Here is an interesting article about an infestation of grasshoppers in Minnestoa in the mid-1870's. The interesting part is the end of the article where they list theories for the quick departure of the grasshoppers.
(Since it is a long URL and I'm not sure how to list it as that you may have to copy and paste rather than just click on it)
Ray
http://www.stolaf.edu/depts/environment ... rmain.html