Colony Collapse Disorder: The Honeybee Mystery Explained

Introduction

The new millennium has given us new problems to solve. Climate change, global warming, and Arctic meltdown are just a few starring environmental concerns. However, over the last few years, a new and equally important problem has arisen in the United States: the sudden and mysterious disappearance of the honeybee. As insignificant as it may seem, the vanishing of honeybees presents a huge crisis — one that could have a significant impact on the entire agricultural economy of the nation. According to a USDA report, bees contribute around $15 billion per year to the nation's agricultural economy and are involved in the pollination of more than 130 crop varieties. Speculation on the disappearance of honeybees has ranged from parasite infestation to virus attack to pesticide usage and even stress factors. While scientists are still investigating the actual cause — or the possibility of several interrelated factors — the crisis has prompted agriculturists and entomologists to look at methods that could reverse the situation. This paper provides a brief overview of the problem, examines the potential causative factors, and explores possible solutions to the crisis.

Colony Collapse Disorder

Colony Collapse Disorder (CCD) is a term now familiar among beekeepers across the nation. In essence, CCD means that adult worker bees desert honey-filled beehives, leaving no clue as to their whereabouts. With no dead bees found around the hive, this strange phenomenon threatens the entire agricultural scenario of the United States. First reported in 2006, researchers are still unable to pinpoint the exact cause of this strange bee behavior. According to a recent survey conducted by the National Agricultural Research Service, beekeepers around the country reported a loss of 37% of their colonies in 2007. This is a shocking development and a wake-up call for agriculturists and environmental researchers. As the U.S. Agricultural Committee recently reported: "The syndrome is mysterious in that the main symptom is simply a low number of adult bees in the hive. There are no bodies, and although there are often many disease organisms present, no outward signs of disease, pests, or parasites exist." [Benjamin P. Oldroyd, 2007]

Loss of bee colonies is not, in itself, a new problem. Colony losses have been documented previously — the most recent notable case being the 1995 Pennsylvania event, where beekeepers reported a loss of around 53% of their colonies. Bee populations tend to fluctuate naturally from year to year, but what is alarming now is the widespread reporting of CCD across the entire stretch of America and parts of Europe, with some beekeepers reporting losses of 80–100% of their colonies. [Benjamin P. Oldroyd, 2007] In attempting to solve the mystery, researchers have focused on every possible contributing factor, including parasites, viruses, pesticides, and the vastly changed and commercialized beekeeping practices and their impact on bee behavior.

It is well known that the chief pest attacking honeybees is Varroa destructor, a phoretic parasite that lives on adult bees. Studies have shown that Varroa destructor can transmit the deadly Deformed Wing Virus (DWV) to honeybees, resulting in the development of crippled wings. Research has shown that the viral load of the infested mite has a direct bearing on the severity of symptoms; at lower loads, DWV is well tolerated and honeybees remain non-symptomatic. [Sebastian Gisder, 2009] However, acute mite infestation is highly visible to beekeepers and does not fully explain the sudden onset of CCD. There are also reasons to suspect that infestation by the other parasite Acarapis woodi, or tracheal mite — which is very widespread in the United States — may be one of the chief contributing causes of CCD.

Parasite Infestation and Viral Infections

Honeybees are susceptible to a variety of viral infections, but in most cases they remain symptom-free. Research has shown, however, that under unfavorable conditions involving greater stress, parasitic infection, or environmental or nutritional imbalance, these viral infections become more potent and fatal. One of the main symptoms of DWV infection is severe trembling.

Researchers initially argued that the absence of distressed, trembling worker bees would exclude paralysis virus infection as a cause of the recent CCD. However, a German study by Iqbal and Mueller reported that DWV infection could cause symptoms other than the trembling and shaking typically observed. Their study found that DWV infections interfere with associative learning and impair memory in Apis mellifera. Associative memory is critical for honeybees, and any impairment can block their normal foraging behavior. [Javaid Iqbal, 2007]

More recently, attention has shifted to a new pathogen known as the Israeli Acute Paralysis Virus (IAPV). Studies have shown that this virus arrived via infected Australian bees imported to the United States in 2005, following mounting pressure from the almond growers' association. Because almonds depend entirely on bees for pollination, the shortage of domestic bees forced the U.S. government to lift its 1992 embargo on bee imports. A study by Cox-Foster identified IAPV as a clear marker in samples from CCD-affected migratory bee operations. Confirming evidence also came from IAPV-infected cockroaches found in CCD-affected beehives. Researchers then infected German cockroaches with infectious fluids obtained from affected bees; the cockroaches began dying within four days, and analysis of the dead insects enabled researchers to isolate the IAPV — but not other viruses. As W. Ian Lipkin of Columbia University, who performed much of the genome sequencing for this research, stated: "This clearly proves that the virus is capable of causing disease in insects. But we don't know if it's [IAPV] necessary or if it's sufficient to cause CCD." [Myrna E. Watanabe, 2008] Scientists believe that bees weakened by Varroa mite attack may be more susceptible to pathogens such as IAPV.

Even so, the virulence of viruses remains an important factor. Healthy bee populations — such as Australian bees — are infected with IAPV without showing symptoms. To further investigate IAPV's role, Cox-Foster and colleagues administered infectious preparations from CCD-affected bees to healthy Hawaiian bees. Cox-Foster reported, "We are beginning to see some mortality in the colonies." [Myrna E. Watanabe, 2008] The researchers hope that further sequencing of the genetic material of the Hawaiian bees will provide additional evidence in this direction.

Environmental Insults: The Role of Pesticides

As the mystery of honeybee disappearance continues to baffle researchers, agriculturists and entomologists are seriously considering the role of environmental pollutants — particularly pesticides and their metabolites — as causative factors. Research has focused on both in-hive chemicals and agricultural pesticides, including the bio-pesticides in genetically modified crops. Christopher Mullin, professor of insect toxicology, and a team of researchers have been studying pesticide metabolites in a variety of samples including pollens, wax, nectar, and the bees themselves. As Mullin stated: "We're seeing a lot of chemicals, a lot of residues. We're still trying to understand it." [Myrna E. Watanabe, 2008]

In commercial beehives, pesticides are used to control parasites such as V. destructor and the hive beetle (A. tumida). However, V. destructor acquired resistance to the widely used pesticide pyrethroid fluvalinate, which led to the switchover to the organophosphate coumaphos. In 2001, a commercial beekeeper from Maine complained that Checkmite+, a formulation of coumaphos, no longer controlled V. destructor effectively. To assess possible resistance development, researchers conducted assays on bee colonies in Maine, Florida, and Maryland using Checkmite+ strips. Results showed a higher mite mortality rate in Maryland colonies (80–100%), which had only been exposed to Checkmite+ for one season, compared to Maine (13%) and Florida (7–80%). This clearly indicated the development of resistance to coumaphos in the Maine and Florida mite populations. [Pettis J.S., 2003] This led to the introduction of a new chemical compound, Amitraz — a triazapentadiene — whose effects have not yet been fully understood. Some beekeepers use combinations of pesticides, and studies have confirmed traces of fluvalinate in honey and wax samples from colonies treated with Apistan strips. [Gatien, 2003]

Beyond in-hive chemicals, bees are also exposed to agricultural pesticides when they feed on pollens. Although these pesticides undergo rigorous testing before commercial approval, risk factors for non-target ecosystems are not always fully understood. With newer pesticides continuously replacing older ones that have lost effectiveness due to pest resistance, risks for non-target species continue to rise. A notable example is a study conducted in France implicating the neonicotinoid insecticide Imidacloprid as the cause of significant bee colony losses. Imidacloprid is considered safe due to its low mammalian toxicity and is used extensively for pest control on large farms. While some studies report higher pesticide levels in plant pollens and nectar, others have failed to detect residual levels of the chemical or its metabolites. As Kimberly Stoner, a Connecticut-based entomologist, explained: "The pesticide is put on the seed, and the plant takes in that pesticide and moves it through the vascular system of the plant. Bees are potentially picking them up in pollen and nectar at low levels that don't kill the bees, but that might affect their behavior and immune system." [Susan Salisbury, 2008]

A study by Laurent and Rathahao (2003) analyzed Imidacloprid concentrations in sunflower plants grown from pesticide-treated seeds. The study was prompted by reports of unusual bee behavior in colonies feeding on these plants. Based on radioactivity measurements, the researchers found that the plants absorbed approximately 10% of the pesticide applied to the seeds, with 75% of the radioactivity concentrated in the cotyledons. Radioactivity measurements in upper leaves were 20 times lower than in lower leaves. The study confirmed that seed treatment affected the entire plant through vascular translocation of the pesticide, indicating a significant potential for bee poisoning. [Laurent, 2003]

Other studies have similarly reported negative effects of Imidacloprid on memory and brain metabolism in honeybees. Decourtye et al. (2003) showed that oral administration of Imidacloprid affected the olfactory responses of honeybees in a proboscis extension reflex (PER) test. When administered 15 minutes after a PER test, Imidacloprid was found to compromise medium-term olfactory memory. Elevated levels of cytochrome oxidase indicated pesticide-induced changes in brain metabolism and neuronal transmission. [Decourtye et al., 2003]

Conclusion

Another study pertaining to IPM in bee colonies focused on controlling Varroa destructor in the northeastern United States. This research by the Sustainable Agriculture Research and Education (SARE) program found that formic acid fumigation produced optimal results when bee colonies had fewer drone broods or were entirely broodless. The study recommended that beekeepers remove drone brood prior to fumigation, since mites tend to seek refuge in brood cells — where they are better protected than when on adult host bees, where they are most susceptible to treatment. Removal of drone brood is therefore a necessary precondition for successful pest control through fumigation. [SARE]

Loss of bees is a serious threat to national agriculture and the food supply. The mystery of the honeybee vanishing remains a largely unsolved puzzle. Thus far, no single cause has been established for Colony Collapse Disorder, and scientists are beginning to approach it as a multifactorial problem. A conjunction of factors is strongly suspected in the sudden disappearance of bees, including pest infestation, the toxic effects of pesticides, viral attacks (IAPV, DWV, and others), and the stress induced by the frequent relocation of bees for commercial pollination services.

Environmentally sustainable agricultural practice is the only way to prevent such catastrophic developments in the future. Pesticides not only incur considerable costs but also affect the natural purity of hive products and increase toxicity risks from the consumers' perspective. Most importantly, the indiscriminate and systemic use of pesticides threatens to destroy a keystone species — the honeybee that underpins our agricultural economy. Albert Einstein reportedly said, "If honeybees become extinct, human society will follow in four years." [Gunther Hauk] The alarming developments witnessed by beekeepers over recent years have once again drawn attention to the consequences of indiscriminate interference with natural ecosystems and the neglect of sustainable agricultural practices in favor of short-term profit.

Adoption of an integrated pest management system is indispensable in efforts to revive honeybee populations while limiting the damage caused by pests to below economic injury levels. The onus rests with us to protect this vital insect species that plays a major role in supporting our lives and our food systems.