Fruit flies - yuck!
Catch them in four different easy-to-make traps.

Got an infestation? Control it right now!
I compare the effectiveness of four different types of traps.

Step 1: Inverted Cone Fly Trap

This is the only trap here that can catch other types of flies besides fruit flies.
To make it, cut off the top of a soda bottle and insert it into the remaining part inverted. Tape or hotglue it in place. Put some liquid bait in the bottom.

The flies will fly in and will follow the walls back up, not finding the way out.

Instead of the plastic cone some people use a paper cone,
a plastic bag with the corner cut off,
or a piece of saran wrap with a small hole in it.

some people use these Alternative Baits:
juice or sugar water'
sugar water with yeast
apple cider vinegar
pickle juice

for other types of flies: meat, fish, canned cat food, or whatever the flies seem to like.

Step 2: Soap Bubble Trap

The flies are attracted by the bait and get stuck in the soap bubbles.
I used red wine as bait and added some dish soap. Put your hand over the top and shake it up as shown to make the bubbles. Then put it near your infestation.

Step 3: Soapy Bait Trap

This one is even easier.
Put a dab of soap on your fingertip and touch that to the surface of your liquid bait.
The soap breaks the surface tension and the flies fall into the liquid when they touch it.

Step 4: Wine Bottle Trap

This one is just a wine bottle with wine left in the bottom.

Step 5: Which Is the Best?

I caught one or two flies in each of the first three traps.
The wine bottle wasn't supposed to be a trap. I left it out by accident at the other end of the counter. All the flies flew in there and died.

Now I can't do any more experiments.

Fermentation of wine releases carbon dioxide which is heavier than air. I suppose they flew down into the CO2 and suffocated.

And that's it!
After having a fruit fly infestation for almost a year, an accidental trap wipes them out!!
Here's a photo down the neck of the bottle.