As LordOmar stated above there was a huge, media noted, movement of 'killer bees' northwards. What happened was the American Beekeeping Association took action.
In a nutshell the bred HUGE quantities of nice, docile, drones, and shipped them south. (This is noteworthy in itself, as large drone populations are not helpful in honey production!) They supplied border and southern USA beekeepers with these drones, and kept them stocked up. This along with greater hive numbers (which were also shipped south) allowed the great plan to progress:
As the killer bees moved northwards, they mated with these docile drones, and thus the AHB strain was somewhat diluted. It was a long-term plan, but it well and truly paid off.