PENN STATE (US)—Honeybees in colonies affected by colony collapse disorder (CCD) have higher levels of pathogens, but what causes the bees to become infected in the first place is still puzzling, according to a new study by an international team of researchers.
“Although pathogens seem likely to play a critical role in CCD, that role may be secondary, much like AIDS patients die from secondary diseases,” explains lead author and Penn State entomologist Dennis vanEngelsdorp.
“Our results suggest that this condition may be contagious or the result of exposure to a common risk factor that impairs the bees’ immune systems, making them more susceptible to pathogens,” says vanEngelsdorp, who is acting state apiarist for the Pennsylvania Department of Agriculture.
The researchers collected samples of adult bees, wax comb, pollen, and developing larvae from 91 colonies in 13 apiaries in Florida and California. They quantified more than 200 variables, including the presence of parasites such as varroa and tracheal mites; infection by bacteria, viruses, and fungi; pesticide levels; nutritional factors; and bee physiology. No single factor was consistently found in colonies suffering from the disorder.
None of the screened pathogens had a higher prevalence in colonies that had CCD and there also was no significant difference in the prevalence or in the total load of varroa or tracheal mites and Nosema, a protozoan that causes disease in bees.
But overall, CCD colonies were coinfected with a greater number of pathogens. For instance, 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, but there was no association between increased pesticide levels and CCD and in some cases, insecticides were even more prevalent in non-CCD colonies.
The study suggests that future research should focus on interactions among pesticides and pathogen loads, says Jeff Pettis, an entomologist with the ARS Bee Research Laboratory.
“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,” he explains.
Researchers from the University of Liege and Gembloux Agricultural University in Belgium, North Carolina State University, and the U.S. Department of Agriculture’s Agricultural Research Service contributed to the study. The study’s findings were published in the online journal PLoS One.
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