Google's answer....

They are not fruit flys, they are sewer flys...and it has nothing to do with running water in it...although the salt and baking soda may work.

Our plumber says the deep down there may be a crack in the line, where a sewer fly can come in.

Remedy would be to jack hammer the line and put in a new one...or

Keep a stopper on the drain.

Eve

Keep the fruit in the fridge, not the bath tub. 

Call a plumber and as Eve said, put the drain stopper in!

Eve has this one..... those who have that issue are not called "lucky"...

I don't like to put anything down my drain besides water. I use a stopper

Close to drain? Maybe pour some bug repellent, drano or bleach down the drain?

John Peak Roy Kelley has a great way to keep them away.

I have never heard of this so thank you to Roy Kelley for his answer

My first thought is... did the plumber use the proper tub drain trap when it was installed? I don't see how they can make it through the trap when there is water in it... unless the bypass plug is missing which I have seen in some homes John Peak 

Roy provided my solution!

Roy Kelley has hit the nail on the head.  If they're coming from your drain, they're not fruit flies... they're drain flies (also known as sewer flies)... Roy has given you a good link to deal with the infestation.

I would keep putting things in that drain until nothing came out...

I wish we knew.

That is a new one on me.

Bury the body somewhere else farther away from the house.

What's a fruit fly. Haven't seen any flies in my house in forever.

I was going to say pour vinegar down it. That will pickle them but Roy Kelley has a real answer.

And people say that Q&A is boring .....

I have never heard of such a thing. But Roy Kelley seems to have a receipe for it. 

There's a plastic zip it drain cleaning tool that costs $3.50 use that to remove hair and debris on regular schedule. Half bleach and water solution, wear gloves and old clothes. Scrub the tub and surounds and let the solution in the drain. Repeat for two days  This kills the eggs they lay on surfaces and reduce number in drain. Put stopper in afterwards. The problem is as Eve says- moisture somewhere from a crack or leak is meeting soil. 

 Gosh, never heard of this but the answers are interesting.  

You have great advice here, we have not had that problem. A

Mike & Eve Alexander and Roy Kelley have this one. 

I have heard the banking soda and vinegar trick too. Roy has the right answer and something i do in my tub to get rid of blocks

Draino, or that recipe that Roy shared.

Ew. I have never heard of this. -Kasey

   Cracked pipe somewhere.  

Hadn't heard of this one but it seems Roy Kelley has the answer.

No idea

Never have ever had this issue, so reading comments.

Have not heard of this one before but good question.

Roy has nailed this one.

That is a new one. Obviously there are some issues with that drain. In my area huge roaches can crawl in from those drains. So gross! 

John Peak - some of the solutions I was not aware of! 

I was thinking of vinegar - but looks like it is not the right solution.

I have never had this issue.

Well, I would think that any kind of a killing agent dumped down would help

Call a pest control company!

not here

Run the water?

Sounds like Mike & Eve Alexander have the answer