Africanized bees extend their distribution in California
Africanized bees extend their distribution in California
Wei Lin 1 2
Jakob McBroome 1 2
Mahwish Rehman 0 2
Brian R. Johnson 1 2
0 Research Officer, Department of Entomology, Agricultural Research Institute , Tarnab, Peshawar , Pakistan
1 Department of Entomology and Nematology, University of California , Davis, CA , United States of America
2 Editor: Peter K. Dearden, University of Otago , NEW ZEALAND
Africanized honey bees (Apis mellifera) arrived in the western hemisphere in the 1950s and quickly spread north reaching California in the 1990s. These bees are highly defensive and somewhat more difficult to manage for commercial purposes than the European honey bees traditionally kept. The arrival of these bees and their potentially replacing European bees over much of the state is thus of great concern. After a 25 year period of little systematic sampling, a recent small scale study found Africanized honey bees in the Bay Area of California, far north of their last recorded distribution. The purpose of the present study was to expand this study by conducting more intensive sampling of bees from across northern California. We found Africanized honey bees as far north as Napa and Sacramento. We also found Africanized bees in all counties south of these counties. Africanized honey bees were particularly abundant in parts of the central valley and Monterey. This work suggests the northern spread of Africanized honey bees may not have stopped. They may still be moving north at a slow rate, although due to the long gaps in sampling it is currently impossible to tell for certain. Future work should routinely monitor the distribution of these bees to distinguish between these two possibilities.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
Funding: This work was funded by a grant from the
University of California Agriculture and Natural
Resources (Edwin C. Pohle Fund), and a Hatch
grant to Brian Johnson (CA-D-ENM 2161-H).
Competing interests: The authors have declared
that no competing interests exist.
Honey bees are a part of many urban and natural landscapes. In their native range, they are
the most populous, large colony bees and they play many important ecological roles [
their introduced range, which includes essentially all land inhabited by human beings with the
exception of the two poles, they are a major component of both ecological communities (often
as invasive species) and economic systems in which they are used for large scale pollination
and food production [2±4]. Many people identify with the honey bee due to its close
relationship to human beings and are hence more willing to listen to environmental concerns
regarding pollinators at large if they are packaged in the context of keeping honey bees healthy [5±6].
Africanized bees (AHBs) were initially brought to Brazil from South Africa for breeding
purposes, but quickly escaped and spread north replacing European honey bees (EHBs) as
they moved [7±10]. They arrived in the US in 1990 in Texas and in California in 1994 [
The arrival of these bees into California is of concern for two main reasons [
]. First, many
parts of California are strongly agricultural and Africanized honey bees can interfere with the
use of European honey bees for commercial pollination [
]. Second, many regions of
California are densely populated and Africanized bees are a public health threat as their
defensiveness can lead to serious injury and even death [
The purpose of the present study is to document the current distribution of AHBs in
California, with emphasis on quantifying their presence in populated areas. Recently, a small scale
study found AHBs in the San Francisco Bay Area, which raised much concern in the local
popular press [
]. The present study was ongoing before and during that study and uses the same
genetic methods. It, however, has a larger scope in terms of number of samples tested across
the northern part of the state. It thus can be seen as an extension of Kono and Kohn [
both seeks to validate its basic findings and to extend the sampling to more areas. Our goal was
to sample from the area of known distribution up to counties in which we could no longer
find AHBs. We used a simple genotyping test that makes use of a SNP present in EHBs missing
in AHBs [16±17]. As this test was also used in the past, it makes for strong continuity between
past measures of AHB distribution and the present measure. We chose to bias our samples to
urban areas, although we sampled in rural areas as well, because large numbers of managed
EHBs in rural agricultural areas may make detecting the much smaller AHB presence unlikely.
Further, urban landscapes are particularly favorable for honey bees in California due to large
scale planting of ornamental flowers making food available year-round (something strongly
lacking in natural landscapes). Urban landscapes have an abundance of nesting sites as well,
while the non-forested landscapes of much of California are not so favorable. Thus, urban
landscapes are both where one of the major problems exists (due to the danger of AHBs) and
they are also the landscapes most favorable to honey bees (both EHBs and AHBs) suggesting
intensive sampling in such areas should be productive for determining the distribution of
Materials and methods
Collection sites and methods
In urban landscapes, we collected from floral sites separated by at least several kilometers. In
rural, and or natural landscapes, we collected bees from wild flowers every 3-5km by driving
along roads and highways. We did not collect near farms or from crop plants. Collection
locations and number of bees caught at each site are in S1 Table. Bees were collected on flowers by
placing a new plastic bag over the flower. Bees flew into the bag, which was then sealed. Bees
were then immediately put into 95% ethanol in individual 1.6 ml tubes. Each bag was used
once and discarded. For each collection site, the date of collection and the latitude and
longitude of the location was recorded. At the end of each collecting day tubes with bees were placed
into a refrigerator for storage until DNA extraction.
The flight muscles of bees were used for DNA extraction using the Qiagen DNeasy blood and
tissue kit according to the manufacturer's instructions. To determine if the bee had a mtDNA
background of either European or African origin, we followed the protocol of Pinto et al [
In short, we amplified a region of the cytochrome b gene with PCR using primers given in
Pinto et al [
] and recently used by Kono and Kohn [
]. The PCR product was then digested
with BglII at 56ÊC for 3 hours. European bees have a cleavable site for this restriction nuclease
within the amplicon, thus EHBs can be diagnosed by two bands on a standard agarose gel.
AHBs exhibit a single band. Any unclear results (faint or double bands and so forth) were
thrown out and the test repeated with backup material (tissue or DNA depending on the
nature of the PCR failure).
2 / 8
We collected and tested 2699 bees from throughout northern California. This spanned the
range from where AHBs bee were known to be found (and were common in our dataset) to
the region past which we were able to identify AHBs. Fig 1 shows the current distribution of
AHBs in California and Table 1 shows the counts of Africanized bees found in each county.
The northern counties of Napa and Sacramento now appear to be the northernmost range of
AHBs. Neighboring counties along the coast were sampled with some depth but AHBs were
not found. Some counties in the central valley and foothills (Yolo and the counties in the Sierra
Nevada mountain range below Placer county but above Mariposa) were not sampled. It is
Fig 1. Current distribution of AHBs in California. Percentage of AHBs in each county sampled by the present study
only are shown. Counties highlighted in red have been found to be inhabited by AHBs. Counties in pink had no AHBs,
but had poor sampling.
3 / 8
likely that AHBs are present in these areas, but future work will have to determine whether
this is the case. Counties north of Sacramento (Butte and Shasta, for example) were sampled
but AHBs were not found.
California is a very large state. This, and the difficulty of finding bees in some regions,
limited sampling in some counties. Essentially, some counties were poorly sampled and could in
principle have AHBs present. Fig 2 explores this issue with a simple analysis using the binomial
distribution. Shown is the probability of having sampled no AHBs given their actual presence
at various levels in the environment from 2.5 to 40%. For four counties: Yolo, Sutter, Place and
Butte, sampling was too limited to conclude that AHBs were not present at even moderately
high levels (up to 30±40%). For the other counties in which AHBs were not found, AHBs are
likely not present at levels above 5±10%, but could be present at lower frequencies. It is
impossible to know the actual probabilities associated with the sampling procedure used, but the
idealized situation shown in Fig 2 should be qualitatively correct, meaning roughly speaking
AHBs could be present in all of these counties at low levels and could be present at even high
levels in the four highlighted counties.
This study was motivated in part by numerous reports to the UC Davis Apiculture program
from exterminators in the San Francisco Bay Area complaining of very aggressive bees.
4 / 8
Fig 2. Probability of sampling zero AHBs for all counties sampled without AHB presence using the binomial distribution.
For Yolo, for example, there was over 80% chance of sampling zero AHBs assuming AHBs are present at 2.5%. Yolo, Sutter,
Placer, and Butte were sampled with insufficient depth to rule out significant AHB presence.
Exterminators are often at the vanguard in noticing AHBs in an area, as they are called in to
remove unwanted wild or feral bees. Further, it has been nearly 20 years since a large-scale study
looked carefully at the distribution of AHBs in California. We found that AHBs are now much
further north than they were in the 1990s, being now present in both Napa and Sacramento.
AHBs were common in our samples around the Monterey area and in parts of the central valley
around Stockton, but they were at very low abundance in Napa and Sacramento. Thus, while the
edge of the distribution of AHBs may extend further north than Sacramento, the density of
AHBs in these regions is likely quite low. Perhaps the chief take-home message from this study is
that the spread of AHBs north may still be occurring at a slow rate. Thus, studies such as the
present one should probably be conducted at least once a decade to track their spread.
Kono and Kohn [
] published a recent study using the same methods we used here. They
samples over a larger area (up to Oregon), but sampled a smaller number of bees per site and
fewer sites. They found AHBs in the East Bay Area (near Oakland and across the bay from San
Francisco), which attracted considerable local attention. The present study extends the
distribution up to Sacramento and Napa. We also found AHBs over most of the Bay Area, while
Kono and Kohn found AHBs in only 2 samples from the East Bay. We thus filled in the
distribution of AHBs in the Bay Area and extended it further north.
Interestingly, San Francisco was well sampled in this study and no AHBs were found. This
is not due to a lack of forage in this highly urban area. Although San Francisco is the second
most densely populated city in the United States, ornamental plants are commonplace on
streets and parks and honey bees were quite easy to sample there. We sampled in several large
parks with numerous trees (Golden Gate Park, for example,) which provide natural nesting
sites. While AHBs may be in San Francisco at low numbers, they are certainly not common
and are probably quite rare. Why this is the case is intriguing and could be the focus of future
work by local researchers in this area.
5 / 8
Given the extremely rapid spread north of AHBs starting in Brazil in the 1950s (reaching
the US in the early 1990s), followed by a very slow rate of spread over the next decade, many
researchers suggested that perhaps AHBs had reached their natural northern limit in Central
]. This was a reasonable assumption given that AHBs are tropical in origin and
their spread south into temperate Argentina slowed and stopped when AHBs entered colder
climates where presumably EHBs are more competitive [11, 18±19]. The present study
suggests that although AHBs have slowed down in their rate of spread, the spread may not be
over. California has a Mediterranean climate and is relatively warm throughout the year
through most of the central valley and along the coast almost to the Oregon border. It is not
clear what the natural northernmost distribution would be in this context. While it is not likely
that AHBs will be very common as far north as Redding, it seems reasonable to speculate that
they could become common in the Sacramento and Napa areas as these areas do not differ
much in climate from regions to the south where AHBs are already common.
We conclude with a discussion of the testing approach used here and by Kono and Kohn
(2015). Africanized bees have been shown to be almost exclusively African in their mtDNA
haplotype, but up to 20% European in their nuclear DNA [20±21]. European bees outside of
the Iberian peninsula, in contrast, are almost exclusive European in their mtDNA and in their
nuclear DNA [22±23]. For these reasons, the simple mtDNA test used here has become the
standard for tests for Africanized bees in the United States [23±24]. Essentially, a large wave of
bees with almost exclusively African mtDNA is spreading into a region with almost exclusively
European mtDNA. We can see this in the results of the present study in that in the
northernmost samples no African mtDNA was found, while it was abundant in the southern samples.
In general, the present study is best seen as showing that there is a good likelihood that
Africanized bees are present in northern California. However, comprehensive studies are now
necessary to determine the scope of the problem, which could be small or large. Ultimately,
because the present study is only the first step in what must be a series of studies, it is not
suitable grounds for policy decisions regarding the large beekeeping industry present in northern
S1 Table. Sample information and genotyping results.
We thank Catherine Chen, Yu-Ju Chen, and Stephanie Wu for help with genotyping. We also
thank three anonymous referees for valuable feedback on the manuscript.
Conceptualization: Brian R. Johnson.
Formal analysis: Wei Lin, Brian R. Johnson.
Funding acquisition: Brian R. Johnson.
Methodology: Wei Lin, Brian R. Johnson.
Project administration: Brian R. Johnson.
Supervision: Brian R. Johnson.
Investigation: Wei Lin, Jakob McBroome, Mahwish Rehman, Brian R. Johnson.
6 / 8
Writing ± original draft: Brian R. Johnson.
Writing ± review & editing: Brian R. Johnson.
Winston ML. The biology and management of the Africanized honey bees. Annu Rev Entomol. 1992;
7 / 8
1. Seeley TD . Honey bee ecology . Princeton University Press, Princeton, N.J; 1985 2.
Goulson D. Effects of introduced bees on native ecosystems . Annu Rev Ecol Evol S . 2003 ; 34 : 1± 26 .
3. Aizen MA , Harder LD . The Global Stock of Domesticated Honey Bees Is Growing Slower Than Agricultural Demand for Pollination . Curr Biol . 2009 ; 19 : 915 ± 918 . https://doi.org/10.1016/j.cub. 2009 . 03 .071 PMID: 19427214
4. Calderone NW . Insect Pollinated Crops, Insect Pollinators and US Agriculture: Trend Analysis of Aggregate Data for the Period 1992±2009 . Plos One 2012 ; 7 : 27 .
5. Klein AM , Vaissiere BE , Cane JH , Steffan-Dewenter I , Cunningham SA , Kremen C , et al. Importance of pollinators in changing landscapes for world crops . P R Soc B . 2007 ; 274 : 303 ± 313 .
6. Oldroyd BP What's killing American honey Bees? Plos Biol . 2007 ; 5 .
7. De Jong D. Africanized honey bees in Brazil, forty years of adaptation and success . Bee World 1996 ; 77 : 67 ± 70 .
8. Sheppard WS , Smith DR . Identification of African-derived bees in the Americas: A survey of methods . Ann Entomol Soc Am . 2000 ; 93 : 159 ± 176 .
9. Schneider SS , Hoffman GD , Smith DR . The African honey bee: Factors contributing to a successful biological invasion . Annu Rev Entomol . 2004 ; 49 : 351 ± 376 . https://doi.org/10.1146/annurev.ento. 49 . 061802.123359 PMID: 14651468
10. Rangel J , Giresi M , Pinto MA , Baum KA , Rubink WL , Coulson RN , et al. Africanization of a feral honey bee (Apis mellifera) population in South Texas: does a decade make a difference? Ecol Evol . 2016 ; 6 : 2158 ± 2169 . https://doi.org/10.1002/ece3. 1974 PMID: 27069571
11. Rinderer T. E. , Oldroyd B. P. , and Sheppard W. S. Africanized bees in the United States . Sci Am . 1993 ; 269 : 84 ± 90 .
12. Visscher K , Vetter RS , Baptista FC . Africanized bees, 1990 ±1995: Initial rapid invasion has slowed in the U .S. Cal Agr 1997 ; 51 : 22 ± 25 .
14. Schumacher MJ , Egen NB . Significance of Africanized bees for public health: a review . Archives of Internal Medicine 1995 ; 155 : 2038 ± 2043 . PMID: 7575061
15. Kono Y , Kohn JR . Range and Frequency of Africanized Honey Bees in California (USA) . Plos One 2015 ; 10 .
16. Nielsen DI , Ebert PR , Page RE , Hunt GJ , Guzman-Novoa E . Improved polymerase chain reactionbased mitochondrial genotype assay for identification of the africanized honey bee (Hymenoptera: Apidae) . Ann Entomol Soc Am . 2000 ; 93 .
17. Pinto MA , Johnston JS , Rubink WL , Coulson RN , Patton JC , Sheppard WS . Identification of Africanized honey bee (Hymenoptera: Apidae) mitochondrial DNA: Validation of a rapid polymerase chain reactionbased assay . Ann Entomol Soc Am . 2003 ; 96 : 679 ± 684 .
18. Kerr WE , Delrio SD , Barrionuevo MD . The southern limits of the distribution of the Africanized honey bee in South America . Am Bee J . 1982 ; 122 : 196 ±&.
19. Taylor OR , Spivak M. Climatic limits of tropical African honeybees in the Americas . Bee World 1984 ; 65 : 38 ± 47 .
20. Clark KE , Rinderer TE , Franck P , Quezada-Euan JG , Oldroyd BP . The Africanization of honeybees (Apis mellifera L.) of the Yucatan: a study of a massive hybridization event across time . Evolution . 2002 ; 56 : 1462 ± 1474 . PMID: 12206246
21. Collett T , Ferreira KM , Aria MC , Soares AEE , Del Lama MA . Genetic structure of Africanized honeybee populations (Apis mellifera L.) from Brazil and Uruguay viewed through mitochondrial DNA COI-COII patterns . Heredity . 2006 ; 97 : 329 ± 335 . https://doi.org/10.1038/sj.hdy. 6800875 PMID: 16955114
22. Pinto MA , Sheppard WS , Johnston JS , Rubink WL , Coulson RN , Schiff NM , et al. Ann Entomol Soc Am 2007 ; 100 : 289 ± 295 .
23. Meixner MD , Pinto MA , Bouga M , Kryger P , Ivanova E , Fuchs S. Standard methods for characterizing subspecies and ecotypes of Apis mellifera . J Apicult Res . 1991 ; 52 : 1± 28 .
24. Delaney DA , Meixner MD , Schiff NM , Sheppard WS . Genetic characterization of commercial honey bee (Hymenoptera: Apidae) populations in the United States by using mitochondrial and microsatellite markers . Ann Entomol Soc Am . 2009 ; 102 : 666 ± 673 .