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Penn State Study on Varroa Mites Published - June 2005

June 3, 2005

UNIVERSITY PARK, Pa. - A paper written by a postdoctoral scholar and a professor of entomology at Penn State about their study on varroa mites suppressing the immune systems of honey bees was recently published in a prestigious scientific journal.

"Impact of an ectoparasite on the immunity and pathology of an invertebrate: Evidence for host immunosuppression and viral amplification," written by Drs. Xiaolang Yang and Diana Cox-Foster, appeared in the "Proceedings of the National Academy of Sciences of the United States of America", one of the world's most-cited multidisciplinary scientific serials. The paper can be found in its entirety at Web site http://www.pnas.org/cgi/content/abstract/0501860102v1.

Dr. Yang started his work on vorroa mites in honey bees as a graduate student in entomology at Penn State and was the topic of his Ph.D. thesis. According to Dr. Yang, now a postdoctoral scholar who specializes in insect immunology and pathology, honey bees are essential to the pollination of over 90 fruit and vegetable crops worldwide and are valued at over $14 billion annually in the U.S. "Populations of honey bees are in jeopardy due to the 1987 introduction of varroa mites, an external parasite similar to a tick and the major killer of all bee colonies in the U.S.," he explains. "In Pennsylvania, an average of 53 percent of managed honey bee colonies died between 1995 and 1996 from parasites like varroa, and in many regions of the U.S. nearly all feral honeybee nests have died. Many beekeepers are giving up keeping bees due to the mites, thus increasing the cost for growers who must lease hives to help pollinate their crops."

Current methods for varroa control have not been effective because many varroa mites have developed resistance to traditional pesticides. In addition, pesticide applications can cause adverse effects on exposed honey bees and potentially contaminate honey.

Varroa mites have been implicated in several viral diseases in honey bees, including deformed wing virus (DWV). "It is unclear how varroa mites kill honey bees, but researchers hypothesize that the mites compromise the immune systems of honey bees, making them more susceptible to disease. In our research, we used honey bees, varroa mites and DWV to examine these interactions," says Dr. Cox-Foster.

The bees used in the research project were from a mite-infested colony at the University Park Apiary at Penn State. According to Yang and Cox-Foster, the bees were divided into three groups according to mite infestation and wing deformity, a typical symptom of a varroa mite infestation. "After running several tests, which included exposing a group of bees to a bacterium, we were able to conclude that expression of genes responsible for regulating immunity in bees was significantly suppressed in mite-infested bees, thus making them more susceptible to diseases and that the degree of wing deformity was significantly related to varroa mite density," Yang explains. They have demonstrated the first example of an ectoparasite suppressing the immune system of its invertebrate host, much the same as ticks immunosuppresing their vertebrate hosts. Drs. Yang and Cox-Foster also discovered that in order for the bees to become highly infected with DWV, both exposure to the mites and bacteria was needed. "This may explain why, in a statewide survey in Pennsylvania and experiments in South America, bee colonies treated with antibiotics survived significantly better than the untreated colonies. Bacterial colonies are often found on varroa feeding sites in some bee pupae," Dr. Cox-Foster explains.
The researches say their work gives insight as to how parasites such as varroa mites can affect of the immunity and pathology in honey bees. "We hope this may lead to the development of methods to control varroa mite-related damage to honey bees," Dr. Yang reports.

The work was supported by the Graduate Student Competitive Grant sponsored by the College of Agricultural Sciences at Penn State and by a grant from the Pennsylvania Department of Agriculture.
Established in 1963, Penn State's Department of Entomology has grown into a well-balanced department providing undergraduate education, graduate student training and extension outreach education. Twenty faculty and more than thirty graduate students work on a variety of research topics, including chemical ecology. The department is part of Penn State's College of Agricultural Sciences. For more information, contact the department at (814) 865-1895 or visit the department's Web site at http://www.ento.psu.edu/.