Well Grahm, I was thinking asteriod impact a la Char's Counterattack (gasp, a Gundam reference) which y'know, kills everything. The Blue are insect therefore really can't survive in a cold zone, given their mutation factor if it was a gradual drop I can dig it, but a sudden freeze would kick the crap out of them.
If you're not gonna drop an asteriod, at least get some OWPs up and blast their hives from orbit, then clean up, like sensible people do.
Oh, and yeah the military from the Animatrix was really gay.