I asked this same question last spring as it was my first year. The mosquito man typically comes after dark at which time the bees are not flying. My bees are about 200 ft. from the road from which they spray. I am sure its not good for the bees, but i have not had any trouble with my three hives thus far.
I don't know which chemical they are spraying, but the mosquito's seem to take a bath in it and keep on moving.