Non-toxic and safe to use in horse water troughs, Mosquito Dunks from Mary's Tack & Feed kills mosquito larvae without harming horses, livestock, fish and birds. Just place a Mosquito Dunk disc in your water trough, duck pond or other standing water once a month. BTI, the active ingredient in Mosquito Dunks, is toxic only to mosquito larvae but is completely non toxic to pets, horses, wildlife, fish and humans. In fact, it is labeled for Organic gardening by the USEPA.