How do I get rid of flying insects near my cat's litter pan?

I don't know where you live, but I'm in southern California and in my neighborhood, everybody seems to be having the same problem (including me). I also tried pouring a bunch of bleach down the drain, but the very next day, the little insects were back. What I'm doing now, until I can figure out a better solution, is stuffing the overflow holes on the sink with little wads of toilet paper so that the bugs can't enter from there, and I'm keeping the sink drain plug in the closed position so they can't enter from the drain either. When I need to use the sink, I turn the water on BEFORE opening up the drain, so that if there's a bunch of insects just under the surface of the plug, the rush of water when I open the drain plug will push them back down the drain. Then when I'm done using the sink, I just close the drain plug again. It's a hle, but for the past few days it's been working.