My barrel is theoretically sealed so the water goes from the down pipe into the barrel. In practice it isn't really mosquito proof. I sometimes take the down pipe out and put in one of those mosquito dunks and sometimes I just curse them and let them be. I have noticed that it only really gets mosquitoes during long dry spells - because I guess the mosquito larvae don't do well with turbulent water which this must be when it rains.Thank you! I didn't think of using rain runoff to kind of naturally help clean and work the run cover. Don't have to worry about a frozen rain barrel here...mosquitos are a different question...