There is a (safe) bacterial product, developed in Israel years ago. Paralysis of the gut of insects (tomato worms, caterpillers)...one strain definately works on skeeters.
Bt (BT (Bacillus thuringiensis)
See www.gardensalive.com for the 'donut shaped' disks that float in ponds, specifically for skeeter control.
Mosquito Dunks®
Kills mosquitoes without sprays or making the water harmful to birds or pets. Last 30 days.
Elminate mosquitoes in standing water without pesticides, and without making the water harmful to birds, pets, fish and wildlife. Mosquito Dunks contain Bt israelensis, a naturally occurring biological organism which kills the mosquito larvae that feed on it. Use in bird baths, ponds, rain barrels, gutters - anywhere standing water collects. One doughnut covers 100 square feet and lasts 30 days.
No affiliation with Gardens Alive, just a happy customer for over a decade. Plus I hate mosquitoes.
Gwen
bug free in L.A.
EDITED TO ADD: for large ponds, see more info at http://www.nal.usda.gov/bic/BTTOX/bttoxin.htm