Have you tried Bleach

That's a little harsh isn't it? A few drops of dish detergent lowers the water's surface tension and causes larvae to drown. If the water is being saved for garden use, you can screen it. If it is a larger, permanent area, you could use Mosquito Dunks, an EPA registered larvicide. If the area is small enough, you might be able to fill it or, if there is enough grade, you might be able to drain it.

Been involved with mosquito control since 1953, and this is the first time I have heard of bleach as a larvicide.