Killer bees, as they've been dubbed by the news media, arrived in the U.S. in 1990, and now inhabit the southernmost areas of California, Arizona, Nevada, New Mexico, and Texas. In recent years, killer bees have also been found in Florida, particularly in the Tampa area.
So what are killer bees? Killer bees are more properly called African honey bees (AHBs), or sometimes Africanized honey bees. Actually a subspecies of Apis mellifera, the European honey bee, African honey bees earned their "killer" reputation for their more aggressive tendencies when defending their nests.
African honey bees are quicker to respond to potential threats, and do so in considerable numbers. Their venom is actually no deadlier than that of European honey bees, but what they lack in venom quality they make up for in quantity. African honey bees may inflict ten times as many stings during a defensive attack as their calmer cousins.
In the 1950's, biologists in Brazil were attempting to breed a honey bee that would produce more honey in tropical environments. They imported honey bee queens from South Africa and established experimental hybrid colonies near Sao Paolo. As sometimes happens with such experiments, some of the hybrid bees – Africanized bees – escaped and established feral colonies.
Because the African honey bees were so well suited to tropical and subtropical environments, they continued to thrive and spread throughout the Americas. The killer bees expanded their territory northward at a rate of 100-300 miles per year for decades.
The arrival of killer bees in the U.S. in 1990 didn't really live up to the decades of hype. Campy 1970's horror films depicting attacking swarms of killer bees, coupled with news media hysteria, probably led people to believe that the world would be a much more dangerous place once the killer bees flew across the border. In truth, killer bee attacks are relatively rare, even in areas where the African honey bees are well established. A fact sheet from the University of California-Riverside notes that just 6 deaths occurred in the U.S. as a result of killer bee stings in the first ten years after their arrival.