You don't have to aim it at a point, a line is good enough. A cyclotron should be sufficient to generate a disk-shape of radiation, which could be aimed downwards, via Bremsstrahlung. You could aim that just outside feeding places so that the backside of the mosquittos remain exposed, should be possible to do that on a large scale to make millions of them. You can make it easier if you consider that it's sufficient if their offspring is infertile and reduce the dose for the initial generation, then just blast the followup generation with a general dose.