Prevent your rain barrel, ornamental pond, trough, or bird bath from becoming a nursery for mosquitoes with Mosquito Dunks, a safe and harmless disk that is dropped into a standing water source to eliminate larvae for up to one month. The active ingredient, Bt-israelensis, is a bacterium that targets mosquito larvae specifically, so it's completely safe for use in ponds with your koi and in rain barrels used for garden watering. The Dunks will not be affected by repeated wetting and drying. Replace your Dunk every 30 days to kill mosquito larvae before they mature. Pack of two.