Scotch pine trees in the midwest are dying 
from the disease known as pine wilt.
The culprits:

Infected trees can die in as little as 3 weeks
Several organisms are involved in pine wilt disease. The nematode Bursaphelenchus xylophilus is transported to the trees by Pinesawyer beetles, and feed on Blue-stain fungi as well as cells lining the resin canals of the tree. Bark beetles help to introduce the Blue-stain fungi into the tree, allowing the nematodes to feed and multiply.

Bursaphelenchus xylophilus 
Pine wilt nematode

The nematode, B. xylophilus is probably native to the midwest, but the trees it attacks have been introduced.
Effects on 
susceptible trees:
During warm periods in the summer, the nematodes spread throughout the tree and multiply very rapidly. As they destroy the resin canal cells, the tree's water-moving system becomes clogged and resin flow slows, then stops. Wilt symptoms develop and the tree dies. 
Why the midwest?
Periods of drought and hot temperatures place introduced tree species- such as Scots pine- under stress, contributing to their susceptibility to disease. Native pine species are not usually afffected by pine wilt disease. 
Avoid planting Scots pine in areas (Iowa, Missouri, Illinois, eastern Kansas, SE Nebraska) where pine wilt is a threat.
What to do:
Dead pines should be cut promptly and burned, buried or chipped. Do not keep the wood for firewood. Wood chips pose minimal risk of spreading pine wilt, but beetles will continue to emerge from logs kept for firewood. Pinesawyer beetles lay their eggs under the bark of the pine, and a new generation emerges in the spring, ready to infect new pine trees.
Pictures of trees killed by pine wilt
Map: Confirmed cases of pine wilt in the Lincoln NE area
Nematode description and Photo Gallery