Research from the United States, which looked at the effects of mixtures of agricultural chemicals on honeybees, has linked the presence of numerous chemicals with bee deaths in colonies, including some that researchers previously thought were safe.

Bee species - including domesticated honeybees - are dying off across the northern hemisphere, and though the pressures behind the losses are multifaceted, with scientists suggesting habitat loss, climate change and disease are playing parts in the decline of different bee species, the use of certain agricultural chemicals has also been implicated.

Research methods used to assess the impacts of farm chemistry on bees have been subject to intense debate, with environmentalists and farm interests clashing over the validity of studies carried out in labs, or ones in which bees live in outdoor colonies, but are fed by researchers, and even some which try to replicate true field conditions (the UK government’s widely panned 2013 study which used colonies in fields was criticised as the trials were contaminated with neonicotinoid insecticides that weren’t being studied).

For their study, researchers from the University of Maryland and the US Department of Agriculture examined the effects of multiple real-world pesticide exposures within a colony. Most studies, and even official risk assessments, look at products’ impacts on bees in isolation, and the researchers said theirs is the first to systematically assess multiple pesticides that accumulate within bee colonies.  

They studied 91 colonies, owned by three commercial beekeepers, over the course of a full agricultural season. The bees were used as commercial pollinators, being transported from farm to farm, and began their journey in Florida, moving up the coast with some layover periods and time aside for honey production. A total of 93 different pesticide compounds found their way into the colonies over the course of the season, accumulating in the wax, in processed pollen known as bee bread and in the bodies of nurse bees.

At every stop along the beekeepers' itinerary, researchers assessed three different parameters within each colony: the total number of pesticides; the total number of "relevant" pesticides (defined as those above a minimum threshold of toxicity); and each colony's "hazard quotient” - a measure devised by other scientists work out the total hazard posed to each colony based on the cumulative toxicity of all pesticides detected.

The researchers found that the number of different pesticides within a colony - regardless of dose - closely correlated with colony death, and findings suggest that some fungicides, often regarded as safe for bees, correlate with high rates of colony deaths. All three areas of assessment correlated with a higher probability of colony death or queen failure. The highest number of pesticides accumulated in the colonies early on, shortly after beekeepers placed colonies into early season flowering crops like apples and blueberries, though honey production stopovers and holding areas (between commercial pollination work) offered the bees some respite from further contamination.

University of Maryland entomology professor Dennis vanEngelsdorp, who was a senior author o the paper, commented, ”Our results fly in the face of one of the basic tenets of toxicology: that the dose makes the poison. We found that the number of different compounds was highly predictive of colony death, which suggests that the addition of more compounds somehow overwhelms the bees' ability to detoxify themselves.”

Fungicides implicated in colony death

The results also suggest that some fungicides, which have led to the mortality of honey bee larvae in lab studies, could have toxic effects on colony survival in the field. The fungicides most closely linked to queen deaths and colony mortality disrupted sterols - compounds that are essential for fungal development and survival.

Dr Kirsten Trainer, also from the University of Maryland, said, "We were surprised to find such an abundance of fungicides inside the hives, but it was even more surprising to find that fungicides are linked to imminent colony mortality. These compounds have long been thought to be safe for bees. We're seeing them at higher doses than the chemicals beekeepers apply directly to the colonies to control varroa mites. So that is particularly concerning."

The current study borrowed a concept from human cancer research: the "exposome," or the sum total of chemicals an organism is exposed to over its lifetime. But instead of looking at individual bees, the researchers assessed each colony as a single "superorganism" that functions as a single unit for study.

Within this framework, the researchers tracked the death of queen bees, which is a life-threatening event for the colony as a whole. In some cases, a colony is able to create a new queen, but if those efforts fail the entire colony will die. In the Maryland study, colonies with very low pesticide contamination in their wax experienced no queen events, while all colonies with high pesticide contamination in the wax lost a queen during the beekeeping season. Dr Traynor commented, "This is a huge problem for beekeepers currently. Not long ago, a queen would typically last up to two years. But now many commercial beekeepers replace the queens in at least half of their colonies every spring in the hopes that this will prevent premature queen deaths. Even with such measures, many queens still don't make it through one season."

However, despite their concerning findings, the team did not find any significant contribution from neonicotinoids, extremely commonly used systematic insecticides, three of which have been heavily restricted in the EU over evidence of their effects on bee health. Prof vanEngelsdorp elaborated, “We just did not find neonicotinoids in the colonies. There were some trace residues of neonicotinoids in a few samples, but not nearly on par with other compounds. However, it's possible we did not test the right matrix - we did not test nectar, for example - or that the product breaks down faster than others in the collection process or that neonicotinoids are simply not very prevalent when crops are flowering."

The researchers said industrial practices have changed since they collected the data for the study, and so recommend further research be carried out to reflect this. They said their research nonetheless offers some important insights for beekeepers and farmers alike.

Prof vanEngelsdorp said, ”We have to figure out ways to reduce the amount of products that bees are exposed to while still helping farmers produce their crops. This will require careful examination of spray plans, to make sure we only use the products we need, when we need them, in order to reduce the number of products bees are exposed to while pollinating different crops."