Oct 1, 2012Scientists still puzzling over CCD causes
When it comes to solving the puzzling syndrome known as Colony Collapse Disorder (CCD), which has been attacking honey bee colonies since 2006, the best that can be said is that there is good news and bad news.
The good news is that the rate of honey bee losses seems to have leveled off rather than continuing to increase. The bad news is that the cause or causes of CCD remain unclear.
In the United States, the problem surfaced in October 2006, when an increasing number of beekeepers began reporting losses of 30 to 90 percent of the hives in their apiaries with no apparent cause. The defining characteristic of CCD is the disappearance of most, if not all, of the adult honey bees in a colony, leaving behind honey and brood but no dead bee bodies. This definition has recently been revised to include low levels of Varroa mite and other pathogens, such as Nosema, as probable contributing factors.
For the last three years, self-reported surveys of beekeepers have found that winter colony losses have averaged about 30 percent, with around one-third of those losses ascribed to CCD, according to Jeff Pettis, research leader of the Bee Research Laboratory in Beltsville, Md., who heads up the Agricultural Research Service’s (ARS) CCD research effort. This compares to colony losses that were averaging 15-20 percent before CCD.
“The faint good news in the survey numbers is that the CCD problem does not seem to be getting worse,” Pettis said. “But – and this is a big ‘but’ – 33 percent losses each year are probably not economically sustainable for commercial beekeeping operations.”
While many possible causes for CCD have been proposed, reported and discussed – both in scientific literature and popular media – no cause has been proven.
“We know more now than we did a few years ago, but CCD has really been a 1,000-piece jigsaw puzzle, and the best I can say is that a lot of pieces have been turned over,” Pettis said. “The problem is that they have almost all been blue-sky pieces – frame but no center picture.”
The bee lab’s scientists have been looking for the cause or causes of CCD within four broad categories: pathogens, parasites such as Varroa mites or Nosema, environmental stressors such as pesticides or lack of nectar diversity, and management stressors. The researchers have been analyzing samples from healthy and CCD-struck colonies and applying a variety of stressors from the four groups 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 the causes has stood up to detailed scrutiny,” Pettis said. “Every time someone has proclaimed a potential smoking gun, further investigation has not been able to make the leap from correlation to cause-and-effect for one reason or another. Other times, there hasn’t even been a scientific correlation.”
ARS’ research work, along with that of university and other scientists, “pretty well supports the idea that CCD is caused by multiple factors – possibly working individually, but more likely in combination,” Pettis said. “But we still can’t say whether it’s the same set of factors in every CCD incident or – if it is the same factors – that they are occurring in the same sequence in every case.”
Pathogens continue to stand out as one of the important puzzle pieces, according to ARS entomologist Jay Evans, also with the Bee Research Laboratory. He was part of a team that used genetic analyses to look for correlations between bee health and bee pathogens and activity levels of honey bee genes.
Two picornalike viruses – acute bee paralysis virus and Kashmir bee virus – along with deformed wing virus, black queen cell virus and two species of Nosema, were found to be more abundant in CCD hives. Infection by multiple picornalike viruses could result in honey bees having reduced abilities to synthesize certain proteins, the lack of which would leave the bees more vulnerable to additional stresses like pesticides, nutrition problems or other pathogens – which sounds like a possible root cause of CCD, Evans said.
When the researchers looked at the bees’ turning on of detoxification and immune genes, which would have reflected exposure to either pesticides or disease, respectively, there was no significant difference between CCD and non-CCD colonies.
The team did find considerable differences between CCD hives on the West Coast and the East Coast.
“Finding Kashmir bee virus in a hive was the best predictor of CCD in the Western United States, while deformed wing virus, an unrelated RNA virus, was a better predictor in the East,” Evans said.
Evans and Bee Research Laboratory colleague Judy Chen were also part of an international team that closely followed 29 European honey bee colonies, carefully monitoring for pathogens, parasites and bee proteins. This study found that four factors appeared to be the best predictors so far of winter honey bee loss: presence of the microsporidian Nosema ceranae; levels of the protein vitellogenin, which strongly reflects the bee’s protein status and plays an immune system role; Varroa mite infestation; and the presence of deformed wing virus, a virus often associated with Varroa mites.
Finding Varroa mites to be a good predictor of winter declines in this study was not really surprising, according to Evans, though other studies in the U.S. have not found as high a correlation between CCD and Varroa mites.
“Even if Varroa mites themselves do not directly cause CCD, we know they can transmit multiple viruses to honey bees,” Evans said, “and higher total pathogen levels (rather than infection by any specific pathogen) have been our best correlation with CCD so far. But should the pathogens be considered primary causal agents – multiple pathogens specifically causing CCD – or are they indirectly involved because they weaken bees, making these bees more vulnerable to something else we don’t know yet?”
Pesticides as cause?
Pesticides – individually and in general – have been repeatedly nominated as a cause of CCD, often without direct scientific data to support the idea. In a pesticide survey conducted by USDA and university scientists that analyzed wax, pollen and bee samples for the presence of 121 different pesticides or their metabolites, the most commonly found pesticides were fluvalinate and coumaphos. While about 60 percent of the 259 wax and 350 pollen samples did show the presence of at least one systemic pesticide, almost all were at levels well below what is considered lethal to honey bees.
There was no overall pattern of exposure among the samples for a specific pesticide or class of pesticides. The study did not look specifically at the pesticides as they might be related to CCD, but if a specific class of pesticides was involved, a pattern of residues should have been discernible, said Pettis, who co-led the study.
Not all pesticide impact is about directly killing honey bees, however. Sublethal doses of the pesticide imidacloprid – one of the neonicotinoid group of pesticides – were found to make honey bees more susceptible to the gut parasite Nosema, according to a study by Pettis and University of Maryland researchers Dennis vanEngelsdorp, Josephine Johnson and Galen Dively.
The researchers fed three generations of honey bee colonies either 5 or 20 parts per billion (ppb) of imidacloprid, which is used to protect a wide variety of crops and ornamentals from many insects. The dosages used in the study were intentionally well below the levels that have been documented to kill honey bees after short-term exposure, and reflected levels that have been measured in the environment.
After the third generation, newly emerged adult bees from these colonies were exposed to spores of N. apis and N. ceranae, gut parasites that have been a growing problem for U.S. beekeepers since the 1990s.
There was up to a fourfold increase in the levels of Nosema in honey bees from the imidacloprid-exposed colonies, regardless of whether 5 or 20 ppb were fed.
“While these increased Nosema levels were found in individual bees, there was no measurable impact at the colony level,” Pettis said. “Imidacloprid was chosen for this study because of its widespread use and beekeepers’ concerns about it. But it was only found in 3 percent of the pollen samples checked in the pesticide-survey study, usually at very low levels, and no connection with CCD has ever been made scientifically.”
To better account for such sublethal impacts, Pettis is working as part of an international group of scientists and regulators to help EPA develop recommendations for pesticide-testing guidelines that factor sublethal effects into test protocols. ARS scientists have already developed a sublethal-impact cage assay that has been provided to EPA.
Could imidacloprid and Nosema together be the cause of CCD, as some claim?
“This study did not look for nor establish any connection between either imidacloprid or Nosema and CCD,” Pettis said. “But the effect of the combination of imidacloprid and Nosema demonstrates that there are many complex interactions between stress factors that need to be considered in looking for a cause of CCD and high honey bee mortality in general.”
By J. Kim Kaplan, Agricultural Research Service