I remember reading in Charles C. Mann's book, "1493: Uncovering the New World Columbus Created", about the after-effects of the "Columbian Exchange" (i.e. all the effects, including biological, of the New World and Old World meeting). It goes something like this:
Africans developed resistances to many mosquito-borne diseases.
Whites / Native Americans / Others not so much.
Mosquitos carrying such diseases were prolific in the New World (Was this since always, or only after the Exchange?)
African slaves, as opposed to other slaves of other races/regions, were able to survive and work the fields with much lower mortality. Thus the specific demand for African slaves.
The latitude (climate variation) at which mosquitos carrying such diseases do not survive as well is roughly around the Mason-Dixon line.
Thus the difference and support for African slavery in the South?
What do you think?
The Mason Dixon line itself was in no way influenced by mosquitoes.
The line was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon to resolve boundary disputes between Pennsylvania, Maryland, Virginia and Delaware.
At the time the line was delineated, slavery was legal in all these colonies.
Mason and Dixon surveyed their line in accordance with an interpretation of early colonial charters, and subsequent treaties and agreements between the colonies. They made no reference to the suitability or otherwise of habitats for mosquitoes in their surveying work.
When Pennsylvania abolished slavery in 1781, the Mason Dixon line became a boundary between free territories and slave territories. Eventually all states North of the Mason Dixon line abolished slavery (although not until 1846, when New Jersey became the last of the states North of the line to abolish slavery). Thus, the Mason Dixon Line became a symbol of the border between freedom and slavery.
The hypothesis you cite theorizes that environmental factors (favorable habitats for mosquitoes) made the economic benefits of slavery greater in some areas (mosquito prone ones) than in less mosquito prone areas. These environmental differences were theoretically demarcated somewhere geographically proximate to the political boundary which was the Mason Dixon Line.
To assess this hypothesis, one would need to look at several factors:
Were mosquitoes less prevalent to the North? (I can tell you that there are plenty of mosquitoes well north of the Mason Dixon Line today.)
Were African slaves considerably more resistant to mosquito borne diseases? (By the way, I believe there is a hypothesis that mosquito borne diseases (yellow fever and malaria) were introduced into North America through the importation of African slaves. If true, this would imply that using African slaves preceded any hypothetical benefits from higher African resistance to mosquito borne diseases.)
Did any such possible resistance confer substantial economic benefits associated with employing enslaved Africans? (If Africans, why slaves? Why not hired Africans?)
Were economic factors the main factors in determining whether states supported or abolished slavery?
I am dubious about this hypothesis, but have not done enough research to have a firm opinion.
In any event, whether slavery was more economically viable in more Southern and mosquito plagued regions or not, none of this had any influence on the location of the Mason Dixon Line.