The bees are dying at an alarming rate. Along with pesticides, parasites, and poor nutrition, scientists blame the colony collapse phenomenon on disease. However, one of the most dangerous diseases has just been shown to be quite innocent, revealing the actual threat.
For a long time, scientists blamed the Varroa destructor mite for transmitting deformed wing virus, a disease that does exactly what its name says. They even suspected that these mites make DWV even more deadly to the bees. But in a paper published Tuesday in the journal Proceedings of the Royal Society B, University of Sydney researchers show that it isn’t the virus that’s the problem — it’s the mites.
Viruses like DWV are normally found in bee populations, experiencing seasonal spikes as they’re transmitted through feces. But despite what was once believed about DWV’s danger, the team hypothesized that it isn’t the virus itself that’s super-deadly — it’s the mite. The mite, they thought, actually supercharged transmission rates among members of a bee colony as the mites bit different bees and passed the disease around.
To test this idea, the researchers extracted biological material from bee pupae and then injected it into others, over and over, simulating a mite biting many individuals in succession. Then, they introduced DWV to the colony and observed its spread, as well as the spread of even more deadly viruses transmitted by varroa mites, called Sacbrood virus (SBV) and Black queen cell virus (BQCV).
Like they hypothesized, DWV rapidly spread through the colony when they simulated the mite bites with the repeated injections, helping to explain how V. destructor mites and DWV seem to go hand-in-hand. More importantly, however, they showed that the mites have been the more destructive force all along. For a virus to spread, its hosts must survive: Dead hosts mean dead ends for a virus, so it actually pays off for a virus to not immediately kill its host. As the very fatal viruses SBV and BQCV spread in the colony, many of the pupae they infected died — which meant that SBV and BQCV levels spiked in the experimental colony then quickly dropped off. But DWV, a much less deadly virus, persisted in the population because it didn’t kill its hosts.
“The arrival of V. destructor quickly selects for an increase in the prevalence of the most virulent viruses until they become so virulent their transmission grinds to a halt due to the death of the brood and thus the mites,” write the authors. “Now more benign viruses such as DWV can make their appearance. Hence, instead of V. destructor directly causing a change in virulence of DWV, DWV is simply more favourable to the mite’s lifecycle and therefore given the upper hand after more virulent species have been selected against.”
In short, varroa mites spread DWV as well as other viruses, but the more virulent infections subsist quickly, giving way to what has always appeared as steady rates of DWV.
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