The scientists behind new research findings on the impacts of certain pesticides on bumblebees' foraging ability have said risk assessments need to include effects on wild pollinators.

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On the same day that a groundbreaking study from researchers in the Netherlands suggested that neonicotinoid pesticides could be implicated in declines of insect-eating birds, an international study has strengthened evidence of the pesticides' impact on bees.

Neonicotinoids are the most widely used pesticide family worldwide (making up 30 percent of the global pesticide market), though growing evidence of their impact on the health of insect pollinators, which are in decline across the Northern Hemisphere, led the EU Commission to ban their use on flowering crops and at certain times of the year in December.

Embattled neonicotinoid manufacturers in Europe maintain that there is no clear evidence that their products could be contributing to pollinator decline, and maintain that disease and habitat loss are the primary drivers of falling bee numbers.

However, a study by researchers from Universities in Canada and the UK and published in the British Ecological Society's journal Functional Ecology has this week added further evidence to the case against neonicotinoids.

Scientists fitted bumblebees with tiny radio frequency tags to monitor their movements. They found that long-term exposure to the pesticides hampers bees' ability to forage for pollen.

The research is important, as most of the body of evidence on the effects of pesticides comes from research on honeybees; these creatures are commercially kept and their populations maintained. Research into the effects of the chemicals on wild pollinators – and their potential impact on populations – is therefore of great importance.  

The research by Nigel Raine, a professor at Guelph University's School of Environmental Sciences, and Richard Gill of Imperial College London, details the effects of four weeks' exposure to the pesticides on individual bees' day-to-day behaviour, including pollen collection and which flowers worker bees chose to visit.

Though bees in the experiment were housed in a laboratory at Royal Holloway University, they had access to the outside world to forage. Bees were exposed either to neonicotinoid imidacloprid at concentrations expected to be found in the pollen and nectar of flowering crops visited by bees in the wild (10ppb), or pyrethroid cyhalothrin at the maximum label-guidance concentration for spray application to oilseed rape in the UK (37.5ppm).

"Bees have to learn many things about their environment, including how to collect pollen from flowers," said Raine. "Exposure to this neonicotinoid pesticide seems to prevent bees from being able to learn these essential skills."

Raine and Gill used radio frequency identification (RFID) tags similar to those used by courier firms to track parcels. They tracked when individual bees left and returned to the colony, how much pollen they collected and from where. Bees from untreated colonies got better at collecting pollen as they learned to forage. But bees exposed to neonicotinoid insecticides became less successful over time at collecting pollen.

Neonicotinoid-treated colonies even sent out more foragers to try to compensate for lack of pollen from individual bees, they said.

Besides collecting less pollen, said Raine, "The flower preferences of neonicotinoid-exposed bees were different to those of foraging bees from untreated colonies."

"Although pesticide exposure has been implicated as a possible cause for bee decline, until now we had limited understanding of the risk these chemicals pose, especially how it affects natural foraging behaviour," Raine addedd. "If pesticides are affecting the normal behaviour of individual bees, this could have serious knock-on consequences for the growth and survival of colonies."

The researchers suggested their findings show there is a need to reform current pesticide regulations, including adding bumblebees and solitary bees to risk assessments that currently cover only honeybees. "Bumblebees may be much more sensitive to pesticide impacts as their colonies contain a few hundred workers at most, compared to tens of thousands in a honeybee colony," Raine said.