We did some research and found that they actually breed and lay their eggs in the drains. So we tried covering up the drains, but they still kept coming, probably from the vents in the sinks.
Back to the internet. Then we found this great solution and thought we'd share it with you here:
-Put about a half inch of apple cider vinegar into a jar, such as a pint or quart canning jar or something similar.
-Add a few drops of dish soap to the vinegar. This lowers the surface tension of the liquid and helps to drown the little buggers when they land on the vinegar.
-Put a paper cone into the jar.
Here's a short youtube video about this Fruit Fly Trap:
Fruit Fly Death Trap Video
-Set the jar somewhere close to your sinks, both bathroom and kitchen.
Here's the cone:
PDF version of the Fruit Fly Cone
Print it out full size, in landscape mode, on 8 1/2 X 11 inch paper, cut it out, roll it up into the cone shape and tape it at the joining edge.
The fruit flies will smell the vinegar and apparently they can't resist it, so they fly or walk down the cone and into the jar. They can't seem to figure out how to get back through the small hole in the cone. Eventually they land on the vinegar to take a drink and that's where you will find them the next day, DEAD!
We also found that the gestation period for the eggs is 10 days.
So you need to keep the vinegar/cone traps active for at least 3 or 4 egg laying cycles to make sure that you got all the parents and kiddies.
When no more flies are trapped over a 10 or 12 day period, you are DONE.
It was very gratifying to see the dead bodies adding up in the jar, but we exchanged the vinegar every 4 or 5 days or so. That way we could see our progress toward total extinction.
The next year we were much more careful about having mature fruit in the house. So we didn't have a reoccurrence of last year's problem. But we do get some house flies and other flying insects from time to time, so we put a trap in our well-used breezeway just in case.
We note that it has trapped some other, larger flies too, but I think it's the perfect solution for the fruit fly.
In subsequent years, we have had the fruit flies show up from time to time. Up till 2018, we would put our two house traps (kitchen sink area and one bathroom) in place as soon as we saw them.
However--- for 2019, I put the traps in place before any flies showed up so we caught the very first ones and never had a "hatch" as far as I can tell.
Just another tip:
Don't leave the peelings from your fruit and vegetables sitting in the kitchen garbage container! That's just as bad a leaving the fruit on the table.