This mosquito subject comes up periodically on the DIS. As PinkBudgie said, this is a big state (with all different types of terrain, landscapes, bodies of water and environments), so, yes, there are mosquitos. But it won't be the infestation you are used to, Disneylovers2000.
And being a lifelong SoCal resident who neither lives near water nor in the mountains (or any of the expected places)...I have been chomped to bits by the little critters at times. I have seen them simply flying around near patches of grass. They just go wherever it looks inviting.
But this does not happen to everyone or in every area. I always say that if the mosquitos
want to find you they will find you. If they don't want to find you, they won't.
In Disneyland, while I think I have been bitten once or twice on distant past Summer DLR trips, it was rare. I've never been bitten on a late Fall/Winter DLR trip.
It's not impossible to encounter a mosquito here or there at DLR because there are bodies of water and that can attract mosquitos, but Disneyland does a very good job of keeping bugs at bay in general, or keeping them out of the general public's view, in any case. I think it has to do with the fact that they keep the grounds so impeccably groomed, swept and cleaned most of the time.
There are bees at DLR. There are spiders. There are even giant roaches at DLR (everyone see deejdigsdis' latest Trip Report - and be careful if you eat at Rancho del Zocalo!

). Most of the critters stay hidden or safely at a distance. So there may be an occasional mosquito here and there too, but they probably won't find you and they will probably not be the enormous, scary kind that you will see in other states/countries!