A yearly survey of beekeepers by the USDA and national Bee Informed programme released on Thursday (14 May) shows that, although fewer colony losses were recorded in the United States over the winter of 2013-2014 than in recent years, beekeepers feel that losses are still higher than what they would consider to be a sustainable level.

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Many species of insect pollinators – which help three quarters of flowering plants, and a third of the edible plants on which humans depend, to reproduce – are in decline across the Northern Hemisphere. The reasons behind declines are complex and multi-faceted, but there is strong reason to suspect that the effects of climate change, habitat loss and the use of certain pesticides are all contributing factors. To date, monitoring and risk assessments have mainly focused on commercially kept honey bees in managed hives, though the limited data available on wild pollinators suggest species including wild ground-nesting bees, hoverflies and a number of butterfly species are also being hit hard.

According to the USDA survey results, total losses of managed honey bee colonies from all causes amounted to 23.2 percent nationwide. Beekeepers' groups claim loss-rates of 18.9 percent would be "acceptable for their economic sustainability." Overwinter loss rates were 30.5 percent across the States in 2012-13, but only 21.9 percent the previous year. The long-term average is 29.6 percent over-winter losses.

Commenting on the findings, Agriculture Secretary Tom Vilsack said, ""Pollinators, such as bees, birds and other insects are essential partners for farmers and ranchers and help produce much of our food supply. Healthy pollinator populations are critical to the continued economic well-being of agricultural producers. While we're glad to see improvement this year, losses are still too high and there is still much more work to be done to stabilize bee populations."

Jeff Pettis, a survey author with the USDA Agricultural Research Service (ARS), and Dennis vanEngelsdorp, a University of Maryland professor and co-author, said they do not know why bees fared better this past winter than in the previous one.

"Yearly fluctuations in the rate of losses like these only demonstrate how complicated the whole issue of honey bee health has become, with factors such as viruses and other pathogens, parasites like varroa mites, problems of nutrition from lack of diversity in pollen sources, and even sub-lethal effects of pesticides combining to weaken and kill bee colonies," Pettis suggested.

Earlier this month, researchers at Harvard University published new research findings that suggest certain neonicotinoid pesticides affect bees' ability to overwinter successfully. The research built on earlier findings from the same Harvard team.

Neonicotinoid manufacturer Bayer CropSicence has claimed that the Harvard study is flawed, suggesting researchers used unrealistically high levels of neonicotinoid pesticides, which bees would never encounter in field conditions. Agchem companies maintain that their products are not affecting pollinators and two manufacturers, Bayer and Syngenta, have taken the EU Commission to court over a 'partial ban' placed on their products in December.

Nevertheless, public health lawyer Michele Simon told news agency Reuters on Thursday that, in light of mounting evidence linking these products to health impacts on bees and other creatures, "Pesticide companies can no longer spin their way out of this crisis."

In April, an EU-funded project revealed that northern European countries suffer higher rates of overwinter bee die-offs than southern states. Die-off rates in the UK's managed colonies were the second highest of the 17 countries studied, both overwinter and in-season.    

USDA said a complete analysis of its bee survey data will be published later this year. The summary of the analysis is available online here. The Agriculture Department also said it would hold a summit this autumn aimed at addressing the nutrition and forage needs of pollinating insects.