Take a glass and put some fruit-fly bait in the bottom. A piece of tomato or banana works well.
Cover the top with saran wrap and poke some decent-sized small holes in the top [just realized my holes have been too big -- they should be tiny, made with a pencil tip or some such]. Use a rubber band to affix.
Put the trap on your counter and let the hunt begin. Every morning and sometimes every few hours subsequently I have to cover it with my hand and bring it outside to release the catch. (I feel kind of silly letting them go free, but it’s too much trouble to kill them). I find it kind of fascinating that the cup is often hot from the methane emitted by the rotting banana.
It’s gotten hot and sultry, and too many fruitflies buzzing around makes me feel like I’m in Baby Doll.