Even though most rain barrels have screens over the top to prevent mosquitoes from laying eggs in them, they don't prevent mosquito eggs from entering through the screen from your gutters. We quit carrying mosquito dunks because we don't currently sell pond supplies and that is what they are normally used for. However, I ordered some today and should have them at the shop within a week. Mosquito Dunks active ingredient, Bacillus thuringiensis subspecies israaelensescalled BTI kills mosquito and black fly larvae for 30 days or more. While floating they slowly release a long term biological mosquito larvacide killing the larvae before they hatch into adults. Will not effect fish, plants, people or wildlife. Alternative wetting and drying will not reduce it's effectiveness. Use one dunk per 100 sqft of water surface. It is safe and environmentally sound because it is highly selective, killing mosquitoes and black flies. Bti contains no poisonous chemicals and is completely harmless to other living things! When Bti is eaten by the mosquito larvae, it damages the gut cells, eventually entering the body cavity. Once this happens, the larvae die.