Though the issue of the decline of the Western honey bee, Apis mellifera, has been previously discussed in the context of finding replacements for the bee as a pollinator and in terms of colony collapse disorder, causes behind the decline have not yet been discussed in detail. In the work since colony collapse disorder first appeared on the metaphorical radar screen of entomologists and other interested researchers, multiple theories have been posited as to why the bees are suffering these declines. Spires (2013) pointed out more than once that multiple sources reference beekeepers’ accounts of losses in the years since 2006 as being in the thirty to thirty-three percent range, and that about a third of those losses are assumed to be from colony collapse disorder. Whereas the extrapolated ten to eleven percent losses from colony collapse disorder may not seem dramatic on the face of the matter, in truth, that is all it could take to make the difference between maintenance of the status quo versus dramatically increased food prices due to lack of pollinators. Spires also summarizes the suggested list of possible causes for colony collapse disorder and the subsequent population decline of Apis mellifera multiple times, describing the primary factors as pests, disease, toxins or pesticides, bee management stress due to increased relocation of hives for agricultural purposes, foraging issues, and the combination of any of the above factors in ways that lead to increased immune stress on the bees. This is a useful framework to follow when bringing in the other relevant research on the matter, and so this list of possible causes for the decline of the Western honey bee will be used here. However, not all factors can be treated equally, as some have been considerably debunked by now. This narrowing of causes itself has indeed proven useful thus far to scientists in the field of honey bee research.
The varroa mite has been posited as a possible explanation for colony collapse disorder, given that it plays a role in carrying certain diseases and also, of course, puts a load on the bees’ immune systems simply by its presence as a pest. The role of this mite in carrying deformed wing virus (DWV) has been known for a while, and descriptions of this pattern appeared significantly before the 2006 onset of the colony collapse disorder phenomenon. For example: “ . . . Varroa jacobsoni were shown to be highly effective vectors of [DWV] between bees. Adult female mites obtained from honeybee pupae naturally infected with DWV contained virus titers many times in excess of those found in their hosts . . . ” (Bowen-Walker, Martin, & Gunn, 1999, p. 101). This demonstrates one of the ways in which the mite might spread the virus throughout an entire colony simply based on one host, but what is less clear is how DWV relates to colony collapse disorder. Cox-Foster et al. (2007) investigated this connection in greater detail, looking at the mites along with several other factors, explaining, “Candidate pathogens were screened for significance of association with [colony collapse disorder] by the examination of samples collected from several sites over a period of 3 years. One organism, Israeli acute paralysis virus of bees, was strongly correlated . . . ” (p. 283). Notably absent from the list of correlated organisms was the varroa mite and its associated virus. Indeed, from this study, it looks as though disease might be a better factor in which to seek the true source of the decline of the Western honey bee.
The unfortunate trouble with pinning colony collapse disorder on any one disease seems to be that there are multiple candidates for the role, yet none of these shows a strong enough association to figuratively put the smoking gun in its hand. Boncristiani, Evans, Chen, Pettis, and Murphy (2013) described this problem: “The ongoing decline of honey bee health worldwide is a serious economic and ecological concern. One major contributor to the decline are pathogens, including several honey bee viruses. However, information is limited on the biology of bee viruses . . . ” (p. 1). This limited information problem plagues the study of this area. For example, with the aforementioned Israeli acute paralysis virus, only variable patterns have been shown. Boncristiani et al. (2013) explained, “An experimental protocol to test these systems was developed, using injections of Israeli Acute Paralysis Virus (IAPV) into honey bee pupae reared ex-situ under laboratory conditions. The infected pupae developed pronounced but variable patterns of disease” (p. 1). This result is hardly conclusive. However, greater possibilities have very recently come to light.
Another possible disease under study has greater importance for the debate over the cause of colony collapse disorder. The work of Li et al. (2014) found that, “ . . . a plant-pathogenic RNA virus, tobacco ringspot virus (TRSV), could replicate and produce virions in honeybees, Apis mellifera, resulting in infections that were found throughout the entire body. Additionally, we showed that TRSV-infected individuals were continually present in some monitored colonies” (para. 1). From this information and subsequent analogies of TRSV as a sort of honey bee HIV, it does seem that preliminary results may be zeroing in on the cause of colony collapse disorder and the resultant decline in the population of Apis mellifera. However, it is still too soon to confirm whether a mutated pollen-borne plant virus-like TRSV could really cause the bizarre phenomenon of colony collapse disorder. By contrast, an investigation into possible environmental toxins has been fairly conclusive.
Toxins from environmental pesticides and diesel fuel exhaust are now widely agreed not to be the cause of the population decline of the Western honey bee. This avenue was worth investigating in part because the bees are fairly unique in their responses to such toxins. For example, though Hyne and Maher (2003) primarily focused on toxicity in aquatic invertebrates, they also pointed out that, “The honey bee, Apis mellifera, is the only terrestrial insect in which inhibition of head acetylcholinesterase activity has been correlated with a biological change” (p. 270). This unusual property of Apis mellifera might have helped justify why no other insects seem affected by population declines dating to 2006. Ultimately, however, the evidence simply was not there, as discussed by Spires (2013), Markle (2013), and Cowles (2010). The fact that so many broad summaries of the topic identify that the literature supports rejecting pesticide toxins as a factor is a strong indication that this cause can safely be left well alone. At this point, only the more minor possible causes remain to be addressed here.
The other possible factors involved in the decline of the population of Apis mellifera can be dealt with in brief, as they are all considered relatively less likely candidates or else do not have a great deal of information on their roles as of this point. There have been greater stressors to the bees than the relocation of their hives due to increased agricultural demand on beekeepers, as Woteki (2013) pointed out. That means this factor is not as consequential as some others. Naug (2009) did give some credit to the notion that habitat losses may be contributing to poor nutrition in Apis mellifera. However, this result does not explain why other species of bees that have similar foraging habits have not been affected, nor why the onset of colony collapse disorder was so sudden as to be associated with the particular year 2006. As for the interactions of combinations of factors, Neumann and Carreck (2010) explained that these are hard to investigate, though there does seem to be evidence in favor of the connection of multiple factors as the cause of the disorder. Yet these associations are notoriously difficult to examine with true scientific and statistical rigor. More time will probably be needed to truly uncover the nature of multi-factor causes of the population decline.
Ultimately, there remains a great deal to be done in investigating the decline of Apis mellifera and the role of the many suggested possible factors. The possibility that combinations of factors are responsible or that the tobacco ringspot virus has mutated and begun affecting bees seem to be the most likely routes to success in solving the mystery of colony collapse disorder. It is fortunate that probable causes are being found, for only with the reasons behind the decline identified can research into solving the problem truly begin. If this happens soon, it could stave off agricultural troubles caused by a lack of pollinators. Due to the importance of honey bees to humans, it is vital to halt their decline as soon as possible.
Boncristiani, H. F., Evans, J. D., Chen, Y., Pettis, J., & Murphy, C. (2013). In vitro infection of pupae with Israeli acute paralysis virus suggests disturbance of transcriptional homeostasis in honey bees (Apis mellifera) [White paper]. Greenshore, NC: University of North Carolina at Greenshore.
Bowen-Walker, P. L., Martin, S. J., & Gunn, A. (1999). The transmission of deformed wing virus between honeybees (Apis mellifera L.) by the ectoparasitic mite Varroa jacobsoni Oud. Journal of Invertebrate Pathology, 73(1), 101-106.
Cowles, R. S. (2010). The facts about systemic insecticides and their impact on the environment and bee pollinators in Palm Beach, Broward and Martin County. Clippings, 2, 10-13.
Cox-Foster, D. L., Conlan, S., Holmes, E. C., Palacios, G., Evans, J. D., Moran, N. A., ... & Lipkin, W. I. (2007). A metagenomic survey of microbes in honey bee colony collapse disorder. Science, 318(5848), 283-287.
Hyne, R. V., & Maher, W. A. (2003). Invertebrate biomarkers: Links to toxicosis that predict population decline. Ecotoxicology and Environmental Safety, 54(3), 366-374.
Li, J. L., Cornman, R. S., Evans, J. D., Pettis, J. S., Zhao, Y., Murphy, C., . . . Chen, Y. P. (2014). Systemic spread and propagation of a plant-pathogenic virus in European honeybees, Apis mellifera. mBio, 5(1), e00898-13. doi:10.1128/mBio.00898-13.
Markle, S. (2013). The case of the vanishing honeybees: A scientific mystery. Minneapolis, MN: Millbrook Press.
Naug, D. (2009). Nutritional stress due to habitat loss may explain recent honeybee colony collapses. Biological Conservation, 142(10), 2369-2372.
Neumann, P., & Carreck, N. L. (2010). Honey bee colony losses. Journal of Apicultural Research, 49(1), 1-6.
Spires, D. N. (2013). Honey bees and colony collapse disorder (CCD) [Smashwords version]. Retrieved from iTunes.com
Woteki, C. (2013). The road to pollinator health. Science, 341(6147), 695-695.