Prevent your rain barrel from serving as a mosquito breeding ground. These pests can get through any openings larger than those in a window screen. A well-sealed screen will help prevent mosquitoes from entering the barrel and laying eggs. Be sure all openings, including the overflow, are covered with a screen. Regularly examine mosquito netting and replace any that is damaged.
Mosquito larvae may still wash into your rain barrel from your gutters. So, every few months, check your gutters, downspouts, and screens and remove any debris. Also, regularly use all the water in your barrel.
For any mosquito larvae that make it into your rain barrel, you can use a product containing Bti (Bacillus thuringiensis israeliensis), commonly known as a “mosquito dunk.” Bti is a nontoxic bacterium that kills mosquito larvae. It’s safe for your plants, and it will not harm people, pets, amphibians, fish, or birds. You can find this product at most garden-supply stores.