Colony collapse disorder (CCD) is a mysterious and potentially devastating phenomenon in which all or almost all of the adult honeybees in a colony suddenly disappear, leaving behind honey, pollen stores, and a queen and developing brood bees, but no dead bees.
Honeybees are vital to life on Earth. About 90% of all plants, including many crops, are pollinated by bees of various types. Honeybees are the most important insect pollinator. About one-third of the North American diet is directly or indirectly dependent on honeybee pollination.
A single honeybee colony can consist of one queen, a few hundred drones, and 80,000 worker bees. Humans have been raising honeybees as honey producers and pollinators for at least 5,000 years. Although not native to the Western Hemisphere, honeybees were brought over by early missionaries and settlers. Compared to native North American pollinators, honeybees live year-round, are more prolific, and are much easier to manage commercially for the pollination of a wide variety of crops. Professional beekeepers raise the bees to produce honey from nectar and also truck them to farms and orchards, where they pollinate fruit, vegetables, and nuts by transferring pollen from one flower to the next. The largest commercial pollination event on Earth occurs each February, when hundreds of beekeepers converge on California almond farms with 1.6 million hives—60% of all managed colonies in the United States—housing 48 billion bees to pollinate an area the size of Rhode Island.
In the winter of 2006–2007, commercial beekeepers in the United States and elsewhere began reporting unusually high losses—30–90% of their hives, depending on the region. Up to 50% of those losses were consistent with the syndrome that has become known as CCD: the queen and brood remain in the hive, along with honey and pollen stores, but the workers have disappeared, with few bodies anywhere near the hives. Without the workers, the hives are unable to sustain themselves.
Pollinators of all types have been in decline for at least a decade. Globally, some bee species having gone extinct, and some North American species are down to 4% of their previous populations. Feral (escaped) honeybees have been declining in the United States for the past 50 years and are almost gone. Most of this decline has been blamed on habitat loss and possibly on the use of pesticides. In addition, during the 1980s and 1990s, new pathogens began afflicting North American honeybee colonies. Although the loss of honeybee colonies, especially over the winter, is not unusual, the losses that began in 2006 were both extraordinarily high and shared the unique features of CCD.
There have been reports of honeybee disappearances in the past. In Cache Valley, Utah, in 1903, 2,000 colonies were lost to what was called disappearing disease after a hard winter and spring. Similar disappearances were reported in the 1880s, 1920s, and 1960s. In 1995–1996, 53% of Pennsylvanian colonies were lost to an unidentified cause. It is not clear whether these losses were due to CCD or, if they were CCD, whether they were caused by the same factors as CCD in the early 2000s.
CCD presents a serious threat to the planet's food supply. Large-scale agriculture cannot function without commercial beekeeping and pollination, and the precipitous drop in wild bee populations has increased dependency on commercial honeybee colonies. Many crops, including tree nuts, fruits, berries, and vegetables are dependent on honeybee pollination. California's $3-billion per year almond industry is almost completely dependent on honeybees. Because of CCD, commercial pollination costs have tripled, raising food prices.
Risk factors for CCD appear to include:
In 2012, there were about 2.5 million commercial honeybee colonies in the United States, down from five million in the 1940s. However, the need for pollination increased dramatically between 1960 and 2010, with the result that honeybee colonies were transported over much longer distances to pollinate crops. During the winters of 2006–2011, colony losses averaged about 33%, with about one-third of losses attributable to CCD. Before the advent of CCD, annual losses averaged 15–20%. During the winter of 2011–2012, honeybee colony losses in the United States totaled 22%. There were about 500,000 managed colonies in California, up from a 2007 low of 340,000–400,000.
What causes CCD has been the focus of much media attention and hype and a great deal of scientific controversy. Many scientists have come to believe that CCD results from combinations of factors. Even without CCD, management stresses and environmental stresses, malnutrition, and mites, fungi, and viruses can kill one-third of a hive every year.
Some researchers include the presence of pathogens in their definition of CCD. In particular, Varroa destructor mites and fungal pathogens in the genus Nosema, which live in the honeybee gut, have been identified as probable contributors to CCD. It is unclear whether it is Varroa mites themselves or the viruses that they transmit to bees that are directly associated with CCD. A number of different viruses have been identified in association with CCD-affected hives, including some novel strains. Studies have also identified changes in the gut bacteria of bees from affected hives. Thus far, CCD appears to be associated with a higher total burden of pathogenic viruses and microorganisms, rather than with any specific pathogen.
Among the prime suspects as causes of CCD are exposure to lethal or sub-lethal levels of pesticides that are applied to crops to protect them from insect damage. However, when bees, pollen, and beeswax from healthy and CCD-affected colonies were tested for 170 different pesticides, no consistent pattern emerged. The most commonly detected pesticide was coumaphos, which is used to rid the bees of Varroa.
Studies of CCD have most often implicated a class of pesticides called neonicotinoids or “neonics”—clothianidin, thiamethoxam, and imidacloprid. Ironically, these pesticides were developed during the 1990s because they were less toxic to honeybees than organophosphate and carbamate insecticides. Like those older pesticides, neonics attack the central nervous system of insects. However, unlike older pesticides that affect insects during or immediately after spraying, neonics are usually applied directly to seeds. They are systemic neuro-toxins that spread throughout the growing plant and, thus, are toxic to insects feeding on all parts of the plant throughout the growing season. Neonics are present in pollen, nectar, and even the droplets of sap or water that form on plant leaves. Furthermore, neonics are very long-lasting and can persist in the soil for a decade or more.
Early evidence of neonic involvement in CCD was anecdotal. Some commercial beekeepers reported that CCD appeared to occur several months after their hives were first exposed to fields that had been treated with a neonic. This suggested that, rather than the immediately toxic effects of some traditional pesticides, the bees were returning to the hive with pollen and nectar contaminated with low levels of neonic that had progressive effects on the bees developing within the hive. It might be coincidental; however, CCD appeared in the United States after 2004, the year that seed companies began marketing corn seeds treated with five times the previous levels of neonics. These coatings are partially pulverized by air-powered seed planters and can be spread over the fields in plumes. However, CCD has not emerged in all regions of the world where neonics are used.
Studies published in 2012 strengthened the case for neonics as primary drivers of CCD. Research indicated that neonics at approved dosages and exposures have sub-lethal effects on honeybees, causing them to become disoriented and unable to return to their hives. Furthermore, neonics appear to reduce the weight and number of queens in bumblebee hives. One study suggested that there may be neonic residues in high-fructose corn syrup fed to commercial honeybees. Honeybees are requiring more supplemental feeding as urbanization and pollution degrade their environments. In addition, the life span of adult field bees is decreasing. This means that younger bees are forced out to gather nectar and pollen, and these younger bees require more care and feeding to maintain their strength.
If neonics are culprits in CCD, their effects may be indirect. There is evidence that neonics may make honeybees more susceptible to pathogens. Some evidence also suggests that a new class of fungicides that have been widely used on corn since 2006 may greatly increase the toxicity of neonics in bees. Other evidence suggests that fungicides may affect the microbiota in the bee gut, killing off the bacteria that bees need to convert pollen into bee bread or to fight infection. In addition, older pesticides that are toxic to bees continue to be used, and herbicides destroy bee habitat, leading to nutritional stress.
Management stresses and environmental stresses may contribute to CCD by making colonies more susceptible to disease. Overcrowded hives can lead to poor nutrition. Furthermore, colonies are stacked on tractor-trailers and transported thousands of miles each growing season. Since hive orientation is essential for honeybees, this frequent relocation increases stress, in addition to spreading pathogens to bees from other colonies. Environmental stresses include scarcity, lack of diversity, and low nutritional value of pollen and nectar, as well as limited access to uncontaminated water. Despite much media attention, neither cell phones nor cell phone towers have been shown to be associated with CCD or poor honeybee health.
CCD usually occurs when worker bees first leave the hive to forage in winter or early spring. The bees fail to return to the hive. The telltale symptoms are a live queen, honey, and often immature bees in the hive, but no live or dead workers.
Incidents of acute pesticide poisoning of honeybee hives have sometimes been mistakenly identified in the media as CCD. However, acute pesticide poisoning is readily identifiable by piles of dead bees outside the hive entrance. Heavily diseased colonies also have large numbers of dead bees near the hive, in contrast to CCD.
There is no treatment for CCD. A solution is unlikely to be found until the cause of CCD is definitively identified.
CCD may be on the decline. It is possible that it is a cyclical syndrome that is undergoing a natural decline following a peak or that colonies are developing natural resistance. The U.S. Agricultural Research Service also introduced new management recommendations, including a new bee diet of supplemental nutrients for winter and times of drought, which may be benefiting the bees.
With the definitive causes of CCD unclear, prevention has focused on improving honeybee health and habitat and countering known risk factors. The public can help prevent CCD by avoiding the indiscriminate use of pesticides. If used, pesticides should not be applied during midday hours, when honeybees are most likely to be foraging for pollen and nectar. Planting pollinator-friendly plants—plants that are good sources of nectar and pollen—may also help prevent CCD.
See also Drought ; Environmental Protection Agency (EPA) .
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Margaret Alic, PhD