It’s not the mosquito that cannot survive in colder climes; it’s that the parasite is only transmissible in certain temperatures.
In 1493, the sequel to 1491, Charles Mann explains succinctly that the malarial parasite (plasmodium) needs to reach a certain stage in its life-cycle before it can be transmitted successfully through a mosquito’s bite. The stages of the life-cycle are temperature-dependent – the colder it is, the longer the stages take. At a certain average temperature, the plasmodium takes so long to mature that the mosquitoes die (of old age, if not predation) before it can be transmitted.
Since average temperatures can be plotted on a map, there is a “malaria line” north of which malaria is not transmitted (although that line can move as temperatures change). Maybe that’s what Cecil was trying to say.
Historically in the US the upper limits of that line have closely coincided with the Mason-Dixon line that allegedly defined “the South.” The relevance of this is that, as Mann explains, the very early plantations were worked by various mixtures of European indentured servants, Native Americans (either enslaved or poorly paid), and imported slaves (overwhelmingly West African). West Africans have resistance to malaria; Europeans and Native Americans did not – so massive die-offs of labor afflicted plantations EXCEPT those with African slaves. Without in any way excusing the morality of the choices involved, this was a contributing cause driving the South toward an economy based on the African slave trade.