Africanized Bees in North America


by
Michael R. Kunzmann
National Biological Service
Stephen L. Buchmann
John F. Edwards
Steven C. Thoenes
Eric H. Erickson
U.S. Department of Agriculture
The honeybee genus Apis likely has the greatest breadth of pollen diet of any insect and, because of its human-caused cosmopolitan distribution, the species directly affects the reproductive biology of about 25% of the world's flowering plants (Schmalzel 1980; Buchmann et al. 1992). This situation has profound consequences for agribusiness, native plants and animals, and ecosystems. In 1956, bee geneticist Warwick E. Kerr imported queen bees of an African race (Apis mellifera scutellata) into Brazil to breed a more productive honeybee that was better adapted to the Neotropical climate and vegetation (Kerr 1967). The following year, 26 of Kerr's Africanized honeybee queens were inadvertently released into the surrounding forest (Winston 1987). Since then, the Africanized hybrids have been expanding their range northward, with an average rate of between 330 and 500 km (200 and 300 mi) each year (Fig. 1).

Fig. 1. Migration of Africanized honeybees.
The first U.S. Africanized honeybee colony was reported in October 1990, at Hidalgo, Texas, along the international boundary. By fall 1993, Africanized honeybees (AHBs) had extended their territory north and west into numerous counties of Arizona, New Mexico, and Texas (Fig. 2). Since the first U.S. AHB swarm was detected, the rate of spread has accelerated to over 600 km (375 mi) per year in the southwestern United States (Guzman-Novoa and Page 1994).

Fig. 2. Confirmed presence of Africanized honeybees in (colored) counties of Arizona, New Mexico, and Texas, January 15, 1994.
European honeybees (EHBs) were introduced into North America as early as the 16th century by Spanish conquistadors and missionaries (Brand 1988). Today, one of the three most common subspecies or races of the EHB, the Italian honeybee (A.m. ligustica), is nearly pandemic throughout North America because of its popularity with professional and hobbyist beekeepers. As a consequence, these non-native bees have become naturalized and have been a part of the North American arthropod biota for about 3,500 bee generations, or at least the past 200 years (Buchmann et al. 1992). European honeybees are commonly seen visiting agricultural food crops, cultivated flowers, and roadside wildflowers to gather nectar and pollen. They are even common in areas far from human population centers. These bees are also the preferred, "managed" pollinator for over 100 U.S. agricultural crops (e.g., fruits, vegetables, and some nuts), most of which depend on or benefit from insect pollination. The value of these pollination services by EHBs is estimated at $5-$10 billion annually in the United States (Southwick and Southwick 1992).
Africanized and European honeybees represent divergent subspecies within the mellifera species of the genus Apis. Both have nearly the same biochemistry, morphology, genetics, diet, and reproductive and other behaviors. Their diet includes pollen and spores from most seed plants. Both EHBs and AHBs are social bees living in perennial colonies. They are active on most days collecting nectar, water, pollen, and plant resins for their subsistence. These honeybees "hoard" excess honey as energy-rich carbohydrate reserves in hexagonal wax combs. Energy from honey consumption partially supports brood-rearing and, most importantly, supplies the energy necessary for foraging flights by thousands of adult worker bees.
Africanized and European honeybees exhibit different foraging strategies (largely tropical versus temperate attributes). Africanized honeybee colonies in Africa, and now in much of the Neotropics, are attuned to finding and exploiting isolated mass-flowering tropical trees, and also use pollen and nectar from the nocturnal flowers of bat-pollinated flowering plants. Some tropical Apis species even migrate to follow nectar and pollen flows across the floral landscape. Consequently, these bees depend on increased colony mobility (reproductive swarming and abandoning the hive) as behavioral responses to seasonal floral richness or dearths. EHBs are better at hoarding vast amounts of honey and surviving long, cold winters.
Although preliminary evidence for behavioral differences between the two races have been documented in the Neotropics (French Guiana, Venezuela, Panama; see reviews by Taylor 1977; Seeley 1985; Roubik 1989), the behavioral ecology of AHBs and their interactions with EHBs and thousands of species of native U.S. bees remain largely unknown. Africanized honeybees have slightly shorter developmental times than do European bees, enabling them to produce more bees per unit time compared with EHBs. Africanized bees will also accept smaller cavities to nest in than European bees. This behavior increases potential competition for nesting sites with birds and other animals and also increases the potential for greater numbers of honeybee colonies in an area. Africanized honeybees commonly abandon their hives, often 15%-30% annually or even much greater in some localities. Absconding colonies may travel as far as 170 km (about 100 mi) before selecting a new nesting site (USDA 1994). Thus they have been able to rapidly colonize new areas in the Neotropics.
Africanized honeybees swarm outside a trap in Costa Rica. Courtesy J.O. Schmidt, USDA
The most often-discussed characteristic separating the two races is the AHBs' propensity to vigorously defend their colony and nest site. Although all honeybees respond to threats to their colonies, AHBs respond more quickly and in much greater numbers than do EHBs. In comparison to EHBs, greater numbers of AHBs will pursue intruders for much greater distances to defend their colonies. Recent research reported that 3 to 4 times as many AHBs responded and left 8 to 10 times more stings in a black leather measuring target in stinging experiments (USDA 1994).
Biochemical comparisons of AHB and EHB venoms indicate they are nearly identical. Nineteen stings per 1 kg (2.2 lb) of human victim body weight is the predicted median lethal dose (Schumacher et al. 1992). Massive stinging incidents by AHBs are more likely to result in toxic envenomation. Reported 1993 stinging incidents in Mexico have involved more than 60 human fatalities (one death per 1.4 million). From 1988 to 1992, the Mexican national African Bee Program eliminated 117,000 AHB swarms in densely populated urban areas (Guzman-Novoa and Page 1994). To date, the worst U.S. stinging incident occurred in July 1992, when a 44-year-old man mowing his lawn experienced a massive bee attack resulting in 800-1,000 stings (McKenna 1992).

Ecological Implications
Competition among nectar- and pollen-feeding invertebrate and vertebrate pollinators, resource partitioning, insect and plant community interactions, and ecosystem processes are affected by introduced EHBs and AHBs, with important short- and long-term ecological and perhaps evolutionary consequences. The influence of exotic honeybees on individual species or communities of native tropical (or temperate) plants or animals can only have one of three outcomes: the native species will suffer, benefit, or remain more or less unaffected. The key to understanding these seemingly obvious outcomes is, however, based on obtaining sufficient information to delineate the very complex short- and long-term competitive dynamics between introduced bees, native bees and pollinators, and native plants in diverse, interacting, natural communities.
One observational and manipulative competition study between honeybees, bumblebees, solitary bees, and ants was at midelevations in the Santa Catalina Mountains in the Sonoran Desert near Tucson, Arizona (Schaffer et al. 1983). Dramatic shifts in abundance of ants and bumblebees were detected when honeybees were present (introduced) or sealed inside their hives. The researchers suggested that direct competition between introduced honeybees and native hymenopteran floral visitors was caused by honeybees numerically dominating the site. Initial evidence seems to indicate that honeybees seek out and preempt the most profitable habitats and partially exclude native bees indirectly by rapidly reducing the standing crop of plant nectar and pollen (Agave in this study).
Both species of non-native bees forage vast expanses of territory containing native and non-native floral resources. Estimates of the amount of terrain foraged annually by an average-sized honeybee colony in New York hardwood forests (Visscher and Seeley 1982) are 80-100 km2 (30-40 mi2). Forage area estimates for AHB colonies living in lowland Panamanian rain forests (Roubik 1989) are 200-300 km2 (75-115 mi2), although 90% of these foraging flights are completed within 5 km (3 mi) of the nest (Visscher and Seeley 1982). Even given this restrictive caveat, the amount of "bee pasture" grazed by these aerial herbivores is immense.
In studying honeybee colonies foraging in temperate forests in New York State, Visscher and Seeley (1982) found that these cold-hardy EHB colonies amassed 15-30 kg (33-66 lb) of pollen and 60-80 kg (132-176 lb) of honey each year. To collect this amount of food, a colony must dispatch tens of thousands of foragers on many millions of foraging bouts with the bees flying 20-30 million km (12-19 million mi) overall. Similar studies of AHBs in Panama (Roubik 1989) determined that AHBs placed more emphasis on pollen collection. The Sonoran Desert of northern Mexico and southern Arizona is perhaps one of the richest areas in the world in floral resources because of the relative high plant diversity and the many fair-weather days for worker-bee foraging.
Many important nectar- and pollen-producing plants visited by AHBs bloom at night and are pollinated by bats. Africanized honeybees find and exploit these rich flowers at first light, and we predict that saguaros and other columnar cacti will be heavily used as food plants for AHBs in Arizona. Early Arizona data for AHB colonies illustrate that most AHB colonies have been found in the subtropical climate zones in Sonoran desertscrub.
Determining which plants are used primarily for nectar versus pollen, or both, depends on direct observations of bees on flowers or indirectly by identifying pollen grains in stored nest samples of honey. In Panama, Roubik (1989) found that AHB colonies harvested pollen from at least 142-204 flowering plant species in a forest containing about 800-1,000 species. European honeybees collected pollen or nectar from about 185 plant species from a secondary forest and agricultural area in Mexico (Villanueva 1984). These studies suggest that honeybees are using about 25% of the local flora, but intensively use far fewer species at any given time (Roubik 1989). In Arizona EHBs will often harvest pollen from more than 60 species annually, but of these, only 10-15 are harvested heavily and consistently from year to year (Buchmann et al. 1992). Because of their pollen herbivory and reproductive contact with so many plants, there can be serious long-term ecological and evolutionary consequences of these interactions that we simply do not yet understand.

Ecological Monitoring
Although we have made a case for potential serious, competitive displacement of food resources by honeybees to the exclusion of some native bees and pollinators, there is a little-appreciated yet unique ecological application for using EHB colonies (A. mellifera) as short- and long-term local and regional monitoring devices of vegetation diversity, plant productivity, flowering phenology, precipitation, climate, and general ecosystem health. No expensive equipment is required since the bees do all the "fieldwork." In addition, floral changes in landscapes can be determined from the rich "fossilized" source of pollen dietary information in old, dark brood combs or in 75- to 100-year-old "debris middens" in the Sonoran Desert (Buchmann et al. 1992). Long-term records (some spanning decades) for certain beekeeping locations are invaluable aids to beekeepers, ecologists, and resource managers for ecological evaluation and monitoring.
To validate any AHB range-expansion prediction or to measure potential effects on native pollinators or ecosystem components, we must monitor the bees and evaluate habitats on national and local scales. Information must be collected, integrated, and shared by researchers, individuals, and agencies. Public-and-private-sector partnerships have been developed to exchange AHB information and develop monitoring protocols.
Researchers use geographic information systems (GIS) and global positioning systems (GPS) technologies to track the locations of known AHB and EHB colonies; delineate honeybee habitat parameters such as preferred vegetation community, climatic zone, elevation, and distance to water; investigate potential ecological consequences to native bees and other nectar-dependent species; monitor and detect habitat productivity changes; and develop computer models to illustrate and predict preferred AHB habitats and potential ecological consequences (Fig. 3).

Fig. 3. Known honeybee locations in Arizona displayed with vegetation classes; derived from Brown et al. (1979).

The Future
Knowing how far north AHBs will spread is critical in predicting their ecological effects. There is general agreement that they have a climatic limit, but precise limits of their U.S. range expansion is disputed. Some researchers suggest that AHBs will disperse almost as far north as Canada; others propose that they will go no farther than the U.S. southwestern and southeastern corners. In all likelihood, AHBs will become established as a dominant ecosystem forager in the southern third of the United States, where EHB overwintering behavior is less critical for survival. If conditions are favorable, however, the AHBs may expand into marginally productive or colder habitats in higher latitudes or elevations.
While the ecological range limits and economic consequences of non-native AHB migration into the United States are not precisely known, researchers agree that honeybees are economically important, and that sufficient biological information exists to develop adequate inventory and monitoring programs. Added benefits to honeybee monitoring programs are also important because bee colonies can also serve as excellent indicators of flowering plant productivity, ecosystem stability, and relative ecological health.
For further information:
Michael R. Kunzmann
National Biological Service
Cooperative Park Studies Unit
University of Arizona
125 Biological Sciences E., Bldg. 43
Tucson, AZ 85721
References
Brand, D.D. 1988. The honeybee in New Spain and Mexico. Journal of Cultural Geography 9:71-81.

Brown, D.E., C.H. Lowe, and C.P. Pase. 1979. A digitized classification system for the biotic communities of North America, with community (services) and association examples for the Southwest. Journal of the Arizona Nevada Academy of Science 14 (Supplement 1):1-16.

Buchmann, S.L., M.K. O'Rourke, C.W. Shipman, S.C. Thoenes, and J.O. Schmidt. 1992. Pollen harvest by honeybees in Saguaro National Monument: potential effects on plant reproduction. Pages 149-156 in C.P. Stone and E.S. Bellantoni, eds. Proceedings of the Symposium on Research in Saguaro National Monument, Tucson, AZ.

Guzman-Novoa, E., and R. Page. 1994. The impact of Africanized bees on Mexican beekeeping. American Bee Journal 134:101-106.

Kerr, W.E. 1967. The history of the introduction of African bees to Brazil. South African Bee Journal 39:3-5.

McKenna, W.R. 1992. Killer bees: what the allergist should know. Pediatric Asthma, Allergy and Immunology 6(4):19-26.

Roubik, D.W. 1989. Ecology and natural history of tropical bees. Cambridge University Press, England. 514 pp.

Schaffer, W.M., D.W. Zeh, S.L. Buchmann, S. Kleinhaus, M.V. Schaffer, and J. Antrim. 1983. Competition for nectar between introduced honeybees and native North American bees and ants. Ecology 64:564-577.

 Schmalzel, R.J. 1980. The diet breadth of Apis (Hymenoptera: Apidae). M.S. thesis, University of Arizona, Tucson. 79 pp.

Schumacher, M.J., J.O. Schmidt, N.B. Egen, and K.A. Dillion. 1992. Biochemical variability of venoms from individual European and Africanized honeybees (Apis mellifera). The Journal of Allergy and Clinical Immunology 90:59-65.

Seeley, T.D. 1985. Honeybee ecology: a study of adaptation in social life. Monographs in Behavior and Ecology, Princeton University Press, NJ. 201 pp.

Southwick, E.E., and L. Southwick, Jr. 1992. Economic value of honeybees (Hymenoptera: Apidae) in the United States. Journal of Economic Entomology 85(3):621.

Taylor, O.R. 1977. The past and possible future spread of Africanized honeybees in the Americas. Bee World 58:19-30.

USDA. 1994. African honeybee fact sheet. U.S. Department of Agriculture.

Villanueva, R. 1984. Plantas de importancia apicola en el ejido de Plan del Rio. Veracruz, Mexico. Biotica 9:279-340.

Visscher, P.K., and T.D. Seeley. 1982. Foraging strategy of honeybee colonies in a temperate deciduous forest. Ecology 63:1790-1801.

Winston, M.L. 1987. The biology of the honeybee. Harvard University Press, Cambridge, MA. 281 pp.