Neotropical African bees are derived from an intentional introduction of bees from South Africa into Brazil. These bees are famous for their stinging behavior and they are known as “killer” or “Africanized” bees in the media.
The introduction from South Africa into Brazil of a subspecies of the common honey bee, Apis mellifera. resulted in a spectacular biological invasion that has had profound effects on agriculture, beekeeping, and human and animal health. Establishment of the African bees led to hybridization with bees of European subspecies maintained by beekeepers for honey production and pollination. The resulting apiary bees became extremely defensive, and honey production declined as beekeepers abandoned beekeeping. Stinging incidents that resulted in human and animal deaths soon led the press and the public to refer to these invaders as “killer bees.” The perception that the feral African-derived bees were of hybrid origin led to the term “Africanized bees,” which is a misnomer.
PREVIOUS INTRODUCTIONS
Honey bees (A. mellifera ) are native to the Old World, and all honey bees present in the Americas are descendants of bees introduced from Europe, Africa, and the Middle East. Managed honey bees in the Americas are derived from at least five introduced European honey bee (EHB) subspecies belonging to two major lineages of A. mellifera (west European bees, A. m. mellifera and )ber-ica; eastern European bees, A. m. ligustica, carnica, and caucasica). These temperate subspecies are notably successful in other temperate regions (e.g., Australia) and have been used with some success in tropical areas. However, they are not well adapted to tropical conditions and did not establish large self-sustaining feral populations in the tropics as they have done in subtropical and temperate regions. In many tropical areas, European bees would not persist without human assistance and, in most habitats (with the exception of regions with pronounced dry seasons) honey production was marginal. In 1956, to improve honey production in Brazil, Warwick Kerr intentionally introduced to Brazil A. m. scutellata, a subspecies belonging to the African lineage of A. mellifera. The introduced subspecies became established in southern Brazil in early 1957 and, because this bee was well adapted to tropical conditions, a large feral population soon developed and began to spread at rates of 100-300 miles per year. No other invading species has expanded so rapidly into new habitats.
DEFENSIVE BEHAVIOR
The Neotropical African honey bees (AHBs) are well known for their defensive behavior, and the deaths of hundreds, perhaps thousands, of people (and certainly thousands of domestic animals) have been attributed to these insects. These bees are not always defensive, but under certain conditions they will attack people and animals near their nests in massive numbers, inflicting hundreds and even thousands of stings. The venom (per bee) of AHBs is less toxic than that of EHBs; nevertheless, human victims of massive stinging require immediate medical attention to minimize lysis of blood cells, breakdown of muscle tissue, and kidney damage, which can result in acute kidney and multiple organ failure.
IMPACT ON BEEKEEPING
Beekeepers accustomed to dealing with relatively gentle and manageable EHBs were unable to adapt to the defensive behavior as AHBs advanced through the Americas, and many abandoned beekeeping. Honey production declined and many countries became honey importers rather than exporters. For example, in the Yucatan peninsula of Mexico, an area of intense beekeeping with both EHBs and native stingless bees (Melipona and Trigona), both types of beekeeping declined precipitously following the arrival of AHBs. In much of the Americas, as new beekeepers adapted to AHBs, beekeeping and honey production recovered, most notably in Brazil. AHBs are now used for honey production in many areas of the Americas where EHBs were ineffective, and it appears that Kerr’s goal of improving honey production in the New World tropics will be realized.
DIFFERENCES BETWEEN AHBS AND EHBS
Honey bees subspecies from tropical and temperate regions have evolved adaptations that are suitable for their respective environments. AHBs are smaller, and they have higher metabolic rates, more rapid development, reduced longevity, smaller nest sizes, greater brood production, and lower honey storage than EHBs. These traits combine to limit the ability of AHBs to overwinter in areas where the interval between first and last frost is longer than 3.5 months. In contrast, feral EHBs can exist in areas where this winter interval is 6 months. Adaptations to tropical conditions that give AHBs advantages include higher rates of swarming (reproduction) and the ability to abandon the nest (abscond) and move to new habitats under unfavorable conditions. Also affording AHBs a distinct competitive advantage in the tropics is these insects” abilities to find pollen and nectar, to increase their brood production under conditions in which EHBs are unable to do so, and to mount an intense nest defense that repels predators.
HYBRIDIZATION AND GENETIC DIFFERENCES
When two formerly isolated species or populations come into secondary contact there are four possible outcomes: coexistence with complete reproductive isolation, replacement of one population by the other, fusion of the two populations and complete mixing of the two gene pools (sometimes referred to as “dilution”), and establishment of a more or less permanent hybrid zone. The first scenario, coexistence with reproductive isolation between these biotypes, has not developed anywhere in the Americas. The second scenario seems to be the rule in nonmanaged populations: African-derived bees establish large feral populations and replace any resident European feral honey bees.
Gene flow between neotropical AHBs and EHBs seems to be strongly asymmetrical. AHBs have maintained their genetic integrity, in spite of hybridizing with EHBs, as they have expanded their distribution. Even after 45 years of interaction with EHBs, these bees are indistinguishable in behavior and so similar genetically to bees in the Transvaal of South Africa that it is more appropriate to refer to them as Neotropical African bees or as African-derived bees rather than Africanized bees. Low acquisition of EHB traits into the AHB population can be attributed to pre- and postzygotic isolating mechanisms (i.e., mate selection, queen developmental time, and hybrid dysfunction). For example, AHB queens mate predominantly with AHB drones even in the presence of large numbers of EHB drones. When AHB queens are inseminated with semen from drones of both types, the AHB queen progeny develops faster than the hybrids, assuring that in most cases the next queen would be an AHB rather than hybrid.
Colonies from backcrosses of F1 hybrid queens to either parental genotype have unusual metabolic patterns, low honey storage, and high rates of mortality. This finding suggests possible incompatibility of the nuclear and mitochondrial genomes of these biotypes. EHBs, in contrast, rapidly become Africanized, and nearly all traces of the EHB nuclear and mitochondrial genome disappear from the feral bee populations following the arrival of AHBs. Disappearance of the European traits seems to result from a lack of prereproductive isolation, which results in extensive mating by EHB queens with AHB drones. This is followed by a pattern of queen development that favors hybrid rather than EHB queens. Matings by these F1 queens to AHB drones results in colonies with low fitness and the eventual loss of EHB mitochondrial DNA from the population. Displacement of EHBs therefore seems to result, in part, in a type of “genetic capture” in which one form, A. m. scutellata, eliminates the others by hybridizing with their females. The genetic and population consequences of the interactions between A. m. scutellata and A. mellifera subspecies from Europe suggest that A. m. scutellata deserves the status of a semispecies.
HYBRID ZONES
A hybrid zone formed at the southern limit of AHBs in northern Argentina in the late 1960s and early 1970s. Although many anticipated that a similar zone would form in the United States, this has not happened. Coincident with the arrival of AHBs in the United States, the mite, Varroa destructor, an introduced brood parasite from Asia that kills EHB colonies, spread rapidly throughout the country eliminating feral EHB colonies. At present, feral EHBs, which usually are escaped swarms from managed apiaries, are transitory and persist only for a short time; this precludes the formation of a persistent hybrid zone. In Argentina, it is likely that Varroa, which arrived after the formation of the hybrid zone, has changed the dynamic of the interaction of both types of bee as well.
FUTURE
In the future, the only significant feral bee populations in the United States will consist of AHBs in the southwestern states and possibly Florida, until or unless EHBs acquire sufficient tolerance of Varroa mites to once again establish feral populations (Fig. 1 ). AHBs will not become more European through hybridization or selection and move further north. Barriers to gene flow into the AHB genome, coupled with selection against any AHBs with EHB genes that make them susceptible to Varroa mites, assures the continuation of a nearly separate Neotropical African bee genome. AHBs have many useful attributes but hesitancy on the part of beekeepers to work with these bees, partly because of familiarity with EHBs but also because of concerns of legal issues should their bees be linked to stinging incidents, will keep American beekeepers from adapting this bee to apiculture.
FIGURE 1 The known summer (shaded) distribution and predicted overwintering limit (60-degree line for January) of Neotropical AHBs in the United States.
