I used sand many years ago, when I got them. It was silicone sand, for sand blasting, Sand with tiny particles of glass. The glass cuts the larvae as they become flies and try to fly out. Regular sand may be of help. I just know that when I used it it was the glass in the sand that was the lethal weapon. It did work. I found it messy, and I scratched a glass table a bit with it, so protect furniture, it will scratch. I learned to blow it off with a blow dryer to get it off furniture, after I moved the plant off the table of course. I removed the sand as soon as the gnats were gone. I would not use it today, BTi is what I would try.
