I just read about three interesting, non-chemical ways of keeping a rain barrel free of pesky critters:
1. Put a goldfish or two in the barrel. I especially like this one because of the happy little ecological world it creates. The fish eat the mosquito larvae and then poop out beneficial elements (mainly, nitrogen) for your garden. Things to remember: leave enough of an opening so that the fish can receive oxygen from the surface, and take the poor guys out before the first frost!
2. A thin layer of vegetable oil on the surface of the water can prevent mosquito larvae from clinging to the surface, which is how they breathe. The oil slick will also serve as a barrier for insects to enter the water. Do not use this method with goldfish because it blocks the availability of oxygen.
3. Filters - cover the intake with a piece of old nylons or netting. You will need to clear out the filter regularly and ensure that there are no holes for insects to pass through.
Any other suggestions? What about growing water plants in the rain barrel? Has anyone tried that?
The goldfish is an awesome idea! But how to filter fish waste? Or...is it a good fertilizer?