how to rid mosquitos from rain water

[cirillonb]

The only "official" way to kill mosquito larvae is with the Bt containing material. Up here our health department has them available for fear of West Nile virus disease and other mosquito borne illnesses.
Incidentally, as mentioned above, Bt is the most effective way to get rid of the larvae of the gnat that eats orchid roots and is the active ingredient in Gnatol and others.
Oil should work as the critters need to breath at the surface but use only good cooking oil, ie, the edible kind. That way if some gets into your orchid water, no harm done.
Nick