Yeah they always come out around this time of year. Honestly, we left on vacation for 3 weeks - there was no food left out anywhere but still there was fruit flies in the kitchen somehow. No idea where they came from or what they were eating.
Really easy trick - take a coffee cup or something, pour it like half-full of apple vinegar, and put in a couple of drops of dish soap. They fly to it because of the smell, try to land on the surface but the dish soap causes the surface tension to not be able to hold them and they drown. You'll wake up to a cup full of dead fruit flies.
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