africanized bees in the u.s

The long-anticipated announce-ment came in October 1990. Af-ricanized honeybees, more popu-

larly known as killer bees (because ofsensationalized accounts of their at-tacks on people and animals), had Þ-nally crossed the Mexican border intothe U.S. Less than 35 years after mem-bers of a honeybee subspecies living in Africa (Apis mellifera scutellata) werereleased outside S�o Paulo, Brazil, theirdescendantsÑthe Africanized beesÑhad migrated as far north as southernTexas. Today the bees occupy a rangeof about 20 million square kilometers,encompassing much of South Americaand virtually all of Central America. Andtheir spread continues. They reachedArizona in 1993 and are expected tocolonize parts of the southern U.S. be-fore being stopped by climatic limits,probably by the year 2000.

Their arrival in the U.S. raises manyquestions. How will the newcomers af-fect public health and the beekeeping in-dustry? Why were African bees broughtto the Americas in the Þrst place? Whatallowed their progeny to be so extraor-dinarily successful? And, most impor-tant, can anything be done to minimizethe impact of settlement by African-ized bees in the U.S.? We and othershave devoted a great deal of study to

this last question. That work, particular-ly research exploring the genetic make-up of the insects heading for the U.S., oÝers hope that eÝorts to control mat-ing between Africanized honeybees andhoneybees common in North Americanapiaries can be of considerable value.

One already obvious eÝect of thebeesÕ arrival is heightened concern forpublic safety. Africanized bees typicallydefend their hives much more vigorous-ly than do honeybee strains in NorthAmerica. North American honeybeesdescend from rather gentle subspeciesof A. mellifera that were imported pri-marily from Europe, when early settlersfound that the New World lacked nativehoneybees. Compared with the Europe-an bees, those with markedly Africantraits become aroused more readily andare more prone to sting any person or animal they perceive is threateningtheir nest. They may also attack in larg-er numbers (occasionally by the thou-sands) and persist in the attack longer(sometimes for hours).

Such behavior has reportedly causedone human death in the U.S. and per-haps 1,000 in the Western Hemisphere,and it is responsible for many more fa-talities among domesticated animals.Fortunately, most nonallergic individ-uals will survive an attack if they canrun away and so limit the number ofstings they suÝer. Almost all individ-uals killed by Africanized honeybeeshave died because they could not ßeeÑeither because they had fallen and in-jured themselves or had otherwise be-come trapped.

Beyond posing a public health prob-lem, the bees also promise to threat-en the livelihood of thousands of com-mercial beekeepers (apiculturists) andfarmers. Amateur and professional bee-keepers alike keep their hives outdoors.It is therefore possible that Europeanqueen bees will mate with Africanizeddrones (males) and thereby introduceincreased levels of defensiveness andother costly and troublesome traits intoapiary colonies.

The queenÕs mating choices accountfor the characteristics of a colony be-cause it is she who lays the eggs. Ear-ly in life, she mates in ßight with per-haps 15 drones from other colonies andthen never mates again. When bees areneeded in a colony, the queen lays eggsinto individual cells. Fertilized eggs usu-ally give rise to worker beesÑfemalesthat carry chromosomes from each oftheir parents and are responsible forforaging and guarding the nest. ( If thelarva emerging from a fertilized egg isfed a special diet, however, it can de-velop into a queen.) Unfertilized eggsgive rise to drones; these males bear asingle set of chromosomes (from themother), and they die after mating.

If beekeepers are unable to controlthe infusion of undesirable traitsproduced by mating between Euro-

pean queens and Africanized drones,their proÞts will shrink, partly becausemeasures will have to be adopted to pro-tect workers and the public from ex-cessive stinging. For instance, apiariesmight have to relocate to sparsely pop-ulated areas, and everyone handling thebees will have to wear sturdy protectivegear. Moreover, the bees tend to aban-don hives more readily than do Euro-pean bees; repopulating hives can beexpensive.

Beekeepers could also face a reduc-tion in honey production, which nowamounts to about 200 million poundsannually (representing roughly $100million in sales). Much research suggeststhat under climatic and ecological condi-tions that foster the abundant produc-

84 SCIENTIFIC AMERICAN December 1993

Africanized Bees in the U.S.Africanized honeybees have reached the U.S. from points south.As more of them arrive, they will certainly wreak some havoc

but perhaps not the type their “killer bee” nickname would imply

by Thomas E. Rinderer, Benjamin P. Oldroyd and Walter S. Sheppard

THOMAS E. RINDERER, BENJAMIN P.OLDROYD and WALTER S. SHEPPARDhave cooperated on several studies andexpeditions. Rinderer is a research genet-icist and director of the Honey-Bee Breed-ing, Genetics & Physiology Laboratory inBaton Rouge, La., a part of the U.S. De-partment of AgricultureÕs AgriculturalResearch Service. Oldroyd was a visitingscientist in that laboratory until recent-ly, when he returned to his home base inMelbourne, Australia. There, he is re-search geneticist at La Trobe University.Sheppard, a research entomologist at theAgricultural Research ServiceÕs Bee Re-search Laboratory in Beltsville, Md., spe-cializes in the molecular aspects of hon-eybee population genetics and evolution.

AFRICANIZED HONEYBEES have becomealert to the presence of an intruder neartheir hive, as is evident from the raisedstance of the bee at the right. Such bees,which look virtually identical to otherhoneybees, are descendants of a honey-bee subspecies (Apis mellifera scutella-ta) that was introduced into South Amer-ica from Africa in 1956.

Copyright 1993 Scientific American, Inc.

tion of honey by European bees, Afri-canized bees would be less productive.

Meanwhile beekeepers who rent theircolonies to farmers for the pollinationof such crops as almonds, blueberries,apples and cucumbers would face addi-tional Þnancial losses. Rentals generatean estimated $40 million in fees everyyear, much of which goes to migrato-ry beekeepers, who truck thousands ofcolonies to distant sites. Beyond havingto exercise particular care to protectthe public, beekeepers who maintainedmany Africanized bees could be pre-vented from bringing bees into non-Af-ricanized areas.

Farmers who rely on pollination ser-vices for the production of $10 to $20billion worth of crops could be hurteven more. Their costs would go up be-cause protection of the public would re-quire them to purchase services from a reduced number of beekeepers whosestocks were known to be European; suchbeekeepers might have to travel greaterdistances or might have to charge morebecause of the expense involved in keep-ing their apiaries under control and in

attaining certiÞcation of their success.TodayÕs concerns are an outgrowth of

an unfortunate series of events that be-gan in the mid-1950s, after the govern-ment of Brazil decided to shore up thatnationÕs beekeeping industry. At thetime, European honeybees formed thebasis for a strong beekeeping industryin many places, but not in Brazil. Bra-zil had only a small apiculture indus-try, partly because European honeybeeswere poorly suited to the countryÕs trop-ical climate. Rarely, if ever, did a colonysurvive in the wild, and only consider-able eÝort enabled beekeepers to sus-tain colonies throughout the year.

It is now evident that the poor per-formance by the European bees was re-lated to their misreading of environ-mental cues for reproduction. Directand indirect studies of genetics indi-cate that European bees, like all sub-species of A. mellifera, trace their an-cestry to an Asian species that evolvedthe ability to regulate body temperatureand survive in a temperate climate. Thebees withstood the cold mainly by clus-tering together in sheltered nests and

eating stores of honey they had collect-ed in warmer seasons. Later they ex-panded their range to include Asia Mi-nor, Europe and Africa, ultimately form-ing 20 or more subspecies adapted toparticular locales.

In the course of evolution the behav-ior of the various European subspeciesapparently became highly linked to sea-sonal ßuctuations in day length. Whenhours of daylight begin to increase, her-alding the imminent appearance of ßow-ers, European honeybee colonies expandthe size of their worker populations. Bythe time the ßowers bloom, many work-ers are available to forage for pollen andnectar. Nectar, which contains a greatdeal of sugar, is converted to honeyÑaprime source of energy.

Linkage of the life cycle to day lengthworks well in temperate regions, but inBrazil day length bears little relation tothe availability of pollen and nectar.The rainy periods that are required forabundant production of ßowers do notnecessarily coincide with periods of ex-tended daylight. Consequently, Europe-an colonies can be induced to expand


even when food supplies are too scarceto support large populations.

In 1956 the best solution to BrazilÕsbeekeeping woes seemed to be impor-tation of a honeybee variant more ac-customed to tropical living. The gov-ernment therefore authorized WarwickE. Kerr, then at the University of S�oPaulo at Piracicaba, to bring A. m. scu-

tellata from the highlands of eastern andsouthern Africa for study. Kerr obtained170 queens, although only 46 from SouthAfrica and one from Tanzania survivedthe journey from South Africa to a re-search apiary in Rio Claro. (Rio Clarolies roughly 100 miles from S�o Paulo.)He chose individuals that had alreadymated with African drones and werethus ready to lay the eggs needed tocreate complete colonies.

In 1957, within months after the Af-rican colonies were established, a visi-tor to the experimental apiary removedscreens that had been placed at hiveentrances to block queens from leav-ing. The reasons for the removal are

unclear, but before the act was discov-ered, 26 colonies had abandoned theirhives with their queens. For years, thoseliberated colonies were thought to havebeen the founders of the entire Afri-canized population. Recently, however,scientists have learned that soon afterthe initial release, queens reared fromthe remaining colonies were distributedto beekeepers in Brazil. The additionalreleases undoubtedly helped to ensurethat enough African insects would beavailable to establish permanent feralpopulations of Africanized honeybeesin Brazil.

The freed bees and their descen-dants found Brazil to be a hos-pitable place, and so they thrived.

Compared with European bees, the new-er arrivals were better able to take theirreproductive cues from variations inthe availability of rainfall and ßowersand were better equipped to cope withdry seasons. When ßowers are abun-dant, Africanized colonies engage in a

process known as reproductive swarm-ing: the queen and a good many hivemembers split oÝ to establish a new,growing colony. This swarm leaves theremainder of the original colony with ayoung queen, who repopulates the hive.When ßoral resources dwindle severely,Africanized bees are likely to abscondÑthey gather any remaining honey andabandon the hive en masse, to try toÞnd a more hospitable locale. (Europeanbees, in contrast, swarm perhaps oncea year and rarely abscond.)

As the Africanized bees ßourished inBrazil, they fanned out in all directions,including into areas that previously hadno beekeeping. In the 1960s they beganto draw an increasing amount of atten-tion, especially for their intense nestdefense, and it became clear that theycould be very troublesome.

By 1972 the U.S. government beganconsidering the potential impact of thebees on the U.S. A committee orga-nized by the National Academy of Sci-ences and funded by the U.S. Depart-ment of Agriculture found some causefor worry. Although bees in southernBrazil were surely prone to stinging,they were manageable. Areas that sup-ported beekeeping before the releaseof the African bees continued to do so.But bees in the northÑthose on a tra-jectory headed for the U.S.Ñwere unac-ceptably defensive. Not surprisingly,northern Brazil, which had supportedlittle beekeeping previously, still had lit-tle. Consistent with the northern Þnd-ings, later work indicated Africanizedbees that had journeyed north into Ven-ezuela and beyond kept a strong pro-pensity for stinging. They also retainedtheir tendency to swarm and abscondfrequently.

In the mid- to late 1980s the U.S. gov-ernment, with the cooperation of Mexi-co, decided to try retarding the spreadof the bees into the U.S. by establishinga Òbee-regulated zone.Ó The Þnal plancalled for detecting and killing anyswarms that arrived in parts of Mexicothe bees would have to traverse in or-der to reach the U.S. Combined withweather inhospitable to migration, thateÝort (which proved more diÛcult toimplement than had been hoped) maywell have delayed the arrival of the beesfor a while. But it was clear they werenot going to be stopped altogether.

Interestingly, as anxiety mounted inthe U.S., Brazilians found a way to useAfricanized bees for the intended pur-pose: to strengthen their beekeeping in-dustry. Initially many beekeepers aban-doned the craft. But the Brazilian gov-ernment embarked on a campaign toteach potential beekeepers how to copeand to instruct the public about how to


MIGRATION OF AFRICANIZED HONEYBEES from outside S�o Paulo, Brazil, to theU.S. was accomplished in less than 35 years. (Red lines indicate the farthest pointsof detection in the years indicated.) The insects reached the southern tip of Texason October 15, 1990, and were Þrst spotted in Arizona in 1993.

U.S.

Hidalgo

São Paulo

ARGENTINA

BOLIVIA

COLOMBIA

VENEZUELA

PERU

BRAZIL

TEXAS

ARIZONA

MEXICO

EQUATOR

TROPIC OF CAPRICORN

TROPIC OF CANCER

FRENCHGUIANA

SURINAMEGUYANA

CHILE

ECUADOR

PARAGUAY

URUGUAY

1957

1963

1964

1965

1966

19671968

1971

19741975

19771980

1981

1982

19831984

198519861987

19881989

1990

1991

1993

1992

BELIZEGUATEMALA

HONDURAS

NICARAGUACOSTARICA PANAMA

YUCATÁN


avoid the bees and handle attacks. Nowa new generation of apiculturists hasemerged. Indeed, in some parts of Bra-zil that were once unable to sustain Eu-ropean honeybees, people earn theirlivelihood through keeping Africanizedbees and harvesting their honey. Theseindividuals maintain reasonable traitsin their stocks by destroying queens inthe most defensive and least produc-tive colonies.

Unlike Brazil of the 1950s, the U.S.has little to gain from settlementby Africanized bees. And so the

beesÕ entry into Texas and Arizona hasadded new urgency to the question of whether the introduction of Africantraits in apiaries and in the wild can beminimized. In theory, two major strate-gies might be helpful. Certainly, bee-keepers could protect their stocks tosome extent by practicing Òrequeen-ingÓ frequently. The procedure involvesinducing colonies to accept substitutequeens of a beekeeperÕs choosing, of-ten purchased from breeders of queens.Beekeepers can thereby ensure that theirqueens are European and (if so desired)that they have already mated with Eu-ropean drones. Many apiculturists arealready adept at requeening. They use itto increase the production of oÝspring(replacing old, less productive queenswith new ones) or to control the genet-ic makeup, and hence the characteris-tics, of hive populations.

Another protective strategy, knownas drone ßooding, calls for maintaininglarge numbers of European drones inareas where commercially reared queenbees are mated. Even if the areas havebeen invaded by Africanized �migr�s,the vast number of European maleswould ensure that European queenswould mate almost entirely with Euro-pean drones.

Furthermore, the presence of manyEuropean bees would increase the prob-ability that Africanized queens, too,would mate primarily with Europeandrones. If the queens of successive gen-erations then continued to mate withEuropean drones, the gene pool of thebee populations in the aÝected areaswould consist mostly of European DNA.Then the bees would have predomi-nantly European traits. In the end, suchgentle hybrids might actually prove tobe quite valuable. Some scientists havereported that Africanized bees may bemore resistant to acquiring parasitesand disease. If these advantages couldbe harnessed by breeding programs,they might help bees in North AmericaÞght oÝ a growing invasion of mites.

Of course, the drone-ßooding strate-gy assumes that honeybees bearing es-

sentially African genes and those bear-ing essentially European genes can hy-bridizeÑthat is, mate with each otherand produce viable oÝspring bearinggenes and traits from both parents. Butcan the two groups in fact hybridize?For many years, researchers were un-sure of the answer. Some early studiesin the 1980s that examined morpholo-gy, or physical features, of bees in ar-eas known to have been invaded by thedescendants of A. m. scutellata seemedto indicate that hybridization was in-deed occurring, as did studies of en-zymes. But other work disagreed.

Morphological comparisons aremuch more diÛcult than theymight sound. Even if one ex-

amines the extremesÑAfrican bees liv-ing in Africa and European bees livingin EuropeÑthe two groups look alike.But colonies can be distinguished by a statistical procedure called multivari-ate discriminant analysis. In doing such an analysis, researchers measure manydiÝerent body partsÑamong them, thelength and width of the wings and legsegments, and the angles at which var-ious veins intersect in the wings. Al-though the mean scores for African andEuropean samples will not diÝer signif-icantly on any one measure, combininggroup means for many measures makesit possible to distinguish overall diÝer-ences that do exist.

To assess whether the invading beesin Central and South America diÝered

physically from A. m. scutellataÑwhichwould suggest genetic mixing had takenplaceÑwe compared their Þnal scoreswith those attained for African and Eu-ropean bees. The comparison revealedthat feral populations in Mexico, Bra-zil, Argentina and Venezuela resem-bled both European and African beesto various degrees, although they weremore like the African bees.

Similarly, when we plotted summedmeasures for three diÝerent clusters oftraits against one another on three axes,the point representing purely Africanbees fell far from that representingpurely European bees in North America[see bottom illustration on next page ].The points representing bees fromMexico, Brazil, Argentina and Venezu-ela fell in between, roughly a third ofthe way between those two extremesbut closer to the African value. TheseÞndings suggested that the populationsadvancing toward the U.S. were not pureOld World African stock; they were in-deed hybrids that had acquired someEuropean genes in their travels.

Other conceivable explanations for themorphological Þndings existed, though.One was that a founder eÝect was re-sponsible. Perhaps the relatively few Af-rican bees that were originally import-ed to Brazil happened to carry genesthat caused them to look European. Inthat case, their progeny in the Ameri-cas would also physically resemble Eu-ropean bees even in the absence of hy-bridization. Alternatively, natural selec-

SCIENTIFIC AMERICAN December 1993 87

PROTECTIVE CLOTHING is essential for anyone who works around hives of Afri-canized bees. Compared with the relatively gentle strains of honeybees commonin the U.S. (originally imported from Europe), the Africanized bees are more easilyaroused. Entire populations of hives can come pouring out in a ßash, ready to pur-sue doggedly and sting any perceived intruder.


tion in the Americas may have favoredthe survival of African bees that bychance had European-like physical traits.

The attractiveness of these explana-tions dimmed, however, when the datafrom multivariate analyses were com-bined with results from studies thatcompared enzymes in African and Eu-ropean bees. Jorge A. Lobo and his col-leagues at the University of S�o Pauloat Ribeir�o Pr�to knew that 98 percentof African bees carry one form, or al-lele, of a gene specifying the amino acidcomposition of a particular enzyme:malic acid dehydrogenase. They alsoknew that this same alleleÑand there-fore the isozyme, or enzyme variant, itencodesÑis much rarer in Europeanbees.

When the group surveyed the formsof malic acid dehydrogenase in the beesof Brazil, they deduced that only 70 to80 percent of the insects harbored theallele common in African bees. Studiesof other isozymes yielded a similarpattern. These results led LoboÕs groupto conclude that at least some African-ized bees are the products of hybrid-ization. It is highly doubtful that boththe morphological and the isozyme dif-ferences between Africanized and OldWorld African bees could be the resultof happenstance.

Yet the Þndings favoring hybrid-ization were contradicted by oth-er observations. Notably, bees in

the colonized areas seemed to displayclearly African traits, namely, intensedefensiveness and frequent swarmingand absconding. If hybridization was go-ing on, it certainly was not obvious be-haviorally. The Þrst direct genetic stud-ies raised similar doubts. They com-pared mitochondrial DNA in bees fromcolonized areas with that in Europe-an and African bees. Mitochondria, theenergy factories of cells, contain smallrings of DNA that are distinct from thechromosomal DNA housed in the nu-cleus. Nuclear DNA directs the emer-gence of physical and behavioral traits.Mitochondrial DNA provides about adozen genes required strictly for energyproduction. Most animal species (includ-ing humans) inherit their mitochondria,and thus mitochondrial DNA, exclusive-ly from the mother.

It turns out that African and Europe-an bees diÝer slightly in the sequence ofnucleotides (the building blocks of DNA)in their mitochondrial DNA. Hence, byidentifying known markers of the vari-able DNA segments from bees in Afri-canized areas, investigators were able totrace the maternal lineage of the insectsto either Africa or Europe. (The mark-ers used are DNA fragments that form


MELON PLANTS ( foreground ) are growing close to a residential area in northernCalifornia. They are about to be pollinated by European honeybees stowed in near-by boxes. As Africanized bees settle in the U.S., farmers may have more diÛcultyÞnding European bees to rent for pollination and may have to pay more for thosethat are available. For that reason and others, the incursion of Africanized beesinto the U.S. could shrink annual proÞts of beekeepers and farmers.

MEASUREMENTS OF PHYSICAL FEATURES in European, African and Africanizedhoneybees have been summarized on this three-dimensional plot. The axes repre-sent the collected measures for distinct clusters of traits. One base of each cylin-der sits on the ßoor formed by the x and y axes. The opposite end of the cylinder(shaded black ), lying above or below the ßoor, marks the intersection of x, y and zcoordinates; that base represents the overall morphology of the group. The mor-phology of the Africanized bees (purple) is between that of the European (blue)and African (red ) beesÑwhich suggests that the Africanized bees are by-productsof mating between European and African bees in the Americas.

–0.8

–0.6

–0.4

–0.2

0.0

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–2.0

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0.00.5

1.0

Africanizedbees inBrazil

Africanizedbees inMexico

A.m. scutellata(African bees in

South andCentral Africa)

Apis mellifera carnica(Most widely usedcommercial bee

in Europe)

European bees usedin North American

beekeeping

3.0 1.5X AXIS

Y AXIS

Z A

XIS

Africanizedbees in

ArgentinaAfricanized

bees inVenezuela


when mitochondrial DNA is cleaved bya restriction enzyme. For example, onefragment generated from African DNAappears as two smaller fragments inEuropean DNA.)

The Þrst published reports found Eu-ropean mitochondrial DNA to be vir-tually nonexistent in the bees studied.This absence implied that almost noneof the sampled bees had descendedfrom European queens. If hybridizationhad taken place, one would expect tosee a greater representation of Europe-an mitochondrial DNA.

The results seemed consistent withthe possibility that something was pre-venting hybridization from taking place.Yet it was also possible that the bees inthe studies came from tracts that pre-viously supported few feral Europeanbees. In that case, there would be littlehybridization because almost no Euro-pean bees would have been available tointeract with incoming African bees.

Determining whether hybridizationcould in fact occur required investiga-tion of bee colonies from areas knownto have been supporting European hon-eybees when the newcomers arrived.We therefore traveled to Argentina,which lies west of the thin, southern-most part of Brazil and extends muchfarther south, into a temperate zone.Africanized bees have not become es-tablished in the southern half of thecountry, which supports abundant bee-keeping with European strains. Butthey have established large popula-tions in the northern half, particularlyin the topmost quarter of the country,which has never maintained as manyEuropean bees.

Julio A. Mazzoli, a graduate stu-dent from the University of Bue-nos Aires, helped us Þnd morethan 100 colonies in bridges, trees,

electric utility poles, fruit boxes andother enclosed areas favored by honey-bees. The collection included represen-tatives from areas extending from thenorth into the south [see illustration on

this page]. Back in our laboratories, weevaluated the morphology and the com-position of mitochondrial DNA. Ourcollective results showed decisively thathybridization had taken place. As partof our evidence, we found that a largenumber of the sampled colonies hadphysical features intermediate betweenthose of European and African bees.Further, more than a quarter of the col-onies either displayed African morphol-ogy (reßecting the activity of nucleargenes derived from African ancestors)yet bore European mitochondrial DNA(reßecting the inßuence of a female Eu-ropean ancestor) or else displayed Eu-

ropean morphology yet bore Africanmitochondrial DNA.

The morphological work yielded an-other interesting Þnding: Europeanphysical features were more prominentin the southern, temperate regions ofthe studied territories than in the north-ern corner, where African morphologypredominated. But in a band of fairlytemperate territory between those ar-eas, no single cluster of morphologicalfeatures predominated. This mixture oftraits implied that hybridization hadoccurred extensively in the interveningzone, a conclusion supported by iso-zyme studies. We found relatively fewhybrids outside the transition zone pre-sumably because conditions in the trop-ical north favor survival of bees havingprimarily African traits, whereas condi-tions in the temperate south favor sur-vival of bees having primarily Europe-an traits.

Such selective pressures may lead toa similar pattern in the U.S., where thesouthernmost regions have a subtropi-cal climate and northern areas are tem-perate. European-like bees may be lesscompetitive in the Deep South, and Af-rican-like bees should be less compet-itive in the North. In the interveningcentral regions, there may be a mixtureof hybrids whose gentleness and toler-ance of cold increase with increasinglatitude. It is also possible that hybridswill be abundant in some central areasduring the warm seasons but will dis-appear in the winter.

Because there were few Europeanbees in the tropical regions of Argenti-na, we could not determine whetherthe presence of a suÛciently large Eu-ropean population would cause Afri-can-like bees to mate with them andproduce hybrids that survived and re-produced well in the tropics. We knewonly that such behavior was common-place in temperate areas. If hybridiza-tion does not occur readily in the trop-ics, then the strategy of ßooding com-mercial breeding areas with Europeanbees might prove ineÝective in parts ofFlorida, Texas and other subtropical re-gions of the U.S.

We sought an answer in the Yucat�nPeninsula of Mexico. The peninsula hasan ideal combination of a tropical en-vironment and an extensive, long-es-tablished population of European hon-eybees. In fact, the Yucat�n has thegreatest concentration of commercialhoneybee colonies in the world. Thiswas the Þrst massive population of Eu-ropean bees encountered by the expand-ing populations of Africanized bees asthey migrated north from Brazil.

We again collected samples from alarge tract. This time we relied on the

cooperation of beekeepers, who ownmost of the bees in the Yucat�n. Despitebeing cared for by humans, the honey-bees in the Yucat�n are probably the ge-netic equal of feral bees. Beekeepers ob-tain them by putting out boxes the in-sects can colonize. Owners usually makelittle eÝort to control the genetic make-up of the hive, other than killing older(less productive) queens and allowinghive members to raise a replacement.An occasional beekeeper will, however,practice requeening with pre-mated Eu-ropean queens.

SCIENTIFIC AMERICAN December 1993 89

ARGENTINA can be divided into threezones based on the abundance of Afri-canized bees. In much of the north (pur-ple) the bees are present year-round. Inmost of the south (blue), they are ab-sent. Recent analyses of morphologyand mitochondrial DNA in bees fromtracts crossing all three areas (whitebars) indicate that Africanized bees, Eu-ropean bees and a range of hybrids co-exist in the intervening transition zone.This Þnding has helped conÞrm thatmating between Africanized and Euro-pean bees can yield viable oÝspring.

Africanized zoneAfricanized bees areabundant; Europeanbees are rare

Transition zoneAfricanized bees,European bees andhybrids are all found

Non-Africanized zoneAfricanized bees arerare; European beesare abundant

BUENOSAIRES


All but a few of our samples camefrom colonies that had not undergonerequeening in the two years since Afri-canized bees had Þrst been detected onthe peninsula. Most of the insects stillpossessed clearly European morpholo-gy, but some possessed mainly Africanmorphology, and many had intermedi-ate morphologies indicative of hybridi-zation. Mitochondrial analyses provid-ed still more evidence of interbreeding:a number of colonies displayed eitherEuropean morphology and African mi-tochondrial DNA, or the reverse. Thus,a tropical environment does not ap-pear to pose an unbreachable barrier tohybridization.

The least evidence of African traitsappeared in the few colonies that hadbeen requeened. This simple observa-tion implies that requeeningÑone of thechief tools beekeepers have for control-ling the Africanization of their stocksÑcan certainly be helpful.

Our conclusion that signiÞcant hy-bridization can be achieved in tropicalareas has recently been conÞrmed byRobin F. Moritz of the Technical Univer-sity of Berlin and Michael S. Meusel ofthe Bavarian Agricultural Institute for

Apiculture in Erlangen, Germany. In asurvey of feral Africanized honeybeesin Brazil, they found that 17 percent ofthe colonies had European mitochon-drial DNA. The team also mathemati-cally modeled the eÝects of intensivereproductive swarming by Africanizedbees on the composition of bee popula-tions in areas that originally supportedonly European honeybees. The resultsshow that rapid growth of Africanizedbees, combined with the survival ad-vantage they enjoy in tropical environ-ments, could enable Africanized beesto predominate over hybrid or other

bees with European traits. So it seemsthat eÝorts to foster hybridization inthe subtropical areas of the U.S. mightrequire continuous requeening with Eu-ropean bees.

Nevertheless, the potential of Af-ricanized honeybees to hybrid-ize successfully with European

honeybees is good news for beekeeping.We anticipate that frequent requeeningof commercial colonies and drone ßood-ing in commercial queen-breeding areaswould serve to dampen the acquisitionof unwanted African traits. We shouldnote, though, that there are dissenterswho contend that hybridization eÝortswill fail to prevent the eventual wide-spread introduction of dramatic Africantraits into honeybee populations. Theseobservers hold that Africanized beeswill inevitably come to dominate in re-gions that initially show signs of hy-bridization. Our evidence does not sup-port that view. We found an abundanceof hybrid bees in the transitional zoneof Argentina some 20 years after Afri-canized bees had arrived there.

If we are correct that Africanizationof U.S. apiaries can be limited, then itseems that, with care, the practice oftransporting bees to crops could becontinued safely without leading to thesigniÞcant establishment of Africanizedbee colonies in new territories. Fortu-nately, there are ways to assess the char-acter of individual colonies, and thesemethods could be employed to guaran-tee that colonies moved from place toplace are European.

It is inevitable that the incursion ofAfricanized bees into the U.S. will in-crease the costs of managing commer-cial colonies, at least temporarily. It isalso likely that some African genes willspread through feral and managed beecolonies. Yet vigilance and coordinationby apiculturists have every chance ofpreserving the European behavior ofcommercial honeybee stocks, therebyreducing the damaging eÝects of Afri-canized insects on beekeeping and al-laying the fears of the public.


CLUSTER OF AFRICANIZED BEES hangs from a tree limb, the insectsÕ temporaryhome until they can construct a hive in some protected place. Such bees swarmÑleave their original hive to establish a new oneÑas part of the processes by whichbee colonies reproduce. Frequent swarming by Africanized bees has contributedto their rapid spread through much of the Americas.

FURTHER READING

THE PAST AND POSSIBLE FUTURE SPREADOF AFRICANIZED HONEYBEES IN THEAMERICAS. Orley R. Taylor, Jr., in Bee World, Vol. 58, No. 1, pages 19Ð30;1977.

THE ÒAFRICANÓ HONEY BEE. Edited by Mar-la Spivak, David J. C. Fletcher and Mi-chael D. Breed. Westview Press, 1991.

GENE FLOW BETWEEN AFRICAN- AND EURO-PEAN-DERIVED HONEY BEE POPULATIONSIN ARGENTINA. Walter S. Sheppard, Thom-

as E. Rinderer, Julio A. Mazzoli, J. Antho-ny Stelzer and Hachiro Shimanuki in Na-ture, Vol. 349, No. 6312, pages 782Ð784;February 28, 1991.

HYBRIDIZATION BETWEEN EUROPEAN ANDAFRICANIZED HONEY BEES IN THE NEO-TROPICAL YUCATAN PENINSULA. ThomasE. Rinderer, J. Anthony Stelzer, BenjaminP. Oldroyd, Steven M. Buco and WilliamL. Rubink in Science, Vol. 253, pages309Ð311; July 19, 1991.


africanized bees in the u.s

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