Mosquito Dunks biological mosquito control contains Bti, a natural mosquito larvicide, which kills mosquito larvae, but is harmless to birds, fish, wildlife, and pets. Simply place a Dunk in any standing water to control mosquito larvae for up to 30 days. Each Dunk treats up to 100 square feet of surface water regardless of depth. Perfect for Koi Ponds, Hydroponic systems, animal watering troughs, rain barrels, birdbaths, stock tanks, planter saucers, or any other standing water.