Jan 31, 2011Colony Collapse Disorder still being studied
After the large-scale, unexplained losses of managed U.S. honeybee colonies during the winter of 2006-07, investigators identified a set of symptoms that were termed colony collapse disorder (CCD). In response to this problem, federal and state government, university and private researchers mobilized to define an approach to CCD, which resulted in the formation of the CCD Steering Committee, according to USDA’s 2010 Colony Collapse Disorder Progress Report.
During the past three years, numerous causes for CCD have been proposed and examined, but it’s becoming increasingly clear that no single factor is responsible for the malady. Researchers continue to document elevated pathogen levels in CCD-affected bees, with no specific pathogen linked definitively to CCD, according to the report.
In addition, tests to examine hives for known honeybee parasites (varroa mites, honeybee tracheal mites, Nosema species), which pose significant problems for beekeepers and once were highly suspected to play a major role in CCD, have not revealed these parasites at sufficient levels to explain the problem.
Over the past year, several independent studies have shown that bees are exposed to a wide range of pesticides, and that some pesticides have interactive effects on bee mortality with other pesticides, with bee pests or with viruses. Taken together, these studies support the hypothesis that CCD is a syndrome of stress, caused by many factors working individually – but more likely in combination, according to the report.
Survey and data collection
In response to an immediate need for a baseline of both bee production and health, several survey and data collection efforts have been underway. Together, the different survey efforts have better defined CCD symptoms. Previous studies showed that symptoms included a rapid loss of adult honeybees, excess immature bees present in the combs and the queen still present. Additional findings indicate an absence of damaging levels of the gut parasite Nosema or parasitic varroa mites at the time of collapse. Data on overall honeybee losses for 2010 indicate an estimated 34 percent loss, which is statistically similar to losses reported in 2007, 2008 and 2009.
Following up on efforts from previous years, researchers continue to analyze bee samples for pesticide residues and pathogen loads to determine possible links to declines. Studies continue to demonstrate high levels of pathogens in CCD-affected samples and lower pathogen levels in non-affected samples, consistent with the observation that healthy honeybee colonies normally fend off pathogens – leading researchers to think bee declines are resulting from immune suppression, according to the report.
A large survey of healthy and CCD-affected colonies also revealed elevated levels of pesticides in wax and pollen, but the amounts were similar in both failing and healthy colonies.
Supporting the hypothesis that CCD is caused by a combination of factors is a series of studies within the National Institute of Food and Agriculture’s (NIFA) Coordinated Agricultural Project (CAP). CAP scientists reported that, in addition to the demonstration that pesticide application rates are highly variable regionally, they’ve identified interactive effects among pesticides, the parasite Nosema and viruses – including the Israeli Acute Paralysis Virus (IAPV) and deformed wing virus. Another multi-year study is taking place across multiple states to examine additional interactions.
CAP also identified sub-lethal effects of neonicotinoids and fungicides on bees. It is hypothesized that these pesticides impair the bee’s immune system, which leaves the bee more susceptible to viruses.
Separate research has independently demonstrated synergistic effects between various pesticides and pathogens. A study by Pennsylvania State University (PSU) scientists, funded by the National Honey Board, identified potential interactions among numerous pests, pathogens and pesticides. Scientists also found that one fungicide (chlorothalonil) was linked with “entombing” behavior in bees (a defensive behavior associated with poor health) and had particularly detrimental effects.
PSU recently conducted preliminary studies that indicate IAPV has an East Coast and a West Coast variant. IAPV was believed to be linked with CCD in 2007. The West Coast IAPV variant appears to be more closely related to the predominant strain in Australia. Currently, studies are underway to compare the virulence of the East Coast and the West Coast strains of IAPV, and to better understand any association with other factors linked to CCD.
With the honeybee genome, and with funding from NIFA, University of Illinois scientists have identified potential molecular markers for genes that play a role in CCD. Although application of these results is a long way off, these markers will be highly useful for breeding bees with enhanced tolerance to pesticides and/or pathogens associated with CCD and declining bee health in general.
Factors affecting bee health
Given the knowledge of high pathogen loads in diseased bee colonies, researchers have increasingly focused on possible interactive effects between pesticides and pathogens, including Nosema and bee viruses. Two recent studies have demonstrated increased levels of the gut parasite Nosema following exposure to sub-lethal levels of imidacloprid. As noted previously, a survey of healthy and CCD-affected colonies revealed an elevated level of pesticides in wax and pollen, but the amounts were similar between failing and healthy colonies.
Bioassay studies conducted at the University of Illinois at Urbana-Champaign and University of Nebraska at Lincoln revealed synergistic effects of different chemicals on honeybee health. The studies revealed that these compounds (such as the pyrethroid pesticide fluvalinate, the organophosphate pesticide coumaphos and the fungicide prochloraz, if used in combination) produced an increase in toxicity to bees, over use of the products individually. These studies show that the pesticides interfere with the bees’ detoxification processes, indicating that these pesticides can be safely used individually, but point to specific pesticide and fungicide combinations that may harm bees and should be avoided, according to the report.
Mitigation and management
Significant progress has been made in the area of mitigating the honeybee crisis. This work is highlighted by two national multi-year projects, the ARS (Agricultural Research Service) Areawide Project on Honey Bee Health and NIFA.
In particular, research continues to focus on developing germplasm with the varroa sensitive hygiene trait, shown to protect bees against the most significant bee pest, the varroa mite. As these efforts continue, varroa-resistant bee stocks will become increasingly available to beekeepers and will reduce the overall impact of this major known cause of bee decline.
ARS has also supported several studies informing honeybee diet recommendations for beekeepers. Studies have supported prior findings that supplemental protein feedings can strengthen honeybee colonies, while high-fructose corn syrup may cause colony stress.
Research is ongoing regarding the development of non-Apis pollinators intended to provide alternatives to honeybee pollination. It is important to note that ARS- and NIFA-funded scientists have made significant progress toward developing the blue orchard bee for almond pollination, according to the report.
Pollinator studies have also focused on exploring bumblebees, seeking to determine the relationship between bumblebee and honeybee pests. Studies have revealed that bumblebees are affected by IAPV and other viruses and parasites that affect the honeybee, according to the report.
− By Derrek Sigler