Fruit flys are the first sign something's "wrong" in your house. That orange tucked in the toe of a
Christmas stocking and forgotten, the bag of
onions under the sink that's been there too long, a forgotten
dish under a pile of laundry in the bedroom. You may not even smell a
stink yet or see a
mess, but one morning you'll wake up and have an
infestation of fruit flies. Overnight, they blossom into a huge, buggy swarm of tiny
black and
gray pests. That's the problem with
fruit flies--there's never just one.
They're not stinky, they generally don't spread disease, they don't bite you or your pets, they don't leave gross maggots all about, and they’re not offensive to look at. But they're still one of the most annoying household pests out there, because they are only found in multitudes that make you wonder if you've been teleported back to ancient Egypt in time for the Plagues. And they're inhaleable. That's absolutely no fun.
Now, a single dirty dish usually won't inspire a swarm (although it can later feed an existent one). The culprit is almost always produce left out. Nearly all produce that enters our homes, unless it's been sprayed with nasty goops and gunks, is covered in microscopic bug eggs. These guys eat decaying produce--they don't actively damage foods--and lay their eggs on other pieces--hence the name fruit flies instead of, say, steak flies. (Oof!) Fruit in the fridge usually doesn't hatch, or if it does the hatchlings freeze to death before they emerge into your kitchen. Fruit bowls on the counter, on the other hand, often send forth tiny annoying bugs unless the food is used quite rapidly. When they do, most of the time they also die off without troubling you. But if something's left out and begins to decay, they'll find it. Without fail. And they'll eat, and breed, and eat, and breed, and one morning you'll stumble into the kitchen and wonder who burned what and why you don't smell it, because there's so much gray haze by the stove. Then, after your cup of coffee you'll realize the haze is moving, and it's bugs.
Swarms do not happen without a reason. If there is a swarm, their food source is nearby. If they're near the sink, look in the sink, run the disposal, look under the sink, find the problem. If they're near a trashcan, empty it. Then wash the can itself and spray that sucker with Lysol about seven times. If they're in a place that makes no sense, do some detective work. The swarms ALWAYS stay near their food source. The problem will be no more than two or three feet away, and usually closer. Look from the ground up to the ceiling. Now we've found the problem, and in a perfect world, the flies would be gone. Too bad there's a catch.
Swarms move. If you take away their food source, they WILL find something else to thrive on if you give them the tiniest chance. They will keep relocating, as a whole, until you've eliminated every single thing they could possibly eat. Empty and clean all your trash cans. Do all your dishes. Put all your fruits and veggies in your fridge for now. Check in every cabinet, every drawer. If you didn't leave something there, your roommate, your cat, or your kids may have. Or the house gnomes. You can blame them too. Lysol every surface where there was a problem. Disinfect the hell out of any area they could find food. If you'd like to at least drive them away from your kitchen area so you can use your sink, run about half a head of garlic down the disposal. They don't like the smell and will reassemble somewhere out of your way.
Now wait.
You can't really spray for fruit flies, you can't really swat them, and you can't really do much with them except wait them out. In the meantime keep your place spotless, food wise. The piles of dirty socks in the corner, they won't touch. An apple core in the living room and *POOF* they're right back in full force. But it's a somewhat short game--if you removed all sources of food and egg-laying surface, and you keep the place clean, the swarm will be gone in two weeks or less. That's the lifespan of the average fruit fly from hatching to death. The ones you see in your house will most likely live a shorter time since they're already mature by the time you see them. Most of them will be gone in three to five days tops as they starve to death, but a few stubborn stragglers will hang on longer.
Once you've won the waiting game, just be extra careful to keep food tidy. They'll be back in an eye blink if you don't.