Apparently they are or else the mosquito magnet machines wouldn't be effective. They attract mosquitoes with a pheramone and co2 (from a propane tank) and trap and kill them. The manufacturer says it takes a month or so for the machine to be effective since they have to kill all the mosquitoes in that territory.
If mosquitoes migrates at will the concept would not work.. These machines deplete an area of mosquitoes by killing all the locals. Thus they have a limited range in sq yards.
I have read that a mosquito never migrates more than a 1000 feet or so from where it is born. I find it hard to believe since mosquitoes are so light and any wind would usher them into new territory.
Anyway for better or worse these are the facts I have heard and wondered about.