In Science, Francis Ratnieks and Norman Carreck write about what has been learned about the collapse of bee populations in Clarity on Honey Bee Collapse? (2010 327 (5962): 152)

Over the past few years, the media have frequently reported deaths of honey bee (Apis mellifera L.) colonies in the United States, Europe, and Japan. Most reports express opinions but little hard science. A recent historical survey pointed out that extensive colony losses are not unusual and have occurred repeatedly over many centuries and locations. Concern for honey bees in the United States has been magnified by their vital role in agriculture. The California almond industry alone is worth $2 billion annually and relies on over 1 million honey bee hives for cross-pollination. So what is killing honey bee colonies worldwide, and what are the implications for agriculture?

In fall 2006 and spring 2007, many U.S. beekeepers encountered hives without adult bees but with abandoned food and brood. It was widely believed that these were symptoms of a new and highly virulent pathogen. In the absence of a known cause, the term “Colony Collapse Disorder” (CCD) was coined. What have we learned about this condition since then? Are the symptoms really novel?

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The first annual report of the U.S. Colony Collapse Disorder Steering Committee, published in July 2009 (15), suggests that CCD is unlikely to be caused by a previously unknown pathogen. Rather, it may be caused by many agents in combination—the interaction between known pests and pathogens, poor weather conditions that diminish foraging, lack of forage (16), and management factors such as the use of pesticides and stress caused by long-distance transport of hives to nectar sources or pollination locations. The increasingly technical process of beekeeping itself merits further research as far as its impact on colony health. For example, although pollen substitutes are now widely used, little is known about the interactions between nutrition and disease susceptibility. Further research is also needed to develop effective ways of keeping colonies healthy through good hive management based on appropriate chemical, and other treatments such as “hygienic” bees that remove diseased brood and can be bred using conventional methods. In Europe, the COLOSS (COlony LOSS) network, consisting of 161 members from 40 countries worldwide, is coordinating research efforts and activities by scientists and the beekeeping industry to address these and other issues related to honey bee losses, including CCD (2).

In February 2009, the high pollination fee, combined with a temporary reduction in pollination demand due to drought and reduced almond prices, resulted in a surplus of hives in California available to pollinate almonds. But this leaves no room for complacency. Almond pollinating beekeepers had a poor summer in 2009 in the Dakotas and neighboring states, where hives spend the summer making honey, with heavy rains delaying and reducing the honey crop. This delayed chemical treatments for Varroa mites, and many colonies were probably in worse than usual condition going into winter back in California. It will be interesting to see what happens in February 2010 when the almonds bloom. On a longer time scale, there is a worrying downward trend in U.S. hives, from six million after World War II to 2.4 million today. Is the future of U.S. commercial beekeeping going to be based on pollinating a few high-value crops? If so, what will be the wider economic cost arising from crops that have modest yield increases from honey bee pollination? These crops cannot pay large pollination fees but have hitherto benefited from an abundance of honey bees providing free pollination.