If you want to prevent mosquitos from getting into your
rainwater supply barrel...Take an eyedropper and add two
or three drops of motor oil or 3 and 1 oil to the surface
of the water. It will create a thin surface film of oil
that will prevent any mosquitos from hatching...as it
suffocates their eggs in the water. Simply skim it off
the top with a disposable rag when you want to use the
water.
---Steve