Africanized honey bees, known colloquially as "killer bees," are some hybrid varieties of the Western honey bee species, (Apis mellifera), produced originally by cross-breeding of the African honey bee A. m. scutellata, with various European honey bees such as the Italian bee A. m. ligustica and the Iberian bee A. m. iberiensis. The hybrid bees are far more defensive than any of the various European subspecies. Small swarms of Africanized bees are capable of taking over European honey bee hives by invading the hive and establishing their own queen after killing the European queen.
Western honey bees differentiated into geographic races as they spread from Africa into Eurasia. Migrating bee populations formed the original colonies of bee in western Europe, landing to eventually populate the continent from Africa across the Straits of Gibraltar. There are currently 28 recognized subspecies of Apis mellifera based largely on these geographic variations. All subspecies are cross fertile. Geographic isolation led to numerous local adaptations. These adaptations include brood cycles synchronized with the bloom period of local flora, forming a winter cluster in colder climates, migratory swarming in Africa, enhanced foraging behavior in desert areas, and numerous other inherited traits.
The Africanized honey bees in the Western Hemisphere are of mixed descent from 26 Tanganyikan queen bees of A. m. scutellata, accidentally released by a replacement bee-keeper in 1957 near Rio Claro, São Paulo, in the southeast of Brazil, from hives operated by biologist Warwick E. Kerr, who had interbred honey bees from Europe and southern Africa. Hives containing these particular queens were noted to be especially defensive. Kerr was attempting to breed a strain of bees that would produce more honey and be better adapted to tropical conditions (i.e., more productive) than the European subspecies of honey bee used in South America and southern North America. The hives the bees were released from had special excluder grates to prevent the larger queen bees and drones from getting out and mating with local queens and drones of European descent. However, following the accidental release, the African queens and drones mated with domesticated local non-African queens and drones, and their descendants have since spread throughout the Americas.
The Africanized bees have become the dominant type of honey bee for beekeeping in Central America and in tropical areas of South America due to them out competing the European sub species, and there are claims that they have improved productivity. Unfortunately, the extreme tendency of Africanized bees to swarm has had a significant impact of honey production in managed colonies and in most areas the Africanized bees tend to have certain behavioral traits that make them less desirable for managed beekeeping. Specifically, as compared with the European bee types, the Africanized bee:
- Tends to swarm more frequently and go farther than other types of honey bees.
- Is more likely to migrate as part of a seasonal response to lowered food supply.
- Is more likely to "abscond"—the entire colony leaves the hive and relocates—in response to stress.
- Has greater defensiveness when in a resting swarm, compared to other honey bee types.
- Lives more often in ground cavities than the European types.
- Guards the hive aggressively, with a larger alarm zone around the hive.
- Has a higher proportion of "guard" bees within the hive.
- Deploys in greater numbers for defense and pursues perceived threats over much longer distances from the hive.
- Cannot survive extended periods of forage deprivation, preventing introduction into areas with harsh winters or extremely dry late summers.
Geographic spread 
As of 2002, the Africanized honeybees had spread from Brazil south to northern Argentina and north to Central America, Trinidad (West Indies), Mexico, Texas, Arizona, Nevada, New Mexico, Florida, and southern California. Their expansion stopped for a time at eastern Texas, possibly due to the large population of honey bee hives in the area. However, discoveries of the Africanized bees in southern Louisiana indicate this subspecies has penetrated this barrier, or has 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. On September 11, 2007, Commissioner Bob Odom of the Department of Agriculture and Forestry said that Africanized honey bees established themselves in the New Orleans area. In February 2009, Africanized honeybees were found in southern Utah. In October 2010, a 73-year-old man was killed by a swarm of Africanized honey bees while clearing brush on his south Georgia property, as determined by Georgia's Department of Agriculture. In 2012 state officials reported that a colony was found for the first time in a bee keepers colony in Monroe County, eastern Tennessee.
In tropical climates they compete effectively against European bees and, at their peak rate of expansion, they spread north at a rate of almost two kilometers (about one mile) a day. There were discussions about slowing the spread by placing large numbers of docile European-strain hives in strategic locations, particularly at the Isthmus of Panama, but various national and international agricultural departments were unable to prevent the bees' expansion. Current knowledge of the genetics of these bees suggests that such a strategy, had it been attempted, would not have been successful.
As the Africanized honeybee migrates further north, colonies are interbreeding with European honeybees. There are now relatively stable geographic zones in which either African bees dominate, a mix of African and European bees is present, or only non-African bees are found, as in southern South America or northern North America.
African honeybees abscond, abandon the hive and any food store to start over in a new location, more readily than European honeybees. This is not necessarily a severe loss in tropical climates where plants bloom all year but in more temperate climates it can leave the colony with insufficient stores to survive the winter. Thus Africanized bees are expected to be a hazard mostly in the Southern States of the United States, reaching as far north as the Chesapeake Bay in the east. The cold-weather limits of the African bee have driven some professional bee breeders from Southern California into the harsher wintering locales of the northern Sierra Nevada and southern Cascade Range. This is a more difficult area to prepare bees for early pollination placement in, such as is required for the production of almonds. The reduced available winter forage in northern California means that bees must be fed for early spring buildup.
The arrival of honeybees in Central America is a threat to the ancient art of keeping stingless bees in log gums even though they do not interbreed or directly compete with the stingless bees. The honey productivity of the honey bees exceeds the productivity of the native stingless bees so much that economic pressures force beekeepers to switch.
African honeybees are considered an invasive species in many regions.
Morphology and genetics 
The popular term 'Killer bee' has only limited scientific meaning today because there is no generally accepted fraction of genetic contribution used to establish a cut-off. While the native African scutellata are smaller, and build smaller comb cells than the European bees, their hybrids are not smaller. Africanized bees have slightly shorter wings, which can be reliably recognized only by performing a statistical analysis on micro-measurements of a substantial sample. One problem with this test is that there are also others subspecies, the first Americas naturalized Apis mellifera iberiensis with shorter wings too, is having from ancient hybridated haplotypes that presents six different lineages, five of them correspond to evolutionary lineages from Africa. Some belong to Apis mellifera intermissa but others are having an indeterminate origin; the Egyptian honeybee (Apis mellifera lamarckii), present in small numbers in the southeastern United States, has the same morphology. Currently testing techniques have moved away from external measurements to DNA analysis, but this means the test can only be done by a sophisticated laboratory. Molecular diagnostics using the mitochondrial DNA (mtDNA) cytochrome b gene can differentiate A. m. scutellata from other A. mellifera lineages, though mtDNA only allows one to detect an Africanized colony that has an Africanized queen, and not colonies where a European queen has mated with Africanized drones.
The Western honey bee is native to the continents of Europe, Asia, and Africa. As of the early 1600s, the insect was introduced to North America, with subsequent introductions of other European subspecies two centuries later. Since then, they have spread throughout the Americas. The 28 subspecies can be assigned to one of four major branches based on work by Ruttner and subsequently confirmed by analysis of mitochondrial DNA. African subspecies are assigned to branch A, northwest European subspecies to branch M, southwest European subspecies to branch C, and Mideast subspecies to branch O. The subspecies are grouped and listed. There are still regions with localized variations that may become identified subspecies in the near future, such as A. m. pomonella from the Tian Shan mountains, which would be included in the Mideast subspecies branch.
The Western honey bee is the third insect to have its genome mapped, and is unusual in having very few transposons. According to the scientists who analysed its genetic code, the western honey bee originated in Africa and spread to Eurasia in two ancient migrations. They have also discovered that the number of genes in the honey bees related to smell outnumber those for taste. The genome sequence revealed several groups of genes, particularly the genes related to circadian rhythms, were closer to vertebrates than other insects. Genes related to enzymes that control other genes were also vertebrate-like.
Besides, A. m. iberica haplotype is present in the honey bees of the western United States, Mexico and South America, where the honey bees are not native and they were introduced from Spain during the conquest of America, from populations with African haplotypes, whose origin is indeterminate. Apis mellifera iberica is having hybridization between the north of African and European bees, Apis mellifera mellifera, and Apis mellifera intermissa. Presents six haplotypes different, five of them correspond to an evolutionary lineage from Africa and one from Western Europe. From this, infer the hybrid nature of this subspecies, is similar to that of African populations in the number of alleles detected and the values of genetic diversity. Additionally A.m.intermissa genoma, present in A.m.iberica belongs to a group shown by experiment to have similar mtDNA, this including A. m. monticola, A. m. scutellata, A. m. adansonii and A. m. capensis .
Several researchers and beekeepers describe a general trait of the African subspecies Apis mellifera scutellata, classified by Lepeletier, 1836 - (African honey bee) Central and West Africa, which is absconding, where the Africanized honeybee colonies abscond the hive in times when food-stores are low, unlike the European colonies which tend to die in the hive.
There are two lineages of African subspecies Apis mellifera scutellata in the Americas: actual matrilinial descendants of the original escaped queens and a much smaller number that are African through hybridization. The matrilinial descendants carry African mtDNA, but partially European nuclear DNA, while the bees that are African through hybridization carry European mtDNA, and partially African nuclear DNA. The matrilinial descendants are in the vast majority. This is supported by DNA analyses performed on the bees as they spread northwards; those that were at the "vanguard" were over 90% African mtDNA, indicating an unbroken matriline (Smith et al., 1989), but after several years in residence in an area interbreeding with the local European strains, as in Brazil, the overall representation of African mtDNA drops to some degree. However, these latter hybrid lines (with European mtDNA) do not appear to propagate themselves well or persist. Population genetics analysis of Africanized honey bees in the United States, using a materially inherited genetic marker, found 12 distinct mitotypes, and the amount of genetic variation observed supports the idea that there have been multiple introductions of AHB into the United States.
Consequences of selection 
The chief difference between the European races or subspecies of bees kept by beekeepers and the African stock is attributable to selective breeding. The Africanized honey bee varieties produced a reversion of behaviour similar to non-domesticated species. The most common race used in the United States today is the Italian bee, Apis mellifera ligustica, which has been used for several thousand years in some parts of the world and in the Americas since the arrival of the European colonists. Beekeepers have tended to eliminate the fierce strains, and the entire race of bees has thus been gentled by selective breeding, from bee species naturally selected by the environment.
In central and southern Africa, the bees had to adapt to the environment of sub-saharan Africa—surviving prolonged droughts and having to defend themselves against other aggressive insects, as well as animals like the honey badger, that also will destroy hives if the bees are not sufficiently defensive. In addition, there was formerly no tradition of beekeeping, only bee robbing.
African bees are characterized by greater defensiveness in established hives than European honey bees. They are more likely to attack a perceived threat and, when they do so, attack relentlessly in larger numbers. Also, they have been known to pursue their threat for over a mile. This aggressively protective behavior has been termed by scientists as hyper-defensive behavior. This defensiveness has earned them the nickname "killer bees," the aptness of which is debated. Over the decades, several deaths in the Americas have been attributed to African bees. The venom of an African bee is no more potent than that of a European honey bee, but since the former tends to sting in greater numbers, the number of deaths from them are greater than from the European honey bee. However, allergic reaction to bee venom from any bee can kill a person, and it is difficult to estimate how many more people have died due to the presence of African bees.
Most human incidents with African bees occur within two or three years of the bees' arrival and then subside. Beekeepers can greatly reduce this problem by killing the queens of aggressive strains to breed gentler stock. Beekeepers in South Africa keep A. m. scutellata using common beekeeping practices without excessive problems.
Fear factor 
The African bee is widely feared by the public, a reaction that has been amplified by sensationalist movies (such as The Swarm) and some of the media reports. Stings from African bees kill one or two people per year in the United States.
As the bee spreads through Florida, a densely populated state, officials worry that public fear may force misguided efforts to combat them.
|“||News reports of mass stinging attacks will promote concern and in some cases panic and anxiety, and cause citizens to demand responsible agencies and organizations to take action to help ensure their safety. We anticipate increased pressure from the public to ban beekeeping in urban and suburban areas. This action would be counter-productive. Beekeepers maintaining managed colonies of domestic European bees are our best defense against an area becoming saturated with AHB. These managed bees are filling an ecological niche that would soon be occupied by less desirable colonies if it were vacant.||”|
— Florida African Bee Action Plan
The sting of the Africanized Honey Bee is no more potent than another variety of honey bee, and they have a similar appearance. What makes Africanized honey bees more dangerous is that they are more easily provoked, quick to swarm, attack in greater numbers, and pursue their victims for greater distances. An Africanized bee colony can remain agitated longer and may attack up to a quarter of a mile away from the hive.
Queen management in African bee areas 
In Mexico, where African bees are well established, pollination beekeepers have found that a purchased and pre-bred non-African queen may be used to locally create a first generation of virgin queens that are then bred in an uncontrolled fashion with the local wild African drones. These first generation African queens produce worker bees that are manageable, not exhibiting the intense and massive defense reactions of subsequent generations.
Impact on existing apiculture 
In areas of suitable temperate climate, the survival traits of African queens and colonies outperform western honey bee colonies. This competitive edge leads to the dominance of African traits. In Brazil, the African hybrids are known as Assassin Bees, for their habit of taking over an existing hive of European bees; this habit is most evident when the hive being attacked has a weakened queen, so not all hives are equally vulnerable, and overall rates of hive usurpation can reach 20%.
Gentle African bees 
Not all African hives show overly defensive behavior; some colonies are quiet, 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 they were during the first wave of the African bees' colonization. Now that the African bee has been "re-domesticated", it is considered the bee of choice for beekeeping in Brazil.
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Further reading 
- Collet T., Ferreira K.M., Arias M.C., Soares A.E.E., Del Lama M.A. (2006). "Genetic structure of African honeybee populations (Apis mellifera L.) from Brazil and Uruguay viewed through mitochondrial DNA COI–COII patterns". Heredity 97: 329–335. doi:10.1038/sj.hdy.6800875.
- Smith D.R., Taylor O.R., Brown W.M. (1989). "Neotropical African honey bees have African mitochondrial DNA". Nature 339: 213–215.