Africanized bees are characterized by their agressiveness in establishing new hives and in their vigorous defensive behavior, attacking perceived hunters, including people. Over the decades, hundreds of deaths in the Americas have been attributed to them, many resulting from multiple bee stings. This defensiveness has earned them the nickname "killer bees", the aptness of which is debated. European honeybees also kill people due to allergic reactions, and it is difficult to estimate how many more people may have died than would have in the absence of Africanized bees.
As of 2002 they had spread from Brazil south to northern Argentina and north to South and Central America, México, Texas, Arizona, New Mexico, and southern California. They are spreading north at a rate of almost two kilometers (about one mile) a day.
Recent evidence suggests that Africanized honeybees are less able to survive a cold winter. As this subspecies of honeybee migrates further north, colonies are interbreeding with European honeybees. This appears to be resulting in a dilution of the genetic contribution of the African stock and a gradual reduction of the aggressive behaviors.
The popular term 'Africanized bee' has only limited scientific meaning today because there is no generally accepted fraction of genetic contribution used to establish a cut-off.
The chief difference between the European races or subspecies of bees kept by American beekeepers and the africanized stock is attributable to selective breeding. The most common race used in North America today is the Italian bee, Apis mellifera ligustica which has been kept for several thousand years. Up until modern times, when honey was harvested, it was done by killing a hive. Naturally beekeepers tended to eliminate the fierce strains as they did, and the entire race of bees thus gentled by selective breeding.
In central and south Africa, there was no tradition of beekeeping, only bee robbing. When one wanted honey, one would seek out a bee tree and kill it, or at least steal its honey. The colony most likely to survice the human foray was the fiercest one. In addition bees had to defend themselves agains honey badgers, an animal that also will destroy hives if the bees are not sufficiently defensive. Thus the Africanized bee has been selectively bred for fierceness.
Not all Africanized hives are defensive; some are quite gentle, which gives a beginning point for beekeepers to breed a gentler stock. This has been done in Brazil, where bee incidents are much less common than during the first wave of the Africanized bees' colonization. Now that the Africanized bee has been gentled, it is considered the bee of choice for beekeeping in Brazil. It is better adapted to the tropics, healthier and more industrious than European bees.
If bees start head-butting you, use this behavior as a warning to retrace your steps. Choosing any other path could lead you deeper into the hive's territory.