John, Colorado is an semi-arid area, counting all the winter snow and the summer rains the state averages about 12 inches of moisture a year. I mainly control the mosquitoes just by putting the lids on the barrels. It takes just a few minuted to slide them off when a rain is expected, after all the rains are not all that often. Of course I don't have to do that during the fall and winter months. I also use Summit's Mosquito Dunks. the Dunks kills mosquitoes, but are safe for animals and the water can safely be used to water plants and filling bird baths. - Millet