An invasive parasitic mite has transformed an apparently harmless virus into a global destroyer of honey bee colonies. A new report in Science chronicles the catastrophic effects of the accidental introduction of the parasitic Varroa Destructor mite into the Hawaiian Islands. The deformed wing virus (DWV), which had been carried by 10% of the Hawaiian honey bees affected 100% of the bees after several years of mite infestation and the viral titer became a million times stronger. Several viral variants were selectively replicated by the mite transforming them into colony destroyers over a period of a few years. World trade in honey bees has expanded the range of the Varroa mite from Asia to most of the continents. The authors think that the process they observed in Hawaii had already happened in Europe and the Americas causing bee colonies to collapse.
Many factors are likely to influence the DWV variant population in different colonies, but the arrival of DWV variants that can replicate in the mite means that these strains would rapidly increase in abundance. There have been no major introductions of honey bees into Hawaii, because strict importation regulations have been enacted since the widespread occurrence of Varroa mites. It seems likely that the now mite-associated European DWV variants were already present in honey bee populations before the arrival of the mites. Studies in the United Kingdom and New Zealand (24) have found that DWV infections and colony collapse did not coincide with the arrival and establishment of Varroa, but there was with a 1- to 3-year time lag, which we also observed on Hawaii. This lag appears to be the time required for the selection of virus variants adapted to mite transmission.This study doesn't rule out other possible causes of bee colony collapse such as nicitinoid pesticides. Bees face many more threats than they did fifty years ago because bees are transported globally, spreading diseases and parasites across continents. The introduction of mites into Hawaii offered the unique opportunity to study the disease processes related to mites while other variables remained constant.
But the mite's arrival in Hawaii in 2007 gave scientists a unique opportunity to track its deadly spread. "We were able to watch the emergence of the disease for the first time ever," said Stephen Martin, at the University of Sheffield, who led the new research published in the journal Science. Within a year of varroa arrival, 274 of 419 colonies on Oahu island (65%) were wiped out, with the mites going on to wreak destruction across Big Island the following year.A particular virus, called deformed wing virus (DWV), was present in low and apparently harmless levels in colonies before the mites arrived, the scientists found. Even when the mites first invaded hives, the virus levels remained low. "But the following year the virus levels had gone through the roof." said Martin. "It was a millionfold increase – it was staggering."