does this powder work on mites on snakes?Sevin powder is a insecticide based on pyrethrins, if I remember correctly. Its sold in most garden sections of stores and is pretty harmless to vertebrate animals. Lots of people use it to de-flea their cats and dogs or as a mite/louse treatment for poultry. The bugs need to get it on them to work, it's not like a chemical that leaches into the air and poisons them that way.
I just sprinkled a small amount in the bedding of my gerbils and through their rooting and digging around in the bedding, they would infuse the powder through their coat and kill the mites. Any mites that fell off of them into the bedding was also killed.
And since it is a powder and restricted to the aquarium you're keeping your gerbils, the transmission to another container containing your tarantula would be minimal. Especially if you sealed up your tarantula's container to be air tight, they can survive for a few days like that since they respirate so slowly. You can also position the gerbils or the tarantula in a closed off section, like a closet or in a dresser drawer.