One of my earliest memories is of the chaos caused by a kamikaze bee that flew through an open window in my family’s pickup truck during an otherwise pleasant country drive. It saddens me to imagine a future where children are deprived of the bee-related traumatic experiences that I had growing up, but lately the world’s bee populations have been mysteriously dying. Bees contribute heavily to the agricultural endeavors of humans, so this unexplained bee population decline is problematic. The good news is that we’re starting to understand more about why they’re dying.

Collapsing colonies

Imagine a world without coffee. Bees aid in the reproduction of coffee plants, as well as many other crop plants (Figure 1) [1]. We’re going to have a hard time keeping coffee and many other foods available if all of the bees disappear.

 

Figure 1 ~ Bees are partly responsible for many delicious things. Shown here are bees pollinating the indicated crop plants. Pollination is essential for the fertilization of many crops, as well as wild flowering plants. (Image source: Wikimedia Commons)

The mutually beneficial relationship between bees and some flowering plants is a classic example of co-evolution, where the evolution of one species is affected by that of another. Bees evolved to take on the role of pollen-movers, transporting pollen from flower to flower, facilitating fertilization [2]. Plants evolved to encourage this behavior in bees by producing attractive flowers and snacks like nectar (or caffeine—coffee flowers actually spike their nectar with caffeine, giving bees a little buzz that encourages return trips [3,4]). If you’re looking for a fun way to kill time, you can of course pollinate flowering plants yourself by hand with a paintbrush or cotton swab [5]. However, it would be tough to pollinate an entire coffee orchard using this method, which is why bees are so useful.

In 2006 people around the world started noticing abandoned bee hives [6], which alarmed the agriculture industry—in the US alone, bee-pollinated crops were estimated to be worth $14.6 billion in 2000 [7]. The term “Colony Collapse Disorder” was invented to describe what was happening, and there were a lot of theories put forward to explain it, including radiation from cell phones, genetically modified organisms, pesticides, and climate change [6,7]. These days, however, researchers think that one central cause of the declining bee population is viral infection.

Deformed wings and parasitic mites

Bee viruses are nothing new. Since the 1960’s we’ve known of ~20 viruses that are normally found in bees but that generally don’t trigger dramatic problems like colony collapse [8]. What seems to be happening at the moment is something of a perfect storm: a normally benign virus is teaming up with a hardy parasitic mite and wreaking havoc on bee populations. 

Figure 2 ~ This mite be a problem. A Varroa destructor mite sits on the abdomen of a young bee. If the mite has a high viral load, it may infect this bee with enough Deformed Wing Virus to lead to the symptoms shown in Figure 3. (Image source: Wikimedia Commons)

Under normal circumstances, bees infected with a virus called Deformed Wing Virus (DWV) often lack symptoms and get along just fine. But the presence of a parasitic mite intimidatingly called Varroa destructor (Figure 2) somehow exacerbates symptoms caused by DWV infection. Bee pupae parasitized by DWV-infected mites grow up to become bloated, discolored adults with deformed wings and a much reduced lifespan [8] (Figure 3). V. destructor might also help spread DWV as it colonizes new regions.

Figure 3 ~ Learning to fly, but he ain’t got wings. In contrast to the healthy bee on the left, the bee on the right exhibits symptoms typical of bees infected with high levels of Deformed Wing Virus. Its deformed wings don’t support flight, and it will die in two to three days. (Image source: Wikimedia Commons)

In a particularly fascinating and disturbing study, researchers followed Hawaiian bee populations after V. destructor was first noticed in Hawaii in 2007 [9]. By 2010, the mite was found on two of the four main islands, and its presence was correlated with a decline in both domesticated and wild bee populations. In mite-free regions, domesticated bee colonies appeared to be in good health, and tests revealed the presence of DWV in only 6% to 13% of these colonies. In contrast, in areas infested with V. destructor, 75% to 100% of domesticated bee colonies were infected with DWV, and these bees contained one million times more virus copies than bees from mite-free regions.

How does V. destructor exacerbate DWV infection? One attractive hypothesis is that V. destructor mites somehow suppress the immune systems of their bee pupae hosts. However, this hypothesis has been called into question by studies that failed to detect suppression of the immune system in V. destructor-parasitized bees [8,10]. Another idea is that V. destructor simply delivers very concentrated doses of DWV to its host—bees parasitized by mites that contained high levels of virus had DWV symptoms, whereas those parasitized by mites with low virus levels were fine [11]. It’s still not clear how V. destructor exacerbates DWV infection, but it is clear that it does.

Keeping an eye on beekeeping 

Both domesticated and wild bee populations are experiencing colony collapse, and it has been speculated that domesticated bees have contributed to the decline of wild populations by spreading disease. Domesticated honeybees are traded and shipped worldwide as crop pollinators, and they are also a primary host of DWV [8]. New research from the UK provided hard data suggesting that DWV is indeed spreading to wild bumblebee populations via domesticated honeybees, even in the absence of V. destructor [12,13]. Researchers demonstrated that strains of DWV from domesticated honeybees can infect wild bumblebees, that wild populations are more likely to be infected with DWV when they are neighbors with domesticated bees, and that wild and domesticated neighbors tend to be infected by the same DWV strains.

Are the bees doomed? Are we soon to follow after the coffee plants die? You can take a little comfort in the fact that multiple fluctuations in bee populations have been recorded since 950 AD [7]. However, the recent declines have been comparatively dramatic, possibly facilitated by the ease with which disease can spread in our increasingly globalized world. It seems wise to invest in research aimed at finding better ways to control V. destructor populations, treating or preventing viral infections in bees, and understanding more about the causes of colony collapse [8].

Katherine Rogers is a PhD student in the department of Molecular and Cellular Biology at Harvard University.

References:

1. List of crop plants pollinated by bees (Wikipedia) http://en.wikipedia.org/wiki/List_of_crop_plants_pollinated_by_bees

2. Unraveling the Pollinating Secrets of a Bee’s Buzz (New York Times) http://www.nytimes.com/2013/07/11/science/unraveling-the-pollinating-secrets-of-a-bees-buzz.html?_r=0

3. Wright GA, Baker DD, Palmer MJ, Stabler D, Mustard JA, et al. (2013) Caffeine in Floral Nectar Enhances a Pollinator’s Memory of Reward. Science 339: 1202–1204. doi:10.1126/science.1228806.

4. Bees Buzzing on Caffeine (National Geographic) http://news.nationalgeographic.com/news/2013/03/130308-bees-caffeine-animal-behavior-science/

5. Got pollination problems? Give hand pollination a try! (Bonnie Plants) http://bonnieplants.com/2012/07/pollination-problems-give-hand-pollination-a-try/

6. Ratnieks FLW, Carreck NL (2010) Clarity on Honey Bee Collapse? Science 327: 152–153. doi:10.1126/science.1185563.

7. Oldroyd BP (2007) What’s killing American honey bees? Plos Biol 5: e168. doi:10.1371/journal.pbio.0050168.

8. Elke Genersch MA (2010) Emerging and re-emerging viruses of the honey bee (Apis mellifera L.): Vet. Res. doi:10.1054/vetres/2010027.

9. Martin SJ, Highfield AC, Brettell L, Villalobos EM, Budge GE, et al. (2012) Global Honey Bee Viral Landscape Altered by a Parasitic Mite. Science 336: 1304–1306. doi:10.1126/science.1220941.

10. Zhang Y, Liu X, Zhang W, Han R (2010) Differential gene expression of the honey bees Apis mellifera and A. cerana induced by Varroa destructor infection. Journal of Insect Physiology 56: 1207–1218. doi:10.1016/j.jinsphys.2010.03.019.

11. Gisder S, Aumeier P, Genersch E (2009) Deformed wing virus: replication and viral load in mites (Varroa destructor). Journal of General Virology 90: 463–467. doi:10.1099/vir.0.005579-0.

12. Fürst MA, McMahon DP, Osborne JL, Paxton RJ, Brown MJF (2014) Disease associations between honeybees and bumblebees as a threat to wild pollinators. Nature 506: 364–366. doi:10.1038/nature12977.

13. Managed honeybees linked to new diseases in wild bees (Biotechnology and Biological Sciences Research Council) http://www.bbsrc.ac.uk/news/food-security/2014/140219-pr-managed-honeybees-new-diseases.aspx