ivory liquid dishwashing soap, but it wont last real long.
Or better would be malathion or most any 'oil' based stuff that one might spray on plants for bugs.

Thanks, Mikie!
We went to our local independent hardware store and after the owner stopped laughing, she went to the basement to get bacteria pellets that stop the mosquitos from growing. Didn't want to put oil in there because I don't know what it would do to the sump pump.

I don't know what we would do without our neighborhood hardware store - they have a little bit of everything. The current owners look to be in their late 70s, so we don't know if anyone else will take over.

They are prety hardy mosquitos to be living in those temps!

Here if we find them breeding in our rain water tanks (5,000 litres x 2), we just add half a cup of kerosene to the tanks. Our rainwater is only used for the garden, lawn and pool, so no ill effects for drinking water.

Seems to form a film on the water and the mossies drown. Very popular and recommended treatment here.