i got to agree with GiGa. the sand will cause problems, i had the same bugs and mentioned i have loads of fine sand the kids use in thei sand box, and i was told not to use it as it smothers the soil and can cause problems, but i did read that some growers have used it how ever, but after i was told it would cause problems i decided against the idea.
instead i used sticky trap tape and i did not water for several days to dry the soil out, this helped loads, but i tried something else, i got a sheet of newspaper and cut a slit in it, then i placed it over the top of my pot and slid the stem of the plant into the slit, then i taped it all up around the edge of the pot then i taped the slit up, this left my pots covered with a sheet of newspaper that was sealed and had the stem coming through it, this stopped all bugs withing a week or so, if i needed to water id poke a hole or pull the tape of the slit then water and tape back up once the paper was dry again, i got great results using that method and it did stop the bugs
how ever they have come back with this grow so i have let the pots dry out and just used sticky fly paper to catch any that was flying about, and its kept the numbers down, but when i water i notice the numbers go up again, so im trying to water from the bottom now, i have tried putting the pots in buckets of water so it soaks up from the bottom and leaves the top inch or 2 dry, the other method ive tried is using a dish and stand the pots in a dish and water from the bottom that way and let the pots soak the water up out the dishes,
i have not tried any chemicals yet but might have to if numbers start to creep up much more, i also found that running the sticky tape at pot level caught more bugs than the tape that was at plant level, so i have got tape running on the floor of the grow room and also got it running between my pots at the same level as the top of the pot, i noticed i had more bugs on the tape that was at pot level, it seems they dont like to fly to high, some still fly high but most just fly around the level of the pot, maybe this stops them getting eaten by preditors, but they are not quick enough to dodge a nicely timed clap,