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Varroa Mite Infestation In US Worse Than Previously Believed: Multi-Year Study

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The first multi-year honeybee disease study in the United States has revealed that varroa mite infestations in the country are far worse than what was previously believed, as the population of the deadly pests is more abundant than ever.

Scientists from the Department of Agriculture and the University of Maryland conducted the research to look into the ill effects of diseases that plague honeybees in the United States, including varroa mites, fungal gut parasites and several other debilitating viruses. Their results provided a crucial five-year baseline against which to track future trends.

Researchers had found that the varroa mite has become abundant in the country and is closely linked to several dangerous viruses. Additionally, the Chronic Bee Paralysis Virus used to be rare in the country, but has boosted in prevalence since its first detection in 2010.

The findings of the study are based on a survey of beekeepers and bee colony samples in 41 states, as well as Guam and Puerto Rico. The results span from 2009 to 2014.

Scientists looked at the two major parasites that afflicted honeybees: the varroa mite and the nosema. The latter is a fungal parasite that hinders the digestive system of a bee.

There were clear annual trends in the prevalence of both parasites, the study found. In fact, varroa infestations had peaked in late summer or early fall, while nosema peaked in the winter.

Aside from that, researchers found clear differences in the prevalence of nosema and varroa mites between stationary and migratory beehives.

Those who drive their migratory beehives across the United States every summer to pollinate crops reported lower levels of varroa compared with those who stay put with their stationary beehives all year round.

The reverse was true for nosema, however, as incidence of this fungal infection was fewer among stationary beehives.

More than 50 percent of the beekeeping operations contained high levels of varroa infestation at the start of winter, an important time when bee colonies produce long-lived winter bees that must survive on stored honey and pollen.

"We knew that varroa was a problem, but it seems to be an even bigger problem than we first thought," said Dennis vanEnglesdorp, co-author of the study.

VanEnglesdorp said they were surprised at the high level of varroa especially in the fall, as well as in well-managed colonies under the care of beekeepers who have taken precaution to control the mites. He said the varroa mites' ability to spread viruses presents a direr situation than what they expected.

Varroa mites are vectors for viruses, and lead study author Kirsten Traynor says these pests are basically "diry hypodermic needles." The varroa mite is known to cause a devastating disease among bees called "deformed wing virus."

But there is a small piece of good news. Researchers say three possibly damaging species have not yet found their way into the United States: the Asian honeybee Apis cerana, the slow bee paralysis virus and the parasitic tropilaelaps mite.

Despite emerging efforts that shift focus to poor honeybee health, Traynor said their research is the first to conduct a systematic survey to establish disease baselines so they can track changes over time.

"It highlights some troubling trends and indicates that parasites strongly influence viral prevalence," explained Traynor.

The team's findings are featured in the journal Apidologie.

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