Reader Request: Where have all the honeybees gone?

Spring is in the air, and so too are the bees. But spring is also a reminder that the US and Europe are still experiencing a mysterious phenomenon where more bee hives than usual are not surviving the winter. This topic was a reader request and as I am infinitely grateful to my blog readers, I tried to get on it as soon as possible and was excited to see that there have been new developments in the last months.

Starting in 2006, beekeepers noticed a drastic increase in the failure of bee colonies. The phenomenon came to be known as colony collapse disorder (CCD) where adult bees were simply disappearing, leaving behind hives with a queen and often enough, honey stores.

Honey aside, honeybees play an incredibly large role in pollinating commercial crops, including almonds and many fruit, worth $15 billion in the US (1). At its peak in the 2007-2008 season, 35% of colonies collapsed. Though CCD has been on the decline in recent years, with 23% of colonies collapsing last year, scientists and farmers alike are still concerned in what causes CCD and how to prevent it.

Leading theories include neonicotinoid pesticide use, the parasitic mite Varroa destructor, the parasitic fungi Nosema, and decreased nutritional diversity and availability.

Nosema and parasitic mites were both implicated early on in the investigation, but there are cases of CCD without any infection and conversely, healthy colonies that are infected.

Pesticide use has long been a suspect. Last year a study from Harvard found that clothianidin and imidacloprid led to loss of six of 12 colonies (2). However many in the science blogosphere – as well as the company that makes imidacloprid – have criticized the amount of pesticide tested, the sample size, and the statistical analysis.

Several other studies have found that bees exposed to these pesticides were more susceptible to Nosema infection due to immune suppression (3,4). Neonicotinoid exposure also impaired olfactory learning and memory (5). Amid mounting studies, the European Union banned neonicotinoids in 2013. The US has been slower to change regulation but the Environmental Protection Agency (EPA) said earlier this month that it was unlikely to approve new neonicotinoid pesticide use as it continues to assess pesticide safety for bees.

At the same time, a study published in March by researchers at the EPA, the US Department of Agriculture (USDA), and University of Maryland found that only field doses at the extreme end had a significant effect on colony survival in the three-year study (6). They conclude pesticide use is “unlikely a sole cause of colony declines.”

Thus, despite intermittent news stories suggesting otherwise, no one factor has been able to account for CCD. The USDA, who is leading the federal response, says it is likely a combination of two or more of these factors.

Last month researchers in Australia published a study showing how stress from a variety of sources can rapidly lead to collapse in the colony. They attached tiny radio trackers to bees to examine their foraging behavior (side note: they literally glued them to the bee’s chest). They induced bees to start foraging at a younger age (precocious foraging) by creating colonies with younger demographics (7).

Younger forager bees were less successful at bringing back food and were more likely to die while trying to forage. The consequent decreased food supply led the remaining bees in the hive to start foraging at an even younger age.

Stresses such as starvation and disease are known to cause precocious foraging in bee populations. The researchers used their data to model bee population dynamics under chronic stress and showed that precocious foraging led to a positive feedback loop w

here progressively younger bees were even less successful, leading to a rapid decline in the colony.

While previous models have only been able to account for a slow decline, the researchers say they are the first to “display dynamics of colony population collapse that are similar to field reports.”

“The failure of a honey bee colony is a breakdown of a society…Understanding why and how colonies fail therefore requires more than analyzing how individual bees react to stressors,” said the researchers.

A review published in Science last Month agrees with this assessment saying that while chronic exposure to multiple stressors is driving CCD, “the precise combination apparently differs from place to place.”  (8).

They go on to say “Although the causes of pollinator decline may be complex and subject to disagreement, solutions need not be; taking steps to reduce or remove any of these stresses is likely to benefit pollinator health,” The authors call for growing more bee-friendly flowers and decreasing dietary stress as well as decreased use of pesticides.

References:

1. “Vanishing Bees.” National Defense Resources Council. <http://www.nrdc.org/wildlife/animals/bees.asp> Retrieved April 13^th, 2015.

2. Lu, C. Warchol, K.M., Callahan, R.A. (2014). Sub-lethal exposure to neonicotinoids impaired honey bees winterization before proceeding to colony collapse disorder. Bulletin of Insectology 67 (1): 125-130.

3. Pettis, Jeffery S., Johnson, J., Dively, G., et al. (2012). Pesticide exposure in honey bees results in increased levels of the gut pathogen Nosema. Naturwissenschaften 99 (2): 153–8.

4. Di Prisco, G., Cavaliere, V., Annoscia, D., et al. (2013). Neonicotinoid clothianidin adversely affects insect immunity and promotes replication of a viral pathogen in honey bees. PNAS 110(46):18466–18471, doi:10.1073/pnas.1314923110.

5. Williamson, S.M., Wright, G.A. (2013). "Exposure to multiple cholinergic pesticides impairs olfactory learning and memory in honeybees". J of Experimental Biology 216 (10): 1799–807.

6. Dively G.P., Embrey M.S., Kamel A., Hawthorne D.J., Pettis J.S. (2015) Assessment of Chronic Sublethal Effects of Imidacloprid on Honey Bee Colony Health. PLoS ONE 10(3): e0118748. doi:10.1371/journal.pone.0118748.

7. Perry, C., Sovik, E., Myerscough, M.R., Barron, A.B. 2015. Rapid behavioral maturation accelerates failure of stressed honey bee colonies. PNAS 112(11): 3427–3432, doi: 10.1073/pnas.1422089112.

8. Goulson D, Nicholls E, Botías C, Rotheray EL. (2015). Bee declines driven by combined stress from parasites, pesticides, and lack of flowers. Science. 2015 Mar 27;347(6229):1255957. doi: 10.1126/science.1255957.