowly, followed by an enzyme-based drain cleaner, to clear out the buildup properly. If a bathroom sees light use, flushing its drains with water at least once a week keeps this from becoming a hidden breeding ground.
