Biting flies have all kinds of food sources- I live in an area of the midwest with no animal farms for several miles (lots of trees and water springs though) and the woods are so thick with deer flies that I can't take my dogs with on a walk- their ears will have 6 flies each before we get 50 feet.
Deer, bears, coyote, wolves, raccoons, mice, otters, beaver, rats, muskrats, cougar, bobcats, chipmunks and squirrels, elk and moose, skunks, badgers, porcupines can all be found in nature in the upper midwest, along with various moles, voles, shrew and weasel family members can be found pretty much anywhere outside of a city center.
Since deer fly (and other biting flies) typically need a bit of standing water to breed, they're going to be found pretty much anywhere mammals live. If they decide they don't like striped cows, there's going to be plenty of other options to munch on.
Deer, bears, coyote, wolves, raccoons, mice, otters, beaver, rats, muskrats, cougar, bobcats, chipmunks and squirrels, elk and moose, skunks, badgers, porcupines can all be found in nature in the upper midwest, along with various moles, voles, shrew and weasel family members can be found pretty much anywhere outside of a city center.
Since deer fly (and other biting flies) typically need a bit of standing water to breed, they're going to be found pretty much anywhere mammals live. If they decide they don't like striped cows, there's going to be plenty of other options to munch on.