Mosquito larvae require standing, still water to mature and pupate. That's why fountains on home ponds are so important. Even bird baths have been made with circulating water to prevent mosquito larvae (since bird baths are the most common standing water offender). Dumping the water would be a temporary treatment unless it is done regularly. If possible avoid standing water or, as in the case of rain barrels, prevent access to the water using screening/exclusion. There is a larvacide that is highly effective against mosquitoes yet remains harmless to other organisms, even insects such as bees. This is called Bacillus thuringiensis israelensis. It is commonly available as 'mosquito dunks'. As with all pesticides it is important to use according to their label. If you're interested in reading more about Bacillus thuringiensis (commonly shortened to Bt) here is a link to an article about the history, environmental effects, and heath effects of Bt: http://npic.orst.edu/factsheets/BTgen.pdf