Studies on two widely used neonicotinoid insecticides conducted by researchers at the Harvard School of Public Health suggest that the pesticides may be doing harm to honey bee colonies over the winter, particularly during colder winters.

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The new study replicated a 2012 finding from the same research group. The group had previously established a link between low doses of imidacloprid and Colony Collapse Disorder (CCD), in which bees abandon their hives over the winter and eventually die.

The causes behind Colony Collapse Disorder are unknown, but they are thought to be varied. Whilst pesticide manufacturers have claimed that habitat loss, disease and the effects of climate change are behind bee declines seen across the Northern Hemisphere, an increasing body of evidence suggests that profligate use of certain pesticides may also be playing a part in pollinator decline and CCD.

As well as looking at imidacloprid, the new study also found that low doses of a second neonicotinoid, clothianidin, had the same negative effect. Both chemicals have been subjected to tight restrictions in the EU, though their manufacturers have taken the EU Commission to court in a bid to reverse 'partial bans' on these and a third neonicotinoid, TMX, which came into force in December.

Although some studies have suggested that CCD – and related mortality rates in honey bee colonies – may be a result of pesticides reducing bees' resistance to mites or parasites, the Harvard study found that bees in the hives exhibiting CCD had almost identical levels of parasites and disease as a group of control hives, most of which survived the winter. This suggests that the neonicotinoids examined may be causing some other kind of biological mechanism in bees that in turn leads to CCD.

Commenting on the findings, Chensheng (Alex) Lu, a Professor at the School of Public Health said, "We demonstrated again in this study that neonicotinoids are highly likely to be responsible for triggering CCD in honey bee hives that were healthy prior to the arrival of winter."

The problem created by pollinator decline is a serious one; roughly one third of the food crops on which humanity relies are pollinated by insects. Last month, an EU-funded study revealed that the northern EU states were worse affected by winter die-offs. The UK had the second highest losses both in-season and over-winter. However, the UK government has refused to implement a number of the EU's non-binding recommendations related to the 'partial neonicotinoid ban' and announced that it rejects neonicotinoid science last year.

Harvard study

As part of the Harvard study, Lu and his co-authors from the Worcester County Beekeepers Association studied the health of 18 bee colonies in three locations in central Massachusetts from October 2012 to April 2013. At each location, the researchers separated six colonies into three groups—one treated with imidacloprid, one with clothianidin, and one untreated.

There was a steady decline in the size of all the bee colonies through the beginning of winter—typical among hives during the colder months in New England. Beginning in January 2013, bee populations in the control colonies began to increase as expected, but populations in the neonicotinoid-treated hives continued to decline. By April 2013, 6 out of 12 of the neonicotinoid-treated colonies were lost, with abandoned hives showing signs typical of CCD. Only one of the control colonies was lost—thousands of dead bees were found inside the hive – with what appeared to be symptoms of a common intestinal parasite called Nosema ceranae.

While the 12 pesticide-treated hives in the more recent study experienced a 50 percent mortality rate, the authors noted that, in their 2012 study, bees in pesticide-treated hives had a much higher rate of collapse – 94%. That earlier bee die-off occurred during the particularly cold and prolonged winter of 2010-2011 in central Massachusetts, leading the authors to speculate that colder temperatures, in combination with neonicotinoids, may play a role in the severity of CCD.

"Although we have demonstrated the validity of the association between neonicotinoids and CCD in this study, future research could help elucidate the biological mechanism that is responsible for linking sub-lethal neonicotinoid exposures to CCD," said Lu. "Hopefully we can reverse the continuing trend of honey bee loss."