Just put a bit of washing up liquid in the bucket, they will sink to the bottom and drown. Seriously each one of them will hatch out into a biting buzzing mosquito very soon if you leave them.
sounds like the larval stage of airborne insects such as midges/mossies/gnats etc If you have a pond the fish will love IT or even a tropical tank , just pour the water in and watch ,...if you really dont want to killl the little critters, they will be gone in the spring and feed the birds anyway :-)