I've got a couple snow dio that are quite a few years old, and I used straight baking soda, and just white-glued/watered it to secure it, and so far, no yellowing and no bugs getting in and munching away....
You can also use very fine white sand, usually found in the "rock" section in Michaels craft store, and to think of it, a fellow club member did a huge Ardennes dio, and his main source for snow was the non-sanded white tile grout. The cool thing about this is that there's something in the powder that when it dries, it has a slight sparkle, so it looks like freshly fallen snow, and has that reflective look when the sun hits it...a box will last a life time.
"There you go with those negative waves again!"