Honeybees and Colony Collapse Disorder

▪ 2009

      In 2008 agricultural researchers sought to determine the cause of a mysterious disorder that was destroying colonies of Apis mellifera honeybees. The malady, called colony collapse disorder (CCD), was threatening honeybee populations across the United States, where they were essential for the pollination of many commercially important crops. The disorder had also been reported elsewhere, primarily in Europe. In CCD the sudden failure of a honeybee colony was characterized by the death of its adult bees, which would leave the hive and fail to return. During the few days or weeks over which the adults disappeared, the queen and several attendant workers bees usually remained in the hive, and the hive had plentiful food stores and showed evidence of normal brood rearing. Although sudden honeybee-colony die-offs were known to occur from time to time, an unusual feature of CCD was that honey-seeking neighbouring bees and common hive pests such as wax moths appeared to delay invading hives that had been affected by the disorder.

      The unexplained loss of honeybee colonies that came to be known as CCD was first reported in the fall of 2006 by a commercial beekeeper from Pennsylvania who was overwintering his colonies in Florida. (Subsequent investigations suggested that beekeepers had experienced unexplained colony losses for at least the previous three years.) By February 2007 several large commercial migratory beekeeping operations had reported cases of CCD, with some operators suffering the loss of 50–90% of their colonies. Many of these larger operations were overwintering their colonies in California, Florida, Oklahoma, and Texas. By late February 2007, some nonmigratory operations located in the mid-Atlantic region and the Pacific Northwest also had reported the loss of more than 50% of their colonies. The absence of dead bees in the affected hives made initial investigations difficult and inconclusive.

      Beekeeping is a critical component of modern agriculture, and CCD not only threatens the beekeeping operations that provide pollination service and honey production but also has the potential for crippling the production of many crops that are dependent on honeybees for pollination. In the United States, beekeepers provide pollination service for more than 90 commercially grown crops, including many fruits and vegetables. The economic value of U.S. crops that benefit from honeybee pollination has been estimated at $15 billion annually. In 2006 the California almond-export crop alone was valued at $1.9 billion and required more than one million bee colonies for pollination (out of a total of about 2.6 million colonies in the U.S.). With the number of available colonies for crop pollination in the country in decline, the beekeeping industry faces a tremendous challenge in meeting the demand for pollination services.

      The Agricultural Research Service of the U.S. Department of Agriculture has organized efforts to address the CCD crisis through surveys and data collection, samples analysis, and mitigation and preventive measures. A variety of possible causes of CCD have been suggested. They include chemical contamination of colony food stores or beeswax; poisoning from pesticides, including newly introduced insecticides based on nicotine derivatives; the introduction of genetically modified crops; possible lack of genetic diversity in colonies; and infection of colonies by pathogens or parasites, including known honeybee parasites such as the single-celled Nosema ceranae and the invasive varroa mite (Varroa jacobsoni).

      Some of these suggestions have been discounted, but a 2007 study stated that Israeli acute paralysis virus appeared to be strongly associated with the disorder. The virus—which was first identified in Israel—had not been previously reported in the U.S., but a subsequent genetic screening of preserved honeybee specimens showed that IAPV had been present in honeybees in the U.S. since at least 2002. A study of the virus in 2008 determined that there were three types and that two of them had infected honeybees in the U.S. Although the virus is a consistent marker of CCD, a cause-and-effect relationship has yet to be established, and many scientists suspect that CCD might be the result of a combination of two or more pathogens or stressors. Bee colonies are commonly found to be infested with pathogens and parasites, and the investigation of the interactions of all the possible causative agents has proved to be a challenge for bee scientists.

      Colony stress could contribute to CCD by harming the bees' immune systems and making colonies more susceptible to disease. Possible sources of stress include poor nutrition caused by the lack of plants that are sources of nectar and pollen, the use of honeybees to pollinate crops with little nutritional value for bees, the overcrowding of honeybee colonies, and the repeated transport of colonies over long distances for pollination or honey production.

      Steps to improve honeybee health are being taken that might reduce colony stress. The advent and utilization of improved nutritional supplements could boost honeybee health during periods of nectar or pollen dearth or inclement weather. Several large migratory operations are being periodically sampled for possible early warning signs of colony-health issues that could trigger or lead to CCD. Some recommendations that have been made to beekeepers to improve honeybee health include feeding bees antibiotics to prevent nosema infections, using genetic stocks that show resistance to mites, and applying fumigants such as formic-acid- or thymol-based products only when necessary to control varroa-mite infestation. Another recommendation is to avoid reusing equipment that has been exposed to bee colonies that have died from CCD.

      A new technological advance that could help in discovering the underlying cause of CCD in honeybees is the full sequencing of the honeybee genome, which was published in late 2006. It makes available new molecular approaches and honeybee genomics in the investigation of the impact of possible causal agents on specific genes and honeybee-colony health. Likewise, the advance might help identify new pathogens in honeybees and unravel the complex effects of multiple combinations of pathogens and environmental toxins.

William Michael Hood

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