Mississippi State University Extension Service apiculture specialist Jeff Harris has an unusual list of reasons for bee colony death.
"My top three reasons for bee colony death are Varroa mites, Varroa mites and Varroa mites," Harris said. "This is my sarcastic response to the heavy emphasis in the press on the effects of insecticides and other pesticides on honeybees.
"Please don't misunderstand me. Insecticides and other pesticides kill honeybees, either acutely by direct exposure to the chemicals or as part of a group of stressors that kills honeybees," he said.
However, Harris said, there is no conclusive link between insecticide or pesticide use and the widespread deaths of honeybee colonies that have been occurring in the U.S., Canada and parts of Europe.
"What is lost by an oversimplified view of colony health is that honeybees suffer from myriad parasites, diseases and other stressors that are more commonly associated with the death of the colony," he said. "Most scientists studying honeybees would rank Varroa mites and the viruses they vector to honeybees as, hands down, the number-one killer of honeybees in the world. Most non-beekeeper members of the public have never heard of Varroa mites. Even some new beekeepers don’t know what they are."
Varroa mites are external parasites that lay eggs in the brood cells within the hive and emerge attached to the host when the bee hatches out of its cell.
"Imagine a tick the size of a basketball attached to your neck," Harris said. "Varroa mites attach to honeybees and suck their hemolymph, which is similar to blood in humans."
Varroa mites also transmit diseases to honeybees. Harris estimated mites vector about 18 different viruses.
"Varroa mites reproduce rapidly and reduce the health of the colony to the point the colonies fail, or collapse," Harris said. "We have found colonies with ample stores of honey and either no bees or a handful of bees left in the hive. As scientists, we had no doubt: high mite populations vector high levels of viruses to honeybees that will ultimately kill the colony."
Since 2006, when episodes of high colony mortality were first reported, millions of dollars have been spent on research into the causes of what became known as colony collapse disorder.
"Scientists came to the conclusion that multiple factors cause these unusually high death rates of bee colonies in some commercial operations," Harris said. "It also became apparent that different sets of stressors cause losses for different beekeepers."
Some beekeepers lost colonies because of a combination of inadequate nutrition related to periods of agricultural drought, stress related to honeybee transportation and parasitism by Varroa mites.
"Although insecticides were acknowledged as contributing to the demise of bee colonies, in most of the key studies into the causes of colony collapse disorder, scientists emphasized the factors causing the most significant problems for honeybees were Varroa mites and the viruses they transmit to honeybees," he said.
At first, the primary method for treating Varroa mites was insecticide, but some mite populations became genetically resistant to the insecticides. Other treatment options with limited effectiveness involve mechanical methods, such as drone-brood trapping or freezing, or natural methods, such as dusting colonies with powdered sugar to increase the bees' grooming behaviors, which results in mite removal.