Study: Higher pathogen loads in honey bees affected by colony collapse disorder


UNIVERSITY PARK, Pa. — Honey bees in colonies affected by colony collapse disorder have higher levels of pathogens — and are co-infected with a greater number of pathogens — than their non colony collapse disorder counterparts, but no individual pathogen can be singled out as the cause of colony collapse disorder, according to a new study by researchers from Penn State’s College of Agricultural Sciences, other universities and the U.S. Department of Agriculture’s Agricultural Research Service.

The researchers collected samples of adult bees, wax comb, pollen and brood (developing larvae) from 91 colonies in 13 apiaries in Florida and California and 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 found consistently only in those colonies suffering from colony collapse disorder.


The study’s findings illustrate the complexity of solving the colony collapse disorder problem, according to 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,” said vanEngelsdorp, who also is acting state apiarist for the Pennsylvania Department of Agriculture.

VanEngelsdorp noted higher pathogen loads are likely to have caused colony collapse disorder symptoms, but what causes the bees to become infected with so many pathogens is still not known.

“Although pathogens seem likely to play a critical role in colony collapse disorder, that role may be secondary, much like AIDS patients die from secondary diseases,” he added.

No one of the screened pathogens had a higher prevalence in colonies that had colony collapse disorder. There also was no significant difference in the prevalence nor in the total load of varroa or tracheal mites and Nosema, a protozoan that causes disease in bees.

Great number of pathogens

But overall, colony collapse disorder colonies were co-infected with a greater number of pathogens — viruses, bacteria and microparasites such as Nosema.

For instance, 55 percent of colony collapse disorder colonies were infected with three or more viruses compared to 28 percent of non colony collapse disorder 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 colony collapse disorder.

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 colony collapse disorder colonies, being found in 32 percent of non colony collapse disorder colonies compared to 5 percent of the colony collapse disorder colonies.

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

Future research

Entomologist Jeff Pettis with the ARS Bee Research Laboratory in Beltsville, Md., said the study suggests that future research should focus on monitoring parasite, pathogen and pesticide loads, as well as potential interactions among pesticide and pathogen loads.

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

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