I’ve been trying to get rid of these little bastards for a week. I’ve been policing beer bottles, scrubbing counters, everything - there’s no fruit in the house and nothing tasty for them to be breeding on! I’ve put out glasses of cider vinegar and dish soap and killed hundreds of them, and they keep coming from somewhere!
Er, from that bag of peaches lurking under the bread in a bowl. That my mom gave me. Two months ago.
Urrrrrk.
Oh…my sympathies.
I had the same problem a couple of years ago.
My “patient zero” was the loose and sticky cap to an ancient bottle of Rumplemintz. There must’ve been over a hundred on that thing. Killed them, dumped it, tracked down the survivors over the next week or so.
Take a short glass and put a little cider vinegar in the bottom. Take a dab of dishwashing liquid and rub it around the inside of the top of the glass. They fly in the glass and the dish soap keeps them from getting out and they drown.
Ew. I share your pain. I started getting weekly organic produce boxes about 5 weeks ago, and have never put fruit in a bowl on the kitchen table until this. I’ve also got a lot more fruit peels and vegetable cuttings in the trash. Never really had fruit fly problems before, but now, where the heck do the little suckers come from, anyway? The fruit is nice on display, but keeping everything in the fridge, including peels and cuttings in a bag until trash day, seems to be the answer.
We had a housefly infestation years ago that turned out to stem from a bag of onions. We’d ignored the onions because they looked just fine from above, but a fine little maggot nursery had been established on the underside.
Trust me, few things are worse than finding “patient zero” in your walls.
A few years back I had an apartment in Cedar Rapids, IA. Second floor (top). For reasons only God knows, a racoon decided to pry up the shingles, crawl in, fall into my wall (spare bedroom, closet wall) and die.
The smell was absolutely devastating. It took me a week or two to isolate it. By the time I had actually found it - well, it was spawning maggots left and right.
My apartment managers were uniquely awful and worthless. See, they had already flooded my apartment. I had to threaten small claims court to get them to even partially pay for that, so by this time we weren’t on good terms.
Anyways, this fucking raccoon died in my wall. My whole place smelled like death (literally.) It started spawning maggots.
I complained, loud and hard. I got home one day to find that management’s solution was…
to put a bunch of air fresheners in that particular room.
“GET THE DEAD RACCOON OUT!” I cried. “Eh, we’d have to take apart the wall to do that” they replied. “THEN FUCKING DO IT!” I screamed. “Nah, that would cost a lot of money.”
I had only about three months left, this place had fucked me like five times before - so I decided just to suffer through it. Honestly, they had worn me out with their previous bullshit when their maintenance crew wrecked the hot water heater in my unit and it flooded the place, destroying near everything.
So I sealed up the room. Windows shut, door shut, towels stuffed under the door. I did open it a month or two later and my God - the floor was black with dead houseflies. Easily one of the most disgusting things I’ve ever witnessed.
Fuck this place so hard.
Pssst. Read the OP again.
I once had as patient zero a missing capful of balsamic vinegar lost behind something on the countertop. They must have been in heaven.
Dear lord those things are evil.
One of my little ones somehow managed to get a half-eaten banana under the radiator in the dining room. Took a month to find - not pretty.
Ours was three bananas that had fallen in the crevice between the counter and the fridge.
The best trap I found was to put a piece of banana in a glass, put a funnel in the top of the glass, and tape all along the edge to seal it. (You can make the same kind of thing by cutting off and inverting the top of a plastic bottle and taping it.)
That is the worst smell ever. It happened to me once when they sided an apartment building I lived in and the birds in the eves were trapped. I’m going to go heave now.
It really makes you understand why people used to believe in spontaneous generation. It’s amazing - suddenly there are eleventy billion nasty little fruit flies trying to swim in your drink. I have eaten several.
Oh, by the way, when I finally found that bag of peaches? Smelled surprisingly like peach brandy. Almost tempting.
Wow. That’s some world class negligence there.
Spontaneous generation. 
I had fruit flies all over the place not long ago until I threw out a can of cat food that my kitty never did finish back when the food was still viable. Imagine my relief at suddenly not swatting at a different little winged bastard every 10 minutes.
Well that’s not bad for an apartment community run by pheasants. 
You think you’ve got problems! (Not for the squeamish.)
I have not had a single fruit fly since I bought one of these. I buy bananas all the time and have heard that there are fruit fly eggs on the peels so that’s where they come from, often. But I set one of those traps on the shelf next to the bananas and have not seen one fruit fly. They claim the bait only lasts a month but I have found it lasts a lot longer.