I'm assuming the white stuff is perlite. Do you use it to keep the fungus gnats at bay, and if so, does it work....? I always seem to have problems with the little flying bastards.
Its perlite of course as sprout stated. Yes its to keep white flys and their ilk at bay. It works very well as a preventative but if you have em already then crumbled up mosquito dunks are your best friend.
Cool, I'll have to try the dunks.....they seem to be a little hard to find this time of year. I tried the perlite method on my last grow, but the bugs made it past my defences.
I've been pretty lucky in my current, and past cabs. Never had a bug issue, or mold issue. Small gnats once, but hanged up one of those sticky fly things and it worked perfect.