The Varroa destructor mite is a honey bee parasite that feeds much like a tick on the body of a bee. The mites are about the size of a pin head, dark brown in color and visible on close inspection.
This bee mite probably arose in the Eastern or Chinese Honey Bee population and hopped over to the United States in 1987. They quickly infested western or European honey bees. One sign of infection is the presence of bees with deformed wings. Also, sometimes seemingly healthy colonies become ill and the complete hive collapses in about two weeks.
"The native Chinese bees do not have the same problems," says Dr. Xiaolong Yang, post doctoral researcher in entomology and plant pathology, who raised bees in China. "I do not recall seeing deformed wing bees in the Chinese bee. Chinese honey bees have grooming behavior which can remove the mites from the bees. They get rid of the mites."
While researchers know that the Varroa mite is behind the death of bee colonies, the mechanism causing the deaths is still unknown. Yang and Dr. Diana L. Cox-Foster, Penn State professor of entomology, now believe that a combination of bee mites, deformed wing virus and bacteria is causing the problems occurring in hives across the country.
"Once one mite begins to feed on a developing bee, all the subsequent mites will use the same feeding location," says Cox-Foster "Yang has seen as many as 11 adult mites feeding off of one bee. Other researchers have shown that both harmful and harmless bacteria may infect the feeding location."
Deformed wing virus is endemic among honey bees in the U.S., although when the European bees became historically infested with this virus, is unknown. However, simply having deformed wing virus does not cause bees to emerge from the pupa state with deformed wings, nor does it cause colony deaths.
"A group of Japanese researchers found that a virus that is 99 percent the same as deformed wing, appears in in the brains of aggressive guard bees," says Cox-Foster. "Guard bees that are aggressive better protect the hive, so there may be some positive effect in this virus that allows it to persist in a colony."
The combination of bee mite infestation and deformed wing virus does cause deformed wings in about a quarter of the emerging bees. This, however, does not lead to sudden hive collapse. Something else is involved that makes bee mites so harmful to bee colonies.
The Penn State researchers report their findings in today's (May 17) online version of the Proceedings of the National Academy of Science.
Yang and Cox-Foster looked at how bee mites affect the bee immune system. They injected heat-killed E. coli bacteria into virus-infected bees that were either infested with bee mites or mite free. The dead bacteria was used to trigger an immune response in the bees in the same way human vaccines cause our bodies to produce an immune response. They checked the bees for production of chemicals that disinfect the honey and for other immunity related chemicals.
They also measured the amount of virus in each bee. Surprisingly, they found that the virus in mite-infested bees rapidly increased to extremely high levels when the bee was exposed to the bacteria. The virus levels in mite-free bees did not change when the bee was injected with bacteria.
One chemical, GOX or glucose oxidase, is put into the honey by worker bees and sterilizes the honey and all their food. If bees have mites, their production of GOX decreases.
"As mites build up, we suspect that not as much GOX is found in the honey and the honey has more bacteria," says Cox-Foster. "It is likely that the combination of increased mite infestation, virus infection and bacteria is the cause of the two-week death collapse of hives."
The mites suppressed other immune responses in the bees, leaving the bees and the colonies more vulnerable to infection. The bee mites transfer from adult bees to late stage larva. The virus can be transferred through many different pathways.
"This system is important not only because of what the mites are doing to honey bee populations in the U.S., but because it can be used as a model system for exploring what happens to viruses in animal or human populations," says Cox-Foster. "If we view the colony as a city, then we have a variety of infection modes - queen to eggs, workers to food supply, bee to bee, and parasite to bee."
The Pennsylvania Department of Agriculture supported this work.