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Africanized honey bees (AHB), (known colloquially as "killer bees") are hybrids of the African honey bee, Apis mellifera scutellata (not A. m. adansonii; see Collet et al., 2006), with various European honey bees such as the Italian bee A. m. ligustica and A. m. iberiensis.
The Africanized bees in the western hemisphere descended from 26 Tanzanian queen bees (A. m. scutellata) accidentally released in 1957 near Rio Claro, São Paulo State in the south 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 be better adapted to tropical conditions (i.e., more productive) than the European bees used in North America and southern South America. The African queens eventually mated with local drones, and their descendants have since spread throughout the American continent.
The Africanized hybrid bees have become the preferred type of bee for beekeeping in Central America and in tropical areas of South America because of improved productivity. However, in most areas the Africanized hybrid is initially feared because it tends to retain certain behavioral traits from its African ancestors that make it less desirable for domestic beekeeping. Specifically (as compared with the European bee types), the Africanized bee:
- Tends to swarm more frequently.
- 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 repeated intrusions by the beekeeper.
- Has greater defensiveness when in a resting swarm.
- 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 intrusion into areas with harsh Winters or extremely dry late Summers.
As of 2002, Africanized honey bees had spread from Brazil south to northern Argentina and north to South and Central America, Trinidad (West Indies), Mexico, Texas, Arizona, New Mexico, Florida 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 bee 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.
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 have been unable to prevent the bee's expansion.
Curiously, their arrival in Central America is a threat to the ancient art of keeping stingless bees in log gums. As honey productivity of the Africanized bees far exceeds the productivity of the native stingless bees, economic pressures force beekeepers to switch.
Africanized honey bees have generally been considered as an invasive species in many regions.
Recent evidence suggests that Africanized honey bees may be able to endure cold winters. They have been seen as far as Kansas City, Missouri. More commonly found in the south, in Mexico. There are now stable geographic zones in which either Africanized bees dominate, a mix of Africanized and European bees is present, or only non-Africanized bees are found (as in southern South America). As the Africanized honey bee migrates further north through Mexico, colonies are interbreeding with European honey bees. This appears to be resulting in a dilution of the genetic contribution of the African stock and a gradual reduction of their 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 locales 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. The reduced available winter forage in northern California means that bees must be fed for early spring buildup.
Morphology - difficulty in determination
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. While the native African bees are smaller, and build smaller comb cells, than the European bee, their hybrids are not smaller. They do 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 is also an Egyptian bee, present in the southeastern United States, that 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.
The consequences of selection
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, 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.
Africanized 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 in larger numbers. This defensiveness has earned them the nickname "killer bees," the aptness of which is debated and unfailingly sensationalized by the media. Over the decades, several deaths in the Americas have been attributed to Africanized bees. The venom of an Africanized bee is no more potent than that of a normal honey bee, but since the former subspecies tends to sting in greater numbers, the number of deaths from them are greater than any other subspecies. 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 Africanized bees.
Most human incidents with Africanized bees occur within two or three years of the bees' arrival and then subside. Beekeepers can greatly reduce this problem by culling the queens of aggressive strains and breeding gentler stock. Beekeepers keep A. m. scutellata in South Africa using common beekeeping practices without excessive problems.
The fear factor
The Africanized bee is widely feared by the public, a reaction that has been amplified by sensationalist movies and some of the media reports. Since their introduction to the United States there have been 14 deaths from Africanized bees over the several year period, which makes them less hazardous than venomous snakes. As the bee spreads through Florida, a densely populated state, officials worry that public fear may force misguided efforts to combat them. The Florida African Bee Action Plan states, "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 insure 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. " 
Impact on agriculture
It has been established that in a partially Africanized hive these aggressive bees can even "recruit" more gentle bees in attacks upon intruders. If true, this habit can make Africanized bees dangerous in areas where European bees are kept for agricultural purposes, since an existing queen may be replaced without the usual out-swarming or supersession, conditions more readily observable by the beekeeper. To the extent that the Africanized bees make pollination management more difficult, they are a threat to the production of all crops which require bee pollination.
Queen management in Africanized bee areas
In Mexico, where Africanized bees are well established, pollination beekeepers have found that a purchased and pre-bred non-Africanized 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 Africanized drones. These first generation Africanized queens produce worker bees that are manageable, not exhibiting the intense and massive defense reactions of subseqent generations. This offers a relatively economical method of safe local beekeeping conditions that would otherwise quickly lead to hazardous conditions.
Impact on existing apiculture
In areas of suitable temperate climate, the survival traits of africanized queens and colonies outperform western honey bee colonies. This competitive edge leads to the dominance of African traits. In Brazil, the African bee and its hybrid are known as the Assassin Bee, for their supposed habit of taking over an existing colony of European bees. According to this lore, their queen waits outside while several worker bees infiltrate the hive by bringing in food, where they will then locate and kill the queen. The new queen will then enter and take over the hive.
Support for the assassin report can be found in the fact that of Africanized bees in Brazil, about one third have mitochondrial DNA indicating a female ancestor of African origin. If the Africanization was attributable only (or even mostly) to the well known superior breeding efficiency of Africanized (male) drones, such "mother" DNA would possibly be much less prevalent . Although some bee species (such as the Cape Bee Apis mellifera capensis of South Africa) are known to be able to propagate additional females via egg laying female workers, this is not believed to occur in A. m. scutellata.
The evidence for matrilineal ancestry may, however, be countered by the recognition that there are two lineages of Africanized bees in the Americas; those which are actual matrilinial descendants of the original escaped queens (thus carrying African mitochondrial DNA), and those which have become Africanized through hybridization (thus carrying European mitochondrial DNA, but carrying partially African nuclear DNA). This is supported by DNA analyses performed on the bees as they spread northwards; those that were at the "vanguard" were over 90% African mitochondrial DNA, 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 mitochondrial DNA drops considerably.
Apis mellifera scutellata is well known for sending from the hive numerous tiny swarms - a queen and only a few attendants. Such a small swarm should be incapable of starting a colony on its own, lacking enough workers for all the tasks required to support a queen and in particular incapable of beginning a new colony. For this behavior to have survived through natural selection it likely has some utility to the survival and propagation of the sub-species. The supposed assassin behavior is consistent with this argument and is actually reinforced by it .
Gentle Africanized bees
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.
- Collet, T., Ferreira, K.M., Arias, M.C., Soares, A.E.E. and Del Lama, M.A. (2006). Genetic structure of Africanized 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 Africanized honey bees have African mitochondrial DNA. Nature 339: 213–215.