The Mysterious Case of the Dying Bees

Bees play an essential role in pollinating a large variety of plants, including many edible plants that humans rely on. According to an estimate by the UN’s Food and Agriculture Organization (FAO), out of about a hundred plant species that comprise 90% of all food consumed by humans in the world, 71 species are pollinated by the honey bee (Apis mellifera). The rest of the pollination is performed by other insects, mainly wild bees.

Since 2006, beekeepers in the U.S. have reported an extremely unusual death rate of honey bee colonies: 30%-90% of all colonies did not survive the winter. Many colonies remained with only the queen, a few young worker bees, larvae and pupae, and a large amount of food – with neither an explanation for why the worker bees have not returned to the hive nor an accumulation of dead bees around the hive. Without the worker bees, the colony cannot function, eventually leading to the death of all the bees within it. This phenomenon, in which worker bees disappear from the colony for no apparent reason is called Colony Collapse Disorder (CCD).

Scientists have yet to identify a single direct cause for CCD. However, since the dead or weak colonies are often found in the same area, the cause may be contagious or involve regional exposure to a common risk factor. In recent years, there has been a consistent decrease in the CCD phenomenon, and nowadays it is rarely reported anymore. Nevertheless, loss of bee colonies is still a concern.

While CCD was observed in the U.S., Europe also experienced damage to bee colonies. However, the phenomenon in Europe seems different, with a much lower percentage of bee loss.

Up to 90% of bee colonies did not survive the winter during the CCD in the U.S. during the 1990s | Photograph: Shutterstock

Human impact

Finding many dead bees at the entrance to the hive is clear evidence for having something other than CCD occurring in the colony. Sometimes the cause of colony death can be identified, for instance starvation, disease, parasites, exposure to high amounts of pesticide, or problems with the queen. In other times, it is not possible to pinpoint the cause. The currently accepted hypothesis is that the increase in bee colony death stems from a combination of factors that weaken the colony, making it more vulnerable to common pathogens, such as viruses, the fungus Nosema ceranae, or the parasitic mite Varroa destructor, which, as its name suggests, is very destructive to the honey bees.

Naturally, every year, 10%-20% of all bee colonies do not survive winter, due to diseases or other factors. In worse years, this rate can increase further. Between April 2016 and April 2017, beekeepers in the U.S. reported that a third of the colonies had not survived throughout the year, mainly due to diseases or parasites. In the preceding year, colony loss even reached 40%.

One of the contributing factors is malnutrition due to a reduction in the number of flowers. In a world in which buildings, roads, sidewalks, or even vast lawns, take the place of flowering plants, bees have a difficult time getting the nutrition they require.

Another potential cause is the exposure to pesticides, which is directed against insects, fungi, and weeds. Two main suspects have been highlighted in this regard. The first is the herbicide glyphosate, which was since found safe for bees in the doses they encounter in real life. The second is not one substance, but rather a group of controversial pesticides called neonicotinoids. Some pesticides may actually be beneficial for bees, such as the insecticide Coumaphos that kills the Varroa destructor. A high concentration of this insecticide was found especially in healthy bee colonies.

The effect of neonicotinoid pesticides is controversial since the studies about them are inconclusive. On the one hand, some studies have found negative effects of neonicotinoids on bees, among them, causing navigation difficulties when returning to the hive. On the other hand, other studies have found no such effect.

Long-term exposure

Two recent publications in the journal Science examined the effect of neonicotinoid pesticides on bees under field conditions and over a long period of time.

The first study examined the effect of two types of neonicotinoids, clothianidin and thiamethoxam, which were used in rapeseed crops in Hungary, the UK, and Germany. In this case, the pesticide was used for coating the seeds, so it is uncertain how it managed to affect the bees, if at all. The results were very inconclusive: when the crop was in bloom, these substances had a negative effect on bees in Hungary and the UK, but a positive effect on the honey bees in Germany. It is possible that it was not the pesticide that affected the bees in the study, but different, untested, factors. The seemingly contradicting results between the different regions raise the possibility that there are other contributing factors, such as environmental factors.

One interesting finding from the study was the detection of imidacloprid, a pesticide that has been discontinued many years ago, in the pollen and nectar that were stored in the hives. This indicates that even years after the substance has not been in extensive use, it is still present and can affect the environment.

The second study examined the effect of using different types of neonicotinoids in corn crops in Canada on the adjacent honey beehives. First, the researchers tested the amount of pesticide the bees are actually exposed to, and discovered they were mainly exposed to clothianidin. Presence of imidacloprid was discovered in this study as well, even though it was not in use at the time. Another surprising finding was that the pollen samples that were tested positive for neonicotinoids did not originate from the seeds that were treated with this pesticide – but rather from wild flowers that were not treated at all. Namely, the pesticides found their way to other plants, which were not treated with them, and actually, they were the bees’ main source of pesticide exposure.

Next, the researchers exposed bees to artificial pollen. This pollen was supplemented with clothianidin in decreasing amounts throughout 12 weeks. Then the pollen was given to the bees at similar amounts to those they would collect in real life. They found that this substance lead to elevated rates of worker bee death, and a decrease in social behaviors that aim to prevent spread of diseases and pests in the colony. In addition, they found a correlation between pesticides and a lower chance of finding a functioning queen that lays eggs in the hive, which is essential for maintaining the hive over a long period of time.  

This study also shows the effect of a combination with other pesticides: the lethal potential of neonicotinoids is doubled when bees are also exposed to agriculturally used fungicides.

Do pesticides harm bees? The findings are inconclusive| Photograph: Shutterstock

Complex effects

Whether neonicotinoids are indeed hazardous to bees in general, and particularly to honey bees, is still under debate. Some scientists promote minimization of the use of these substances, while others cautiously state that the results are inconclusive. Robert Paxton, zoology professor at Martin Luther King University in Germany, told sciencemediacentre.org that “These two new studies are pointers to the environmental problems that real-world use of neonicotinoid pesticides can cause. They are both well-replicated studies that use reasonable numbers of colonies across many farms, thus mimicking the long-term effects of chronic (low-dose long-term) and well as potential acute (high-dose short-term) effects of neonicotinoids to bees in the field. Some may argue that these are sufficient to warrant the outright ban of neonicotinoids.”

But there are those who object to these conclusions, some of whom have a personal interest in the matter. Dr. Peter Campbell, Senior Environmental Risk Assessor at Syngenta, which manufactures and sells products containing the neonic thiamethoxam, was also interviewed on sciencemediacentre.org .He mentioned the positive effect of the substance on the bee colonies in Germany, and thus claimed that the observed effects may not be statistically significant.

There is no doubt that there is still a long way before the exact effect of different substances humans spread in the environment on the bee population will be understood. Moreover, this effect is a decidedly complex one, since the environment contains more than just one pesticide, on top of the other substances. Climate change or the treatment given by beekeepers may also have an effect, so it will be difficult to isolate a single factor and quantify its individual effect. In the meantime, we can only hope that we will not pollute the environment with too many toxic substances, and that we will be able to help the bees to continue to exist, since we also depend heavily on them.