Oh, I don't know that just because in a desert to worry less about mosquitoes. I live in the Phoenix area, and here they can be bad at times. Most likely it is due to people not taken care of standing/stagnate water, or a pool not properly taken cared of. It only takes about 72 hours of there being standing water to a whole mess of mosquitoes. So anytime do get some rain, people should be checking for any standing water and take care of it.
I think with Disneyland, Disney does a great job of taking care of standing/stagnate water (as others have also pointed out). Make the conditions bad for them as much as possible. I wouldn't be surprised if they did other methods as well (fogging/sprays/fish).