Research Directions Scientists are looking in four general categories for the cause/causes of CCD:
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[li]Pathogens: Among others, scientists are considering Nosema (a pathogenic gut fungi), Israeli Acute Paralysis Virus, and possibly unknown pathogens as possible culprits in CCD. ARS research has indicated that no one pathogen of any class directly correlates with the majority of CCD incidents. Rather, a higher total pathogen load of viruses and bacteria correlates more directly with CCD than any one specific pathogen.[/li][li]Parasites: Varroa mites are often found in honey bee colonies that are affected by CCD. It is not known if the Varroa mites are directly involved or if the viruses that Varroa mites transmit (similar to the way mosquitoes transmit the malaria virus) are a factor in causing CCD.[/li][li]Management stressors: Among the management stressors that are possible contributors to CCD are poor nutrition due to apiary overcrowding and increased migratory stress brought on by the honey bees being transported to multiple locations across the country.[/li][li]Environmental stressors: Such stressors include the impact of pollen/nectar scarcity, lack of diversity in nectar/pollen, availability of only pollen/nectar with low nutritional value, and limited access to water or access only to contaminated water. Stressors also include accidental or intentional exposure to pesticides at lethal or sub-lethal levels.[/li][/ol]
A survey of honey bee colonies revealed no consistent pattern in pesticide levels between healthy and CCD-affected colonies when pollen, bees, and beeswax were tested for the presence of 170 pesticides. The most commonly found pesticide in that study was coumaphos, which is used to treat honey bees for Varroa mites.
The pesticide class neonicotinoids (clothianidin, thiamethoxam, and imidacloprid) has been accused of being the cause of CCD. The neonicotinoids were developed in the mid-1990s in large part because they showed reduced toxicity to honey bees, compared with previously used organophosphate and carbamate insecticides.
In 2008, Germany revoked the registration of the neonicotinoid clothianidin for use on seed corn after an incident that resulted in the die-off of hundreds of nearby honey bees colonies. Investigation into the incident revealed that the die-off was caused by a combination of factors, including the failure to use a polymer seed coating known as a “sticker”: weather conditions that resulted in late planting of corn while nearby canola crops were in bloom, attracting honey bees; use of a particular type of air-driven equipment used to sow the seeds, which blew clothianidin-laden dust off the seeds and into the air as the seeds were ejected from the machine into the ground; dry and windy conditions at the time of planting, which blew the dust into the nearby canola fields where honey bees were foraging; and a higher application rate than had been authorized was used to treat for a severe root worm infestation.
Several studies that reported a negative impact on honey bees by neonicotinoids relied on large, unrealistic doses and gave bees no other choice for pollen, and therefore did not reflect risk to honey bees under real world conditions. Nor have the studies demonstrated a direct connection or correlation to CCD.
There have been scientific findings that imply that neonicotinoids have sublethal effects on honey bees at approved doses and exposures. ARS scientists and other researchers are looking into whether such sublethal effects may correlate with CCD or other bee health problems and whether they could be a contributing cause of CCD.
ARS held a workshop with the U.S. Environmental Protection Agency in early 2010 to discuss how potential sublethal effects could be documented summarized in Pesticide Risk Assessment for Pollinators: Summary of a SETAC Pellston Workshop.
ARS researchers also have been analyzing samples from healthy and CCD-struck colonies and applying a variety of stressors from the four categories of possible causes to colonies in hopes of provoking a colony response that duplicates CCD.
While a number of potential causes have been championed by a variety of researchers and interest groups, none of them have stood up to detailed scrutiny.
Every time a claim is made of finding a “smoking gun,” further investigation has not been able to make the leap from a correlation to cause-and-effect. Other times, not even a scientific correlation has been demonstrated in the study claiming to have found “the cause” of CCD.
Researchers have concluded that no one factor is the cause of CCD. Most likely, CCD is caused by multiple factors. It is not possible to know at this time if all CCD incidents are due to the same set of factors or if the factors follow the same sequence in every case.
One explanation for CCD being studied is that a perfect storm of environmental stresses may unexpectedly weaken colonies, leading to collapse when the colonies are exposed to the additional stress of a pathogen, parasite, and/or pesticide. Stress, in general, compromises the immune system of bees (and other social insects) and may disrupt their social system, making colonies more susceptible to disease.
Studies are being conducted by ARS scientists and collaborators to look at the combined impact of two or more factors on honey bees—most recently the impact of exposure to the neonicotinoid imidacloprid and Nosema. While the dual exposure indicated some sublethal effects on individual honey bees, the overall health of the colony did not show an adverse effect.