After my initial concerns, I think if memory serves I got bitten (by something) about 3 times during my 8 night stay, and quite frankly I can get bitten more than that of a week at home in London!
I don't even notice them in Orlando. They are a huge issue here in Ottawa in the summers. I get eaten alive (they love me), but not in Orlando (at least that has been my experience at the parks and where we stay).
There's definitely flora/fauna in Florida that you will see that doesn't exist in "Yankee territory". That said, don't expect Universal or Disney to be this bug infested wasteland. I've never been remotely concerned about bites in the Parks themselves (Universal or Disney). These places are clean and I'm sure they regularly spray to keep pests away.
Mosquito magnet here. I have seen one mosquito at Fantasmic at WDW and two at Portofino in the foliage in the past ten years. I have not been bitten. I carry a repellent towelette with me just in case. Haven’t had to use it.
I think they do an excellent job with them. Maybe I just go at good times of the year? The mosquitos back home in Canada (Ottawa) are FAR worse. You wouldn't even want to leave your house some nights without a lot of spray lol.