||What's Buzzing with Africanized Honey Bees?
Tabs on the Invaders
Did They Go?
Closeup of Africanized honey bees (AHBs)
surrounding a European queen honey bee (EHB), marked with a pink
dot for identification. Since AHBs arrived in Texas in 1990, they've
mated with EHBs and spread throughout the Southwest. But rather
than commingling, AHBs tend to replace EHBs, partly because EHB
queen bees mate disproportionately with African drones.
Photo by Scott Bauer
In 1990, a honey bee swarm unlike any before found in the United
States was identified just outside the small south Texas town of
Hidalgo. With that identification, Africanized honey bees were no
longer a problem we would have some day. Africanized honey bees
Beekeepers, farmers who depend on honey bee pollination for their
crops, land managers, emergency responders like fire and police,
and the public all wanted to know what they would be facing as Africanized
honey bees began to spread.
Now, 14 years later, scientists with the Agricultural
Research Service (ARS) and elsewhere have uncovered many answers,
but they have also come upon some new and unexpected questions.
Africanized honey bees—melodramatically labeled "killer
bees" by Hollywood hype—are the result of honey bees
brought from Africa to Brazil in the 1950s in hopes of breeding
a bee better adapted to the South American tropical climate. These
honey bees reached the Brazilian wild in 1957 and then spread south
and north until they officially reached the United States on October
Actually, all honey bees are imports to the New World. Those that
flourished here before the arrival of Africanized honey bees (AHBs)
are considered European honey bees (EHBs), because they were introduced
by European colonists in the 1600s and 1700s. EHBs that escaped
from domestication are considered feral rather than wild.
Entomologist David Gilley is part of the
team investigating the usurpation of European honey bee colonies
by swarms of Africanized honey bees. Because queenless colonies
are particularly susceptible to usurpation, the team maintains a
group of queenless colonies to lure usurpation swarms into their
apiary to be studied. Gilley is shown here requeening one of these
Photo by Scott Bauer
Africanized honey bees are so called because it was assumed that
the African honey bees spreading out from Brazil would interbreed
with existing feral EHBs and create a hybridized, or Africanized,
This has always been a major question for researchers—what,
if any, type of interbreeding would happen between AHBs and EHBs
and how would this affect honey bee traits that are important to
people, such as swarming and absconding, manageability for beekeepers,
honey production, and temper.
Many experts expected that the farther from a tropical climate
AHBs spread, the more they would interbreed with EHBs. But it appears
that interbreeding is a transient condition in the United States,
according to ARS entomologist Gloria DeGrandi-Hoffman. She is research
leader at the Carl Hayden Bee Research Center in Tucson, Arizona,
and ARS national coordinator for AHB research.
"Early on, we thought the mixing would reach a steady state
of hybridization, because we knew the two groups of bees can easily
interbreed and produce young," DeGrandi-Hoffman says. "But
while substantial hybridization does occur when AHBs first move
into areas with strong resident EHB populations, over time European
traits tend to be lost."
||A Mighty Adversary
DeGrandi-Hoffman and Stan Schneider, a professor of biology at
the University of
North Carolina at Charlotte, have been collaborating the past
3 years to figure out why AHBs replace EHBs rather than commingling.
"We've found six biological and behavioral factors we think
are responsible for making AHBs such successful invaders,"
First, AHB colonies have faster growth rates, which means more
swarms splitting off from a nest and eventually dominating the environment.
Second is that hybrid worker bees have higher amounts of "fluctuating
asymmetry"—small, random differences between the left
and right wings—than African honey bees have, even when
raised in the same hive.
An Africanized honey bee (left) and a European
honey bee on honeycomb. Despite color differences between these
two bees, normally they can't be identified by eye. "
Photo by Scott Bauer
"Imperfections like fluctuating asymmetry that increase with
hybridization may end up reducing worker viability and colony survival,"
says DeGrandi-Hoffman. "But this is a controversial factor
right now, and it will take long-term studies of African, hybrid,
and European colonies in the same habitat to truly understand its
But the third factor is undeniably true: EHB queen bees mate disproportionately
with African drones, resulting in rapid displacement of EHB genes
in a colony. This happens because AHBs produce more drones per colony
than EHBs, especially when queens are most likely to be mating,
"We also found that even when you inseminate a queen with a
50-50 mix of African drone semen and EHB semen, the queens preferentially
use the African semen first to produce the next generation of workers
and drones, sometimes at a ratio as high as 90 to 10," she
says. "We don't know why this happens, but it's probably one
of the strongest factors in AHBs replacing EHBs."
|| When an Africanized
colony replaces its queen, she can have either African or European
paternity. Virgin queens fathered by African drones emerge as much
as a day earlier than European-patriline queens. This enables them
to destroy rival queens that are still developing. African virgin
queens are more successful fighters, too, which gives them a significant
advantage if they encounter other virgin queens in the colony. DeGrandi-Hoffman
and Schneider also found that workers perform more bouts of vibration-generating
body movements on African queens before they emerge and during fighting,
which may give the queens some sort of survival advantage.
Technician Mona Chambers measures the dimensions
of an Africanized honey bee wing, one of the traits that differentiate
these bees from European bees.
Photo by Scott Bauer.
AHB swarms also practice "nest usurpation," meaning they
invade EHB colonies and replace resident queens with the swarm's African
queen. Nest usurpation causes loss of European matrilines as well
as patrilines. "In Arizona, we've seen usurpation rates as high
as 20 to 30 percent," says DeGrandi-Hoffman.
Finally, some African traits are genetically dominant, such as queen
behavior, defensiveness, and some aspects of foraging behavior. This
doesn't mean that EHB genes disappear, but rather that hybrid bees
express more pure African traits. The persistence of some EHB genes
is why the invading bees are still considered Africanized rather than
African, regardless of trait expression, she points out.
Northwest Fire District's Captain John Estes
of Tucson, Arizona, uses a wide spray of water and chemical wetting
agent as a means of subduing Africanized honey bees. Looking on
is ARS entomologist Eric Erickson (retired), who taught this control
method to fire departments throughout Arizona.
Photo by Jack Dykinga
A coincidence may have contributed greatly to an overwhelming takeover
by AHBs in areas they've invaded. Just as AHBs began their spread
throughout the Southwest, the U.S. feral honey bee population was
heavily damaged by another alien invader—the deadly Varroa mite,
an Asian honey bee parasite first found here in 1987.
"Varroa mites emptied the ecological niche of feral honey bees
just as AHBs arrived," says DeGrandi-Hoffman. "If they hadn't
been moving into a decimated environment, AHBs might not have replaced
EHBs so quickly."
||Keeping Tabs on the Invaders
An extensive record of the AHB invasion was created by now-retired
ARS entomologist William L. Rubink, who was in the ARS
Bee Research Unit in Weslaco, Texas. From 1990 to 2001, Rubink
continuously sampled honey bee colonies in the Welder Wildlife Refuge,
about 30 miles north of Corpus Christi, Texas.
Once Rubink retired, researchers from Texas A&M University agreed
to preserve and analyze his samples. "We have about 25 square
feet of frozen bees that represent the only real unbroken sampling
of a wild area before and during its takeover by AHBs. Bill had
a great deal of foresight to take these samples," explains
geneticist J. Spencer Johnston, who is with the university.
Entomologist Justin Schmidt examines
an ARS honey bee trap used to lure Africanized honey bee swarms
and prevent their establishment in walls of buildings. Captured
swarms are easily removed or destroyed with soapy water.
Photo by Scott Bauer
The data showed that within 3 years of the arrival of AHBs in the
refuge there was a turnover from predominantly EHB to predominantly
AHB. From 1997 through 2001, the mixture stabilized, with an average
of 69 percent of the colonies made up of African queens mated with
EHB and AHB drones and 31 percent composed of EHB queens mated with
AHB and EHB drones. This produced a genetic mixture rather than a
replacement of EHBs by AHBs. Additional sampling and more analysis
of existing samples will be needed to see whether this mixing continues
or whether the Africanized proportion increases, as has been predicted.
Judith Hooper, researcher with the University
of Arizona, assists entomologist David Gilley with a gas chromatography/mass
spectroscopy procedure for analyzing volatile pheromones from Africanized
honey bees. Pheromones appear to be an important component of successful
Photo by Scott Bauer
In many ways, the spread of AHBs in the Southwest
has been one of the most successful introgressions ever documented.
It's even interested some as a model of how modern humans may have
interacted with the European population of Neanderthals.
"Alan Templeton, a professor of biology and genetics at Washington
University in St. Louis, has been looking at AHB spread as a
demonstration of his model of Homo sapiens' evolution and
spread, which holds that there have been three major migrations
out of Africa, with large amounts of genetic interchange among groups,"
Johnston says. Honey bee generations are short enough that you can
actually follow the invasion and the gene flow, unlike humans, explains
||Where Did They Go?
Just how far and how fast AHBs have spread in the United States
may be one of the most surprising factors in the whole issue.
Some experts predicted the bees would spread throughout the country;
others thought they'd reach only as far north as the latitude of
Houston. Most expected there would be a southern zone where AHBs
would predominate, a northern zone where EHBs would maintain a climatic
advantage, and a large transitional zone between the two. And everyone
expected AHBs to spread across the southernmost tier of states.
But, as of January 2004, AHBs have been found only in southern California,
Arizona, New Mexico, Nevada, and Texas, as well as Puerto Rico and
the U.S. Virgin Islands.
Why AHBs haven't progressed eastward into Louisiana—though
they were expected there years ago—is a mystery. So ARS entomologist
José D. Villa began looking at factors that might correlate
with where AHBs have spread. It isn't just minimum winter temperature
that limits AHB spread, as many believed, says Villa, who is in
Honey Bee Breeding, Genetics, and Physiology Research Unit in
Baton Rouge, Louisiana.
"What immediately jumped out at me was the correlation with
rainfall," he says. "Rainfall over 55 inches, distributed
evenly throughout the year, is almost a complete barrier to AHB
Total annual rainfall alone isn't a barrier; AHBs have been found
in areas of the Tropics with higher rainfall. But in areas with
high rainfall distributed throughout the year, Villa's pattern of
AHB spread fits perfectly.
Villa is quick to point out that this is simply a mathematical correlation
and not proof of cause and effect. But, he says, "you do find
that 55-inches-of-rainfall point right at the edge of where AHBs
stopped moving east about 10 years ago." He's planning experiments
that may uncover the behavioral or physiological mechanism that
Animated map showing the spread of Africanized
honey bees since 1990.
National Atlas of the United States®
How much farther AHBs may spread is still unknown. But if you apply
the 55-inches-of-rainfall limit, there are still niches that the
bees may fill, mainly in southern California. Southern Florida would
be hospitable to the bees given its temperature and rainfall, but
regulatory vigilance could keep them out, since the area isn't contiguous
with the other areas of AHB spread. Alabama, northern Florida, Louisiana,
and Mississippi are unlikely to be troubled by AHBs if the 55-inches-of-rainfall
||Keeping on Beekeeping
||One of the greatest
challenges for Southwest beekeepers has been maintaining their EHB
hives when they are surrounded by AHBs.
Once AHBs spread to an area, beekeepers can no longer allow nature
to take its course in honey bee reproduction. ARS has always recommended
that beekeepers regularly requeen their hives with queens of known
lineage to keep AHB traits out of their apiaries. But, given the African
bees' strong ability to genetically usurp hives, the recommendation
is now to requeen with queens that have already mated with EHB drones.
It's the best way ARS currently has for beekeepers to manage their
hives in AHB areas.
But requeening is a lot of work for commercial beekeepers who maintain
thousands of hives. DeGrandi-Hoffman and Schnieder are currently trying
to discover what triggers AHBs to usurp a hive. They suspect it could
be a pheromone.
"If we can find out what tells an AHB swarm that this EHB nest
can be taken over or that a colony or queen is strong and cannot be
easily usurped, then we should be able to develop a chemical 'no-vacancy'
sign to help beekeepers keep AHBs out," DeGrandi-Hoffman says.
While AHBs do make honey and pollinate plants, two traits make them
undesirable for beekeepers: Colonies regularly abscond from hives,
and they are often too defensive to be easily tended.
Because of AHBs' genetic dominance there has been little dilution
of their strong defensive reaction to threats to their nests, explains
DeGrandi-Hoffman. This defensiveness is probably the bees' best-known
trait. All honey bee behavior runs the gamut from very defensive to
very docile and can change depending on temperature, humidity, cloud
cover, and food supply. But when provoked, AHBs do tend to sting in
greater numbers than EHBs.
"But they're not anywhere near the type of threat that Hollywood
has made them out to be," DeGrandi-Hoffman points out.
||Living with AHBs
||While beekeepers obviously
do not want to work with "hot bees," people in the Southwest
have simply learned to live with AHBs. While many will never come
in contact with the bees, others have had to learn new precautions.
Retired ARS entomologist Eric Erickson, who was with the ARS
Bee Research Center in Tucson, pioneered many safety methods in
areas where people and AHBs collide. He developed the first instructions
for fire departments—often the emergency responders in stinging
incidents. Most firetrucks already carried a surfactant, a soapy liquid
that helps put fires out. Such soaps also kill honey bees when sprayed
directly on them. Erickson also worked out ways to quickly convert
a firefighter's basic turnout gear into a protective bee suit. Fire
departments all over the Southwest are now trained in Erickson's methods.
Erickson also developed instructions for homeowners to help them deal
with AHBs, such as how to prevent honey bees from taking up residence
inside house walls and how to kill unwanted bee colonies. (It is safer,
though, to call an experienced exterminator if at all possible.)
Swarm traps invented by entomologist Justin O. Schmidt, also at the
Tucson bee center, have been a boon.
"We developed a simple, inexpensive trap with a pheromone lure
to attract swarms looking for new nest sites. That's how we're able
to track honey bee colonies as they spread out," Schmidt says.
The traps are also used as prophylactic barriers around golf courses,
airports, schools, and botanic gardens, or anywhere else AHBs might
take up residence and conflict with people. The traps lure swarms
away from high-traffic areas and make them easy to remove.
||Not All Bad
||People usually think
only of AHBs' downside, but they also represent a potential positive.
ARS entomologist Frank A. Eischen at the Honey Bee Research Unit in
Weslaco, Texas, has been studying AHBs for their resistance to Varroa
Eischen maintains an apiary in a remote part of southern Texas. "Maintains"
may not be the right term, because he simply leaves hive boxes out
and lets the bees fend for themselves year after year. All the honey
bees in the apiary have long since been Africanized.
His AHBs, which are never treated, have a slightly better survival
rate against Varroa mites. But that rate varies dramatically.
"I've looked at about 40 colonies. Some have very few mites,
and others are loaded," Eischen says. "But if these had
been EHB colonies without treatment, they all would have died long
He is trying to isolate which mechanism provides the protection from
Varroa mites. He has already ruled out hygienic behavior—the
time it takes worker bees to clean out mites. But if he determines
what AHBs do differently, it might be possible to breed that desirable
trait into EHBs.
||Adapted from Agricultural
Research: What's Buzzing with Africanized Honey Bees?, March
2004, USDA, Agricultural Research Service, by J. Kim Kaplan