Two large-scale studies published on Thursday suggest that common pesticides may harm bee colonies. In some cases, the pesticides contribute to the loss of not only worker bees but also queens.
The studies, in the journal Science, have been eagerly anticipated due to concerns about the impact of neonicotinoid pesticides on pollinators, but the results were not as clear-cut as experts had hoped.
The question is not whether neonicotinoids can be toxic to bees. These chemicals are marketed for their insect-killing power, and bees are, after all, insects. What has been harder to pin down is whether the exposure bees typically experience in the wild is harmful. Tests of neonicotinoids on bees in the lab show that the compounds are dangerous to them, but field studies have largely failed to implicate the pesticides.
To get the bottom of the controversy, two teams conducted large-scale field and lab studies designed to mimic natural conditions across four countries. Researchers in Europe placed bees in neonicotinoid-treated canola fields and monitored their health. Researchers in Canada monitored bees in apiaries within 550 yards of treated corn fields. They sampled pollen and honey for pesticides over five months, and attempted to replicate that exposure under controlled conditions. Both studies showed negative but different effects on bees, and both demonstrated that bees are picking up neonicotinoids from surprising sources.
In the European study, three types of bees were placed in 33 different canola fields across three countries. The differences between bees in treated or untreated fields were largely insignificant, and many of the bees in both groups died before they could be counted. This demonstrates how tough large-scale field studies are to conduct, said Richard Pywell, one of the study authors.
Of the differences that were statistically significant, results varied by country. In Hungary, honeybee egg production decreased and fewer worker bees survived the winter. In the United Kingdom, honeybee worker numbers decreased, and so did drones for a certain species of wild bee. In Germany, drone numbers for that same wild bee species and honeybee egg production actually increased.
Several important factors might help explain these regional differences. Nearly half the pollen picked up by bees in Hungary and the U.K. came from canola. In Germany, that number is closer to 10 percent. That means German bees had a wider range of flowers to choose from, possibly diluting their exposure to pesticides.
Disease rates were also lower in Germany. In the U.K., many of the bees fell prey to the dreaded Varroa destructor mite — a hairy, fanged parasite that many bee experts blame for the majority of losses that occur in commercial honeybee operations. In Hungary, a fungal infestation devastated the bees in the study. The fact that bees in the U.K. and Hungary, but not Germany, were adversely affected by neonicotinoid exposure suggests that the pesticides may exacerbate deadlier pressures bees face, such as parasites.
However, the small number of significant effects “makes it difficult to draw any reliable conclusions,” said Norman Carreck, science director of the International Bee Research Association.
Christopher Cutler, who studies insect toxicology at Dalhousie University, echoed Carreck’s concerns, pointing out that “when many different analyses are conducted” (42 in this case), “a small number of statistically significant effects are bound to emerge by chance.”
In another complication, when the researchers in Europe sampled the nests of bumble bees and solitary bees for pesticide residues, they found contamination with one type of neonicotinoid not used in the study.