Researchers conducted careful studies to uncover contributors to the disorder, in which seemingly healthy bees simply vanish from a hive, leaving the queen and a handful of newly hatched adults behind.
"One of the first things we looked at was the pesticide levels in the wax of older honeycombs," researcher Steve Sheppard said.
The researchers acquired used hives from the U.S. Department of Agriculture, finding that they had "fairly high levels of pesticide residue." When bees were raised in these hives, they had "significantly reduced longevity," the researchers said.
Prior research by scientists from Pennsylvania State University found unprecedentedly high levels of two pesticides in every sample of honeycomb or foundation wax tested, as well as lower levels of 70 other pesticides.
The pesticides found in the highest concentrations were fluvalinate and coumaphos, used to eradicate the bee pest varroa mites, which have themselves been suggested as a cause of colony collapse.
"We do not know that these chemicals have anything to do with colony collapse disorder, but they are definitely stressors in the home and in the food sources," said Penn State researcher Maryann Frazier. "Pesticides alone have not shown they are the cause of [colony collapse disorder]. We believe that it is a combination of a variety of factors, possibly including mites, viruses and pesticides."
The Washington State researchers uncovered another potential cause, which likely interacts with chemicals to contribute to colony collapse: the pathogen Nosema ceranae, which entered the United States around 1997 and has since spread to bee hives across the country. The pathogen attacks bees' ability to process food and makes them more susceptible to chemicals and other infections.
"What it basically does is it causes bees to get immune-deficiency disorder," said beekeeper said Mark Pitcher of Babe's Honey. "So it's actually causing the bees to almost get a version of HIV."