I don’t leave food out. Garbage gets tossed, counters are cleaned. I replaced half the windows in my home and caulked up the rest. I bought a salt gun and have been picking them off but there’s always more.
How do I figure out where these little fuckers are getting in?
Get those sticky paper strips. You might have to wait a few generations, but after they “hatch” they don’t immediately reproduce, but instead look for food first. That’s the small window where you have to get them.
Importantly, you have to keep the strips up at least 1 “cycle” after you’ve caught the last fly!