Expert Commentary

What’s behind bee declines and colony collapse? Latest science on stress from parasites, pesticides, habitat loss

2015 research review in Science examining a broad range of factors that negatively impact bee health, including declining biodiversity, the role of agrochemicals and climate change.

Estimates vary for the economic value that commercial and wild bees provide to the U.S. economy through crop pollination, but they all boil down to “a lot.” A 2014 fact sheet from the White House cites annual figures of $15 billion for commercial bees and $9 billion for their wild cousins. According to a 2014 report from the University of California, Davis, almonds alone add as much as $11 billion a year to the economy of California, the source of 80% of the world’s crop — and 90% of it is pollinated by honeybees trucked from field to field.

But as much as we depend on bees and other pollinators, they’re in trouble: Over the past 60 years the number of commercial beehives in the United States fell by nearly 60%, from 6 to 2.5 million. Historically, 10% to 15% of hives die each winter, but beginning in 2006, beekeepers were reporting loss rates of 30% to 90% — not just expensive, but unsustainable. While there had been reports of similar events as far back as the 1800s, the scale and scope of the phenomenon were unprecedented. Because the cause was unclear, it was given a descriptive name: colony collapse disorder (CCD).

As recently as 2012 CCD was described as a “mystery malady” — no clear villain, just empty hives and panicked farmers. Potential factors were everywhere: decreasing genetic diversity of wild and managed bees as well as the crops and plants they pollinate; the increase in parasitic varroa mites; a possible one-two punch from a virus-fungus combination; and a correlation between fungicides and bee infections. (Accusations were even leveled at cell phones based on the misinterpretation of a small study in Germany.)

Attention has also focused on the industrial substances so omnipresent in conventional agriculture in the United States — herbicides, pesticides and chemical treatments of all sorts. In the late 1990s “systemic” insecticides were developed, including neonicotinoids such as Imidacloprid and Clothianidin. While they were judged safe by the EPA at the time of release, some of the initial research was funded by the companies themselves. Based on more robust studies, the European Union temporarily banned neonicotinoids in January 2013, and subsequent research from the Harvard School of Public Health found that they can have “sublethal” effects on bees. And yet new factors continue to emerge: In January 2015, the tobacco ringspot virus was found to have made the leap from plants to bees and even the varroa mites that afflict them.

Angles for journalists covering these and related issues can include the crop and pollination practices of local farmers, the challenges of sustainably managing crop pests, urban beekeeping, and the status of local varieties of wild bees and conservation efforts. But before reporting, journalists should have a solid understanding of the latest science, which is complex and nuanced.

A 2015 research review published in Science, “Bee Declines Driven by Combined Stress from Parasites, Pesticides and Lack of Flowers,” examines a broad range of risk factors for bees, including declining plant and insect diversity, the role of agrochemicals and climate change. The authors — Dave Goulson, Elizabeth Nicholls, Cristina Botías and Ellen L. Rotheray of the University of Sussex — review 170 studies in all, giving a comprehensive overview of the current state of knowledge on honeybee colony collapse disorder, its larger causes and potential impacts, and steps that can be taken to protect managed and wild insect pollinators.


  • Globally, 75% of crops benefit from insect pollination, a service worth an estimated $215 billion.
  • Leading up to the early years of colony collapse disorder, losses of managed bee colonies were substantial: 25% in central Europe between 1985 and 2005 and 59% in the United States between 1947 and 2005. (For the 2013-2014 season, a survey by the USDA found that 23.2% of U.S. hives were lost that winter.)
  • Individual stressors to bees include industrial chemicals (herbicides, pesticides, fungicides and more), parasites, pathogens, habitat loss, crop monocultures, declining biodiversity, and other factors. All negatively impact bee health, and together the effects can be fatal. “It seems certain that chronic exposure to multiple interacting stressors is driving honeybee colony losses and declines of wild pollinators, but the precise combination apparently differs from place to place.”

Interaction of bee stressors (Science, 2015)

  • Despite losses, the global number of managed hives increased approximately 45% between 1961 and 2008. However, this was primarily due to significant growth in the number of commercial hives in China, Argentina and other countries.
  • Over the last 50 years the demand for insect pollination services has approximately tripled, exceeding any increase in the total number of hives. While this provides sufficient economic incentive for beekeepers to address problems at a global scale, that is not the case for North America and Europe, which continue to suffer substantial losses.
  • While data is limited, the number and range of wild bees appear to have declined substantially. Wild bees are hurt by the same factors as managed bees, and can also suffer from the introduction of non-native bees and competition from managed bees. A related 2015 study in Science found that wild bees are particularly vulnerable to neonicotinoids, reducing their density, colony growth and reproduction.
  • The majority of crop pollination at a global scale appears to be delivered by wild pollinators rather than honeybees. A 2011 study in Agriculture, Ecosystems & Environment found that honeybees provided just 34% of pollination services in the United Kingdom, even as yields had risen 54% since 1984, “casting doubt on long-held beliefs that honeybees provide the majority of pollination services.”

Factors that negatively impact managed and wild bee populations:

  • Pesticides play a significant and well-documented role in the decline of managed and wild bee populations: “Of the 161 different pesticides that have been detected in honeybee colonies, Sanchez-Bayo and Goka (2014) predicted that three neonicotinoids (thiamethoxam, imidacloprid, and clothianidin) and two organophosphates (phosmet and chlorpyrifos) pose the biggest risk to honeybees at a global scale.”
  • “Long-term chronic exposure results in mortality in overwintering honeybees when feeding on food contaminated with concentrations as low as 0.25 [parts per billion]. Sublethal effects of neonicotinoid exposure have also been observed in both honeybees and bumblebees, including reductions in learning, foraging ability, and homing ability, all of which are essential to bee survival.”
  • Managed colonies are typically exposed to dozens of chemicals, and their combination can have unanticipated consequences. For example, “ergosterol biosynthesis inhibitor (EBI) fungicides, have very low toxicity in themselves but may increase the toxicity of some neonicotinoids and pyrethroids by as much as a factor of 1000.”
  • Interactions between chemicals and pathogens can be particularly harmful: “Developmental exposure to neonicotinoid insecticides renders honeybees more susceptible to the impact of the invasive pathogen N. ceranae…. Similarly, Aufauvre et al. (2012) showed that mortality of honeybees was greater when bees were exposed to the insecticide fipronil and infected by N. ceranae than when only a single stress factor was present.”
  • While herbicides have significant economic benefits, “their use inevitably reduces the availability of flowers for pollinators and can contribute substantially to rendering farmland an inhospitable environment for bees.” Pollen from different plants has very different properties, and limited diets reduce bees’ “longevity, physiology, and resistance or tolerance to disease.”
  • Ongoing climate change raises the possibility that pollinator and crop ranges will diverge in space and time. There have been some observations of this already, such as the shifting of some mountain bumblebees in Spain uphill, away from the crops they traditionally pollinate, but the evidence is limited.

“Bees of all species are likely to encounter multiple stressors during their lives, and each is likely to reduce the ability of bees to cope with the others,” the researchers conclude. “A bee or bee colony that appears to have succumbed to a pathogen may not have died if it had not also been exposed to a sublethal dose of a pesticide and/or been subject to food stress (which might in turn be due to drought or heavy rain induced by climate change, or competition from a high density of honeybee hives placed nearby).” While verifying the precise interactions is difficult, “a strong argument can be made that it is the interaction among parasites, pesticides and diet that lies at the heart of current bee health problems.”

Solutions and best practices

The study indicates that many factors have contributed and continue to contribute to losses of managed and wild bees. Because there is no single key factor, taking steps to reduce the severity and number of stressors on bees is crucial. These include:

  • Making a wider diversity of flowers available for bees. “Schemes such as the sowing of flower-rich field margins or hedgerows, or retaining patches of semi-natural habitat among or near farmland, provide clear benefits to bee diversity and abundance.”
  • Reducing the use of pesticides and other industrial chemicals. Current levels of use are not only harmful to bees, they’re “not always justified by evidence that they are necessary to maintain yield.” In particular, “a return to the principles of integrated pest management (IPM), which depends on preventive methods such as crop rotation and views the use of pesticides as a last resort in the battle against insect pests, could greatly reduce exposure of bees, benefit the environment, and improve farming profitability.”
  • Prevent the introduction of non-native bees, parasites and pathogens. The widespread practice of trucking bees long distances to pollinate crops exposes bees to pathogens and acts to spread them to uninfected populations. “Strict quarantine controls should be implemented on the movement of all commercial bees, and there is an urgent need to develop means of rearing commercial bumblebees that are free from disease.”
  • Increase monitoring of wild bee populations. This would serve as an “early-warning system” against the arrival of a pollination crisis. “With a growing human population and rapid growth in global demand for pollination services, we cannot afford to see crop yields begin to fall, and we would be well advised to take preemptive action to ensure that we have adequate pollination services into the future.”

Because crop yields are more highly correlated with an abundance of wild pollinators than managed honeybees, maintaining the diversity of wild bee populations is particularly important. “Increasing honeybee numbers alone is unlikely to provide a complete solution to the increasing demand for pollination,” the researchers note.

Related: The White House is also leading a cross-agency effort to improve the response to promote honeybee health.

Keywords: colony collapse disorder, Bayer CropScience, Monsanto, pollution, biodiversity, insects, bee hives, apiary, bee keeper

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