A higher total load of pathogens — viruses, bacteria and fungi — appears to have the strongest link with Colony Collapse Disorder found so far, according to a new study published by Agricultural Research Service (ARS) and university scientists.

The study was headed by Pennsylvania State University entomologist Dennis vanEngelsdorp and entomologist Jeff Pettis, geneticist Jay Evans and virologist Yanping Chen with the ARS Bee Research Laboratory in Beltsville, Md. They looked at more than 200 individual variables in 91 colonies from 13 apiaries in Florida and California, where many beekeepers overwinter their honey bees. Among the factors for which the researchers screened were bacteria, mites, Nosema (protozoan parasites), numerous viruses, nutrition status and 171 pesticides. Adult bees, wax comb, beebread (stored and processed pollen), and brood were all sampled.

No single variable was found consistently in only those honey bee colonies that had Colony Collapse Disorder (CCD) — a syndrome characterized by the sudden disappearance of adult honey bees in a colony — that has been devastating some beekeepers in the United States and other countries.

Among the colonies that had CCD, no single pathogen among those screened had a higher prevalence. Nor was there a higher or lower prevalence of varroa mites, tracheal mites or spores from Nosema species, nor a higher total load of these parasites and pathogens. Nosema has been implicated in colony die-offs in Spain, but it has not been closely associated with CCD in the United States.

But overall, CCD colonies were co-infected with a greater number of pathogens — bacteria, microparasites like Nosema, and viruses. Overall, 55 percent of CCD colonies were infected with three or more viruses, compared to 28 percent of non-CCD colonies. The researchers also found detectable levels of residues from 50 different pesticides in all of the sampled colonies. There was no association between increased pesticide levels and CCD.

In fact, the pyrethroid insecticide Esfenvalerate, used for a wide variety of pests such as moths, flies, beetles and other insects on vegetable, fruit and nut crops, was more prevalent in the wax in non-CCD colonies. This insecticide was found in 32 percent of non-CCD colonies, compared with 5 percent of the CCD colonies.

Coumaphos, which is used to treat varroa mites in honey bees, was also found in higher levels in non-CCD colonies.

As for pathogen levels, what the study cannot show is whether the higher pathogen load was involved in the cause of CCD or was a result of CCD, according to vanEngelsdorp. Higher pathogen loads are likely to have caused CCD symptoms, but what causes the bees to become infected with so many pathogens is still not known, he added.

While the study’s results don’t indicate a specific cause of CCD, the results do help scientists narrow the direction of future CCD research by showing that some possible causes are less likely, added Pettis.

ARS is the principal intramural scientific research agency of the U.S. Department of Agriculture.

The scientific article can be found athttp://www.plosone.org/article/info:doi/10.1371/journal.pone.0006481;jsessionid=FD7EE265CBA1AC5323F39D61A272CDAF.

A report that summarizes research progress on CCD can be found at http://www.ars.usda.gov/is/br/ccd/ccd_progressreport.pdf.