Kills mosquito larvae for 30 days or longer. Made with BTI, a natural organic ingredient, contains no poisonous chemicals. Harmless to other living things. Simply leave a floating Mosquito Dunk® in each place water collects around your home. Perfect for Koi ponds, hydroponic systems, rain barrels, stock tanks, etc. Each Dunk® will take care of up to 100sq ft of water surface regardless of depth.