I have no remedies to share but I will tell you a disgusting story.
About 21 years ago I moved with my ex-husband and my baby son to a very old house in England, that had been updated a lot in the 70s and seemed to be stuck in that era. The kitchen counters were tiles. After a few months I notice we're having a really bad fruit fly epidemic and no matter what I did I couldn't work out where the hell the little buggers were coming from. One day I noticed that a couple of the tiles in the counter top had lost alot of the grouting and so I got a knife and managed to lift one of the tiles up. Only to find maggots!!!!!
I bleached the crap out of them!!!