Pathogens, Pests, and Economics: Drivers of Honey Bee Colony Declines and Losses
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- Smith, K.M., Loh, E.H., Rostal, M.K. et al. EcoHealth (2013) 10: 434. doi:10.1007/s10393-013-0870-2
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The Western honey bee (Apis mellifera) is responsible for ecosystem services (pollination) worth US$215 billion annually worldwide and the number of managed colonies has increased 45% since 1961. However, in Europe and the U.S., two distinct phenomena; long-term declines in colony numbers and increasing annual colony losses, have led to significant interest in their causes and environmental implications. The most important drivers of a long-term decline in colony numbers appear to be socioeconomic and political pressure on honey production. In contrast, annual colony losses seem to be driven mainly by the spread of introduced pathogens and pests, and management problems due to a long-term intensification of production and the transition from large numbers of small apiaries to fewer, larger operations. We conclude that, while other causal hypotheses have received substantial interest, the role of pests, pathogens, and management issues requires increased attention.
Keywordshoney beecolony lossVarroamanagementpollinator
There has been significant concern over managed honey bee colony declines and losses given their role as crop pollinators and concurrent concern regarding perceived global pollinator declines. Political and socioeconomic factors have contributed to colony declines in Europe and the U.S. over the past half-century. Yet the overall global number of colonies has increased 45% since 1961. Although many environmental and anthropogenic factors remain under investigation for their role in annual honey bee colony losses, the introduction of pests and pathogens, and large-scale shifts in management practices may be significant, under-researched drivers of colony losses in Europe and North America.
There has been significant concern over managed honey bee colony declines and losses, coupled with a growing interest in the long-term fate of global pollinator populations. The Western or European honey bee (Apis mellifera) is a semi-free-ranging managed agricultural species (i.e., managed wildlife species) that pollinates US$215 billion worth of crops worldwide (vanEngelsdorp et al. 2008; Gallai et al. 2009). While the number of colonies has increased 45% globally since 1961, the proportion of pollinator-dependent crops has risen by 300% (Aizen and Harder 2009). To perform the ecosystem service of pollinating agricultural crops honey bees are now managed intensively (particularly in the U.S.) as mobile pollinators, with millions moved annually by truck across the country (Sumner and Boriss 2006).
However, in recent decades both long-term (multi-year) declines and annual losses in honey bee colonies have occurred throughout Europe and the U.S. In this paper, we review the literature on patterns of bee declines and losses, their hypothesized causes, and the environmental, socioeconomic, and agricultural changes likely involved.
Long-Term Colony Declines
Additionally, long-term declines in the U.S. may be artificially heightened by the counting methods used by the USDA National Agricultural Statistics Service (NASS). A widely cited reduction by one million colonies during the 1980s (Fig. 1) coincides with NASS discontinuing the inclusion of colonies from operations with five or fewer hives (vanEngelsdorp and Meixner 2010). Thus, key political and socioeconomic factors including the process of data collection have played a primary role in the steady decline in honey bee colony numbers in Europe and the U.S.
Annual Colony Losses
Hypothesized Causes of Annual Colony Losses
Pathogens and Pests
Pathogens and pests are known to cause annual colony losses and have been linked, hypothetically, to long-term declines in bee populations. Prior to the 1970s, diseases of bee larvae (brood diseases) were the most economically important threats, e.g., American Foulbrood (Paenibacillus larvae). These continue to cause significant losses and have global ranges (vanEngelsdorp and Meixner 2010). However in the 1970s and 1980s the parasitic mite Varroa destructor was introduced to honey bees in Europe and in North America respectively (Rosenkranz et al. 2010). This is now a pan-global pest with the exception of Australia, several isolated islands, and possibly some Central African countries (Potts et al. 2010a). This pest is a key factor in recently elevated colony losses in Canada (Currie et al. 2010; Guzman-Novoa et al. 2010) and much of Europe (Aston 2010; Chauzat et al. 2010; Gajger et al. 2010; Le Conte et al. 2010; Mutinelli et al. 2010; Neumann and Carreck 2010; Vejsnaes et al. 2010; Potts et al. 2010b), whereas regions in which V. destructor is not a significant problem have not experienced similar losses (Neumann and Carreck 2010).
The V. destructor mite transmits pathogenic viruses (Gisder et al. 2009) including deformed wing virus (DWV), which results in overt infection significantly correlated with colony loss (Genersch et al. 2010). Three other viruses, acute bee paralysis virus, Israeli acute paralysis virus, and Kashmir bee virus, also increase in virulence when transmitted by V. destructor and all are recognized as causes of colony loss in regions affected by the mite (Ribiere 2008; Highfield et al. 2009; Genersch and Aubert 2010). However, lack of standardized surveillance by beekeepers, inconsistency among studies, and the complexity of transmission dynamics hampers an assessment of their true significance. For example most studies report viral presence or absence, but do not collect sufficient clinical data over time to assess the relative contribution of a virus in a given colony loss event. Interactions between viruses, honey bee immunology, and strain variation may play a role in how an individual or colony responds to viral infection. Recent analysis of the emergence of V. destructor in the Hawaiian archipelago suggests that introduction of the mite leads to increased prevalence and intensity of infection by DWV, and the dominance of a single virulent strain (Martin et al. 2012). The rise of this virulent strain in Hawaii lagged the introduction of the mite and may explain the lag in honey bee losses relative to the introduction of the vector elsewhere. This strongly suggests that the consequences of both Varroa introduction and the viruses it harbors are significant for global honey bee losses.
The microsporidian Nosema apis is the cause of bee dysentery. A close relative, N. ceranae from the Asian honey bee, has recently emerged in A. mellifera and may have replaced N. apis throughout much of its range (Klee et al. 2007, Chen et al. 2008). N. ceranae has been cited as the cause of significant colony losses in Spain (Higes et al. 2007, 2008), though such virulence has not been demonstrated elsewhere (Gisder et al. 2010). Further research is needed to understand the role of N. ceranae in honey bee losses and any possible additive or synergistic effects it may have with other stressors.
The recent, pan-global emergence of honey bee diseases correlates with broad trends in disease emergence (Daszak et al. 2000). These include a growing number of diseases spread by globalized trade and travel—a process termed “Pathogen Pollution” (Cunningham et al. 2003). The anthropogenic introduction of exotic pathogens and pests is the most common cause of disease emergence in crop plants (Anderson et al. 2004), and is responsible for the emergence of a wide range of diseases affecting livestock (e.g., H5N1 avian influenza, Rift Valley fever—Kilpatrick et al. 2006a; Al-Afaleq and Hussein 2011) and humans (e.g., West Nile virus—Kilpatrick et al. 2006a, b; Al-Afaleq and Hussein 2011). Honey bee pathogens have emerged and spread through trade in bees, bee products, and equipment [e.g., the introduction of tracheal mite (Acarapis woodi) and V. destructor to the Americas (Mutinelli 2011), and the spread of V. destructor and N. ceranae from the Asian honey bee (A. ceranae) to A mellifera]. Recently, the World Organisation for Animal Health (or Office Internationale des Epizooties) has made a number of bee pathogens notifiable under regulations approved by country signatories of the World Trade Organization (Mutinelli 2011). This enables restriction of trade in bees and bee products from countries that have identified these pathogens in their bee populations.
It has been hypothesized that the application of pesticides, in particular neonicotinoid insecticides, plays a significant role in honey bee colony losses. Neonicotinoids are extensively applied to some crops commercially pollinated by honey bees (Johnson et al. 2010). Neonicotinoids are systemic insecticides that can be applied to seeds and absorbed into the growing plant tissue, making them highly effective, but allowing residues to be present in pollen and nectar (Thompson 2010; Creswell 2011). The application of pesticides directly to seed has environmental advantages over widespread spray treatments such as reduced run-off, less frequent non-target species toxicity (Thompson 2010), and reduction of pesticide use per hectare. Acute honey bee mortality events caused by the accidental drift of dust or spray from pesticide applications have occurred (Krupke et al. 2012). While drift reducing technologies have helped reduce the risk (Thompson 2010), a recent study revealed colony losses may still occur due to pesticide drift (Tapparo et al. 2012).
Exposure to pesticides may also result in sublethal effects on the health of individual honey bees that could lead to reduced survival. Recent studies suggest that low-level pesticide exposure can impair immune system functioning, learning ability, memory, foraging behavior, and odor discrimination in honey bees (Desneux et al. 2007; Yang et al. 2008; Alaux et al. 2010; Williamson and Wright 2013). Further, chronic exposure to pesticides that would otherwise have no harmful effects for healthy colonies may interact with pathogens to produce harmful consequences for colonies already vulnerable to disease (Thompson 2003; Vidau et al. 2011; Aufauvre et al. 2012). One study suggested that individual honey bee vulnerability to Nosema infection is enhanced by the presence of the neonicotinoid imidacloprid, but the parent colonies failed to show increased Nosema levels due to exposure in this study (Pettis et al. 2012).
Simulated exposure events on free-ranging foragers showed that exposure of honey bees to the neonicotinoid thiamethoxam at low levels caused mortality, perhaps due to homing failure, and that this may contribute to colony loss (Henry et al. 2012). However, the bioavailability of the active ingredient in the treated bees was not measured in this study, and is a critical issue to investigate in general. Studies have detected low levels of neonicotinoids (1–10 μg kg−1) in the pollen and nectar of treated plants throughout the blooming period (Creswell 2011) and some scientists argue that bees likely ingest nectar repeatedly through several flower visits, thus increasing their exposure (Yang et al. 2008; Creswell 2011). Other research has shown that chronic exposure to the neonicotinoid imidacloprid at similar sublethal doses has negligible effects at the colony level (Nguyen et al. 2009). Field exposure of bees to flowering canola (Brassica napus) grown from seed treated with the neonicotinoid clothianidin had negligible impact on acute mortality, longevity, brood development, and over-winter survival (Cutler and Scott-Dupree 2007). However, control and treatment fields in that study were in close proximity (295 m), with a small radius (1 km) surrounding the test areas for which alternative foraging options were controlled. Elevated levels of the crop fungicide chlorothalonil have been reported in entombed pollen (pollen presumably sealed off within the hive as a protective measure) (vanEngelsdorp et al. 2009a), and the presence of entombed pollen has been associated with increased colony mortality. However, larvae and adult bees that were fed diets supplemented with entombed pollen did not experience increased mortality (vanEngelsdorp et al. 2009a). Similarly, a laboratory study (Girolami et al. 2009) found extremely high concentrations of neonicotinoids in the guttation drops of corn plants and demonstrated that when bees consumed this fluid, death occurred within a few minutes. The guttation drops were supplemented with a sugar solution to stimulate consumption and there is continued debate as to whether honeybees use guttation fluid as a source of water in relevant crop species. Other studies did not find detrimental effects when conducting similar research with sugar beets, and when alternative drinking sources were available as might occur in nature (Keppler et al. 2010; OPERA 2011). While several recent studies have used models to suggest colony level effects (Henry et al. 2012; Whitehorn et al. 2012), a confirmed causal relationship between environmental pesticide residues and CCD or significant colony loss has yet to be demonstrated (Johnson et al. 2010). The most difficult challenge facing experimental researchers lies in deducing whether sublethal effects on individuals play a role in colony loss under real field conditions.
Beekeepers commonly use miticides to reduce the impact of varroasis. Miticides are placed directly in the hive, and are lipophilic and accumulate in the wax, particularly in the comb where the brood develops (Johnson 2009). Exposure may be exacerbated by synergism, which occurs between low doses of the miticides coumaphos and tau-fluvalinate for example due to binding competition for P450, a class of enzymes important for detoxification (Johnson 2009). Some fungicides found in hives inhibit ergosterol biosynthesis and thus synergize the toxicity of pyrethroid insecticides to honey bees (Johnson 2009). Finally, bees exposed to the antibiotic oxytetracycline which is applied within the hive, were significantly more sensitive to coumaphos and tau-fluvalinate (Hawthorne and Dively 2011). Effects of additive exposure to cholinergic pesticides such as imidacloprid and coumaphos have also been found (Palmer et al. 2013). Further, recent studies examining in-hive chemicals have detected a high diversity of miticides and agricultural pesticides in bee colonies (vanEngelsdorp et al. 2009b; Mullin et al. 2010). Although high levels of intoxicants were detected, no direct correlation between pesticides found in hives and colony loss was found (aside from higher levels of coumaphos in healthy colonies) (vanEngelsdorp et al. 2009b). Although recent studies are beginning to address how bees process these toxins by determining what factors affect the ability of honey bees to upregulate P450 detoxification genes in response to in-hive chemicals (Johnson et al. 2012), additional research into the potential additive and synergistic effects is still needed.
Other Causal Hypotheses
Some authors have proposed that multiple drivers act together, or secondarily to broader changes that may affect the ability of honey bees to resist declines, or that they directly cause immunosuppression. These broader changes include poor genetic diversity, weather events, stress, or inadequate nutrition (Potts et al. 2010a). There has been little research on whether long distance transportation of colonies causes stress, but it has not been linked to losses (vanEngelsdorp et al. 2007, 2008, 2010, 2011, 2012) Agricultural intensification, the increase of monoculture acreage, and loss of natural habitat may affect honey bee nutrition, and theoretically colony health; however, this has not been fully investigated (Brodschneider and Crailsheim 2010).
What is the Most Likely Causal Candidate for Annual Colony Losses?
As with any large-scale biological phenomenon, competing and often conflicting hypotheses are difficult to resolve. However, the evidence from the current state of the field suggests that some individual drivers appear to be more important in honey bee annual losses than others. For example, surveys of beekeepers and published reports suggest elevated colony losses in much of Europe and Canada are attributed to V. destructor and its associated pathogens, as well as known environmental and managerial challenges such as weather events and starvation (Aston 2010; Chauzat et al. 2010; Gajger et al. 2010; Le Conte et al. 2010; Mutinelli et al. 2010; Neumann and Carreck 2010; Vejsnaes et al. 2010; Potts et al. 2010b). Similarly, the majority of losses in the U.S. have been attributed to one or more known causes such as weather events, starvation, queen failure, and mites by beekeepers in recent surveys (vanEngelsdorp et al. 2010, 2011, 2012). However, these reports do not take into consideration the presence or absence of rigorous health surveillance in these colonies prior to the loss.
The hypothesis that honey bees have become immunosuppressed, making them more susceptible to pathogens or pests, parallels similar arguments used for wildlife population declines that were later attributed to single causal factors, e.g., amphibian declines due to the emerging disease chytridiomycosis (Daszak et al. 2003). Caution needs to be applied to this hypothesis, particularly when so little is known of honey bee innate and social immunity. Honey bees have approximately one-third of the immunity genes of other insects, but as colonial insects they also rely on social immunity where colonial behavior protects the colony (Evans and Spivak 2010). Additionally, balanced nutrition including pollen and propolis has recently been demonstrated to induce detoxifying enzymes in individual honey bees (Mao et al. 2013). Therefore, studies that experimentally show an effect on immunity in individual honey bees cannot be readily extrapolated to an effect on the colony in the field.
Perhaps the most parsimonious hypothesis for the cause of annual losses is that the relatively recent introduction of Varroa mites, and the resultant expansion of the range, prevalence, or intensity of infection of viral strains they harbor, has had a dramatic impact on honey bees. Thus, rather than immunosuppression of honey bee immunity as the underlying condition, it may be the relative naivety of bee colonies to Varroa and viral strains causing their decline and susceptibility to other factors. This hypothesis is supported by recent findings that the increase in prevalence and dominance of virulent viral strains lags behind Varroa introduction and leads to a complex, but probably significant, relationship between pest introduction, pathogen emergence, and honey bee colony losses (Martin et al. 2012). In a similar scenario, arguments that disease-induced amphibian population declines were only possible due to immunosuppression by another driver have now been largely refuted. In this case, the international trade in amphibians has now been widely accepted as the primary driver of the emergence of this disease (Daszak et al. 2003; Fisher et al. 2012). This is consistent with the increasing importance of trade and travel in the emergence and spread of diseases in humans, livestock, and wildlife globally (Smolinski et al. 2003; Karesh et al. 2005; Fevre et al. 2006; Hosseini et al. 2010) and in other types of biological invasions (Levine and D’Antonio 2003).
Critical Research and Policy Gaps
New Approaches to Investigating Causal Factors
For honey bee colony losses, long-term datasets on bee colony dynamics, land use change, Varroa destructor prevalence, bee pathogen distribution and prevalence, pesticide exposure, and a range of other hypothesized causes are not uniformly available across the geographic range of reported losses. Furthermore, there are differences among regions in the manner data have been collected, and honey bee losses defined and reported. While specific tests of some hypotheses have been published, these are relatively few and are still insufficient to deduce cause. More importantly, there are a large number of hypothesized drivers that have not yet been studied at a scale relevant to understanding the ecology of bee losses.
Varroa destructor Research
Studies to deduce the role of V. destructor in colony losses would require information on the introduction of Varroa into new regions. At the individual apiary level, there are rarely adequate monitoring history and records available of Varroa levels in each colony over time prior to a loss event. Rather, investigation into the role of Varroa in losses is usually attempted after the fact. Until more rigorous health monitoring techniques are implemented, mathematical modeling of the spread of Varroa at a regional level might provide insight, and models could be back-tested by using data on first reports of the pest from states, provinces, and counties following introduction. Metapopulation models which explicitly estimate the connectivity among colonies may provide a way to estimate the seasonal spread of pathogens, their ability to cause the extensive colony loss in some regions, and their likely patterns of persistence. Testing this hypothesis might be aided by the presence of regions where Varroa is absent (e.g., Victoria, British Columbia) and by studies that monitor the impact of the mite over a number of years on longitudinal experimental colonies.
Intensification of Honey Bee Production
The ratio of commercial beekeeper to (small-scale) hobbyist operations has increased in many countries due to the increasing demand for pollination services to commercial crops (Daberkow et al. 2009; Potts et al. 2010b) and rising competition from international honey markets (Le Conte and Navajas 2008). Given the current condition of the U.S. honey market, American beekeepers are less economically rewarded to produce honey and provide their bees with a varied diet compared to providing pollination services. Bees devoted to pollination services are primarily exposed to monoculture nutritional sources (Le Conte and Navajas 2008; Champetier et al. 2010). Between 1987 and 2002 in the U.S., there was a 40% decline in beekeepers owning fewer than 25 colonies, and a 66% increase in colonies managed by commercial beekeepers (Daberkow et al. 2009). Thus, the intensification of bee production in the U.S. is similar to much earlier intensification of livestock production, e.g., the movement from backyard poultry production to commercial rearing in the early twentieth century. This process led to the emergence of agricultural pathogens historically (e.g., Salmonella enteritidis) and currently (e.g., influenza A/H5N1). However, there are important differences between honey bee management and intensively managed livestock. These include a paucity of regulations on veterinary care, disease testing and quarantine of traded animals, use of treatments, nutrition studies to develop dietary supplements, and health and welfare inspection. The manner in which health care, management plans, nutrition modification, and record-keeping are applied in beekeeping operations may need to adapt to the changing levels of intensification to avoid further declines in bee health and increased annual losses. Trained bee health consultants (e.g., extension workers, state apiculturists, veterinarians) may help in this transition. These workers could assist beekeepers in adapting management schemes based on local environmental and disease threats, size of operation, and diagnostic and treatment needs.
There has been a great deal of work on the role of pesticides in bee colony loss leading to significant concern over their effects. Despite this concern and a recent European policy changes on the use of neonicotinoid pesticides, scientific studies examining the overall impact of neonicotinoid pesticides on managed honey bee colony loss are yet to demonstrate colony level losses in field settings and at field doses. Critical issues to be addressed are the lack of comparable losses in some regions where pesticides are used, uncertainty about appropriate field doses, and the methods used in experimental studies. Further research on sublethal and additive or synergistic effects of agricultural chemicals and their residues, using field-based colony level data, and application of spatial analysis methodologies to past and ongoing losses may help elucidate the role of pesticides in losses.
Longitudinal Epidemiological Monitoring and Disease Investigations
The lack of standardized reporting within and among countries makes it difficult to track patterns of colony loss on local or regional scales. Longitudinal epidemiological monitoring that includes long-term studies of patterns of colony loss, disease incidence and pathogen prevalence in single populations, with standardized sampling and data collection protocols would allow effective comparisons among different geographic areas experiencing losses. The COLOSS network (Prevention of Honeybee Colony Losses—http://coloss.org/), the German Honey Bee Monitoring Program (Genersch et al. 2010), the Bee Informed Project (www.beeinformed.org), and USDA’s Managed Pollinator Coordinated Agricultural Project (CAP—www.beeccdcap.uga.edu), have begun efforts along these lines.
Environmental and Ecological Implications
The full range of environmental implications of honey bee colony declines and losses is unclear and some suggest that the role of the managed honey bee in pollinating native and even some crop plants is overestimated (Winfree et al. 2007; Ollerton et al. 2012; Garibaldi and Winfree 2013). Thus, it is difficult to assess how honey bee colony losses relate to global native pollinator declines. Honey bees are an introduced species in many parts of their range (e.g., North America), and they are extensively managed throughout. They do, however, have free contact with wild native pollinators, allowing pathogen exchange, resource competition, and exposure to similar environmental hazards. Yet stressors may or may not affect managed honey bees differently than native or non-managed species. Despite evidence of transmission, the impact of Varroa mites and viruses on native bees at the population level remains unclear. Similarly, pesticides have the potential to affect native pollinators (Potts et al. 2010a), particularly bumble bee (Bombus terrestris) foraging behavior and reproductive success (Gill et al. 2012; Laycock et al. 2012; Whitehorn et al. 2012) although bumble bee life cycle characteristics differ from that of the honey bee. Regardless of the true extent of native pollinator declines (Levine and D’Antonio 2003), it is uncertain if managed honey bee declines and losses are related to native or free-ranging pollinator declines. Regardless, past lessons learned from impacts of agricultural intensification on wildlife should be applied when investigating the potential health links between honey bees and native pollinators, while accounting for the unique nature of managing a semi-free-ranging species.
Causes of long-term declines of honey bees in the U.S. and Europe are most likely due to the loss of beekeepers to the industry and intense competition for bee products. More research is needed on honey bee annual losses, but the global spread of pathogens and pests through the trade in bees and bee products likely plays a key role. These losses compound two critical environmental problems: the role of anthropogenic movement and manipulation of wildlife and their habitat and the role of agricultural intensification on managed species. In many ways, the state of research on honey bee losses mirrors the situation of livestock production in the early twentieth century, with poor coordination of best management practices (including lack of adequate knowledge of health threats and treatments) and a low appreciation of the impact of rapid demographic change on a managed population.
For this issue, some lessons can be learned from amphibian population declines, which were first brought to global attention in the late 1980s and considered enigmatic by their occurrence in presumed pristine locations (Blaustein and Wake 1990). Despite several initial hypotheses of driver interactions (Kiesecker and Blaustein 1995; Davidson et al. 2001; Pounds 2001), the identification of an emerging disease, chytridiomycosis (Berger et al. 1998) led to a marked shift in emphasis. The emergence of this disease, driven by the global trade in amphibians is now widely accepted as the major driver of enigmatic global amphibian population declines (Stuart et al. 2004; Mendelson et al. 2006; Fisher et al. 2012) and has led to the adoption of global policies to reduce its spread (Mendelson et al. 2006; Schloegel et al. 2010). Applying these lessons to the current state of honey bee health suggests that filling research gaps, particularly in management impacts and techniques, will complement additional studies on environmental factors such as nutrition, pesticides, pests, and pathogens to formulate comprehensive solutions to what may become an economically and environmentally costly phenomenon.
We acknowledge the beekeepers who took our survey as well as Jordan Levinson and Chia-Yi Hou of EcoHealth Alliance for assistance. Research was supported by a National Science Foundation Human and Social Dynamics “Agents of Change” award (SES-HSD-AOC “Human-related factors affecting emerging infectious diseases,” BCS—0826779), and by funding from the MARS Foundation, the Margaret A. Cargill Foundation, and Bayer Crop Science. Funders had no role in the data collection, analysis, nor in the preparation of this manuscript.