Africanized bees, also known as killer bees, are hybrids of the African honeybee, Apis mellifera adansonii (or by other reports A. m. scutellata), with various European honeybees such as the Italian bee Apis mellifera ligustica, descended from 26 Tanzanian queen bees accidentally released in 1957 in Southern Brazil from hives operated by biologist Warwick E. Kerr, who had interbred European honeybees and bees from southern Africa. Hives containing these particular queens were noted to be especially defensive. Kerr was attempting to breed a strain of bees that would be better adapted to tropical conditions (i.e., more productive) than the European bee used in North America and southern South America.


While the African source bees are significantly smaller than the European bees, the hybrids are similar to the European bees in size, with only a slightly shorter wing that may be determined only by examination of a large sample.

They have come to be the preferred type of bee for beekeeping in tropical areas of South America and in Central America because of improved productivity, though in most areas the Africanized hybrid is initially feared, because it tends to retain certain behavioral traits from its African ancestors that make it seem less desirable for domestic beekeeping, specifically (as compared with the European types):

1. To have a tendency to frequently swarm.
2. To be more likely to abscond as part of a seasonal migratory pattern in response to lowered food supply.
3. To have greater defensiveness when in a resting swarm
4. To live more often in ground cavities than the European types.
5. To be highly defensive in guarding the hive, with a larger alarm zone around the hive.
6. To have a high proportion of "soldier" bees within the hive prepared for exit and defense.
7. To recruit additional bees from within the hive for defense.
8. To pursue and sting perceived threats in far greater numbers and over much longer distances.

Africanized bees are characterized by their aggressiveness in establishing new hives and in their vigorous defensive behavior, attacking perceived threats to their hive, including people. Over the decades, hundreds of deaths in the Americas have been attributed to Africanized bees, many deaths resulting from multiple bee stings. This defensiveness has earned them the nickname "killer bees", the aptness of which is debated. Allergic reactions to bee venom from European honeybees also kills people, and it is difficult to estimate how many more people may have died due to the presence of Africanized bees.

Most human incidents with Africanized bees occur within two or three years of its arrival and then subside. Beekeepers can help greatly in this process by culling the queens of mean strains and breeding gentler stock. Beekeepers keep Apis mellifera scutellata in South Africa using common beekeeping practices without excessive problems.

As of 2002, Africanized honeybees had spread from Brazil south to northern Argentina and north to South and Central America, México, Texas, Arizona, New Mexico, and southern California. Their expansion stopped for a time at eastern Texas, possibly due to the large number of European bee beekeepers in the area. However, discoveries of the bees in southern Louisiana indicate this species of bees has penetrated this barrier, or have come as a swarm aboard a ship. In June 2005, it was discovered that the bees had penetrated the border of Texas and had spread into Southwest Arkansas. In the summer of 2005, Africanized bees were discovered in six counties of Florida where they had apparently been present long enough to spread widely without detection.

At their peak rate of expansion, they spread north at a rate of almost two kilometers (about one mile) a day. In tropical climates they compete effectively against European bees. There have been many opportunities to slow the spread by introducing non-defensive relatives, particularly at the Isthmus of Panama, but various national and international agricultural departments proved themselves incapable of action, and remain so to this day.

Recent evidence suggests that Africanized honeybees are less able to survive a cold winter. This is seen in the southern limit of the spread of Africanized bees in eastern South America. There are now stable geographic zones in which Africanized bees dominate, a mix of Africanized and European bees are present, and in the southern reaches only non-Africanized bees are found. As the Africanized honeybee migrates further north through Mexico, 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. Thus Africanized bees are expected to be a hazard mostly in the Southern States of the United States. In California they have been seen on the Pacific Coast as far north as Santa Barbara and are expected to eventually occupy the San Francisco Bay Area. Within the Central Valley in 2004 Africanized bees were involved in an attack in Modesto, having previously (2003) been seen in Bakersfield. The cold weather limits of the Africanized bee have driven professional bee breeders from Southern California into the harsher wintering locals of the northern Sierra Nevada (US) and southern Cascade range. This is a more difficult area in which to prepare bees for early pollination placement, such as is required for the production of almonds as the reduced available forage means bees must be fed for early spring buildup.

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 used for several thousand years. Beekeepers have tended to eliminate the fierce strains as they did, and the entire race of bees has thus been gentled by selective breeding.

In central and southern Africa, bees have had to defend themselves against other aggressive insects, as well as honey badgers, an animal that also will destroy hives if the bees are not sufficiently defensive. In addition, there was formerly no tradition of beekeeping, only bee robbing. When one wanted honey, one would seek out a bee tree and kill the colony, or at least steal its honey. The colony most likely to survive either animal or human attacks was the fiercest one. Thus the African bee has been naturally selected for ferocity.

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 and so is healthier and more industrious than European bees.
