Thursday, February 18, 2016

Reemergence of a Virus is Killing Bees

The reemergence of an old virus in a much more dangerous form is threating both wild and managed pollinator populations in the United States and the rest of the world. Pollination is essential for fertilization and for plants to produce seeds and fruit. Without pollination there would be no fruits, no vegetable and no seeds. Though, grasses, conifers, and many deciduous trees are wind-pollinated, most flowering plants that we eat need birds and insects for pollination. The vast majority of plants are pollinated by insects. Some wasps, flies, beetles, ants, butterflies and moths pollinate various flowers, but bees are responsible for the vast majority of pollination. Commercial agriculture uses honeybees raised to pollinate its crops. The native bumble bee is an effective pollinator for dawn blooming flowers like squash and pumpkin.

The honeybee is an immigrant from Europe and an essential element to our monoculture form of agriculture. It is not really surprising since most of our crops and many of our garden plants evolved in areas where honeybees were native, and both crops and insects were brought to the United States with the colonists to become essential parts of our agricultural system. With modern agriculture’s vast fields and groves of a single kind of plant all flowering at the same time; farmers can’t depend on feral bees that happen to nest near crop fields. It was estimated by a Cornell University study that the value of honeybee pollination in the United States is more than $14.6 billion annually.

Over winter, there had always been a certain amount of loss in the honey bee colonies, but during the winter of 2006-2007, a large number of bee colonies died out. Losses were reported to be between 30% to 90% in the impacted beekeeping operations. While many of the colonies lost during this time period exhibited the symptoms from parasitic mites, many were lost, from unknown cause. The next winter, the number of impacted honey bee operations spread across the country. Honeybee colonies died out at even higher rates. The phenomenon was termed Colony Collapse Disorder, or CCD and got the attention of researchers and their government and business sources of funding.

The impacted colonies had low levels of parasitic mites and minimal evidence of wax moth or small hive beetle damage that offered no obvious cause. Since that time researchers have investigated the factors that have contributed to the decline in the honeybee population. Using DNA analysis scientists have now discovered that a reemergence of deformed wing virus (DWV) an endemic honeybee pathogen is an underlying cause of the Colony Collapse. Deformed Wing virus has reemerged in a much more dangerous way. Previous ways to spread the disease were not as effective. A parasitic mite called Varroa destructor native to the Asian honeybee population was introduced into the European honeybee population.

Although separately the virus and the mite were not major threats to honeybee populations, when the Varroa destructor mite carries the deformed wing disease, the combination is deadly, and has wiped out millions of honeybees over the past two decades. The Varroa destructor mite can inject the deformed wing virus directly into the hemolymph system which is an open circulation system where the insect’s “blood” flows freely within body cavities. The infected hemolymph makes direct contact with all internal tissues and organs effectively circumventing the barriers to transmission within and among honeybee colonies. In addition, scientists say there is evidence that Varroa destructor also increases the virulence of Deformed Wing virus turning relatively asymptomatic infection into severe infections associated with disease symptoms and increasing honeybee colony death over winter.

Deformed wing virus not only causes colony mortality in the managed European honeybee population, but also affect feral populations and has been idenfied by researchers as an emerging disease in wild pollinators including bumble bees. Transmission of the deformed wing virus was amplified by in the European honeybee population by the sale and transport of honeybees between Asia and Europe just as the Varroa destructor mite was emerging. Commercial pollinator populations in Europe, Asia and North and South America are interconnected via trade and movement of pollinators. This has resulted in a fairly rapid spread of the deformed winged virus and Varroa destructor mite to the U.S. where honeybee population has been decimated.

Data shows that the recent spread of deformed wing virus and Varroa destructor traveled to Europe from both Asia and Pakistan, and then from there to North America which has emerged as the main hub of transmission to the Americas and Oceania. The virus has little host specificity and is easily transmitted to wild pollinators. DNA research confirms that there is a global pandemic and that deformed wing virus not only kills off managed honeybee populations but also affect feral honey population and has been identified as an emerging disease in wild pollinators, threatening the bumble bee populations.

To prevent the destruction of honeybees and save the native pollinator populations that have been spared on islands such as Hawaii, New Zeland and Australia scientists are now calling for tighter controls. They recommend health screening and regulating the transport and movement of honeybees to maintain the Varroa destructor free refuges so that the European honeybee and native pollinators can be saved.

L. Wilfert, G. Long, H. C. Leggett, P. Schmid-Hempel, R. Butlin, S. J. M. Martin, M. Boots. Deformed wing virus is a recent global epidemic in honeybees driven by Varroa mites. Science, 2016; 351 (6273): 594-597

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