Beating the mosquitoes is easy, and it helps keep things out of the rain buckets other than rain- put a screen over the top. Say two layers of window screen, with layer of lightweight cotton knit between them. No bugs, no leaves, no dust, no bird droppings. And it will help filter out particulates washing off the roof. If you wanted to get really spiffy, two of those, with a centimeter of clean sand between them, to catch even more in the way of pollution and general nasties, plus it will help reduce loss through evaporation.

BTW, I should point out that only the sand is really my idea, my grandfather's father built something like this with flannel and chicken wire on his water barrels for the same reason. It worked at to a Vermont farm, it should work anywhere. <img src="/images/graemlins/smile.gif" alt="" />