Global Bee Collapse: Varroa Mites Are the Culprits, Not "Deadly" Virus

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.

A Big Misunderstanding

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.

This diagram illustrates how researchers took samples from the honeybee pupae and transferred them to others, simulating a parasite transmitting an infection.Royal Society Publishing/ Remnant et al

So Why Are Colonies Collapsing?

This experiment illustrates the dynamic between DWV and the mites, but it doesn’t quite explain why bee colonies are collapsing. For that answer, we look to the mites themselves.

Previous research had suggested that varroa mites increased the destructiveness of the virus, making it deadlier before it infected bees. But as the researchers showed, it’s the mites that are deadly. Inverse reported earlier this year on another breakthrough paper showing that varroa mites are extremely destructive all on their own — beyond their ability to carry diseases.

Scanning electron microscope images of the wounds left by the Varroa destructor mites show clear evidence that they're feeding on bees' fat bodies, not just on blood.PNAS/ Ramsey et al

USDA entomologist Samuel Ramsey, Ph.D., previously told Inverse about his team’s work on varroa mites, which showed that the mites feed on a vital bee organ that bees can’t live without. The mites themselves — not just the diseases they carry — are deadly to honeybee colonies, Ramsey’s team concluded.

“I was very excited, specifically because this is something they’ve believed about these arachnids for more than half a century now, and it’s gone unquestioned for years and years and years,” he said.

The new study goes a step further in explaining the relationship between bees, parasites, and diseases, showing that the picture between the varroa mites and DWV is far more complicated than previously thought.

Of course, there are some limitations to the study, which include the fact that the researchers only injected pupae, and not full-grown adults, while the mites feed on both stages.

Nonetheless, this study sheds light on what is and isn’t driving the worldwide bee collapse — which is crucial to understanding how to solve it.

In short, they write, “our results show that the known association between “V. destructor” and DWV is more complex than initially thought.”

Abstract: The arrival of the ectoparasitic mite Varroa destructor on the Western honeybee Apis mellifera saw a change in the diversity and prevalence of honeybee RNA viruses. One virus in particular, deformed wing virus (DWV) has become closely associated with V. destructor, leading many to conclude that V. destructor has affected viral virulence by changing the mode of transmission. While DWV is normally transmitted via feeding and faeces, V. destructor transmits viruses by direct injection. This change could have resulted in higher viral prevalence causing increased damage to the bees. Here we test the effect of a change in the mode of transmission on the com- position and levels of honeybee RNA viruses in the absence of V. destructor. We find a rapid increase in levels of two viruses, Sacbrood virus (SBV) and Black queen cell virus (BQCV) after direct injection of viral extracts into honeybee pupae. In pupae injected with high levels of DWV extracted from symptomatic adult bees, DWV levels rapidly decline in the presence of SBV and BQCV. Further, we observe high mortality in honeybee pupae when injected with SBV and BQCV, whereas injecting pupae with high levels of DWV results in near 100% survival. Our results suggest a different explanation for the observed association between V. destructor and DWV. Instead of V. destructor causing an increase in DWV virulence, we hypothesize that direct virus inoculation, such as that mediated by a vector, quickly eliminates the most virulent honeybee viruses resulting in an association with less virulent viruses such as DWV.In short, they write, “our results show that the known association between “V. destructor” and DWV is more complex than initially thought.”