The Varroa mite is a parasite that lives on honeybees. Since the 1950s, it has spread worldwide and is the biggest threat to honeybees today as it spreads diseases. An international study shows that Varroa has had a significant impact on the viruses present in bees. The results are crucial for monitoring and researching the health of honeybee populations.
Varroa feeds on the body tissues of pupae and adult bees. Originally from Asia, this mite initially had the Asian honeybee as its host. However, when the European honeybee was introduced to Asia, Varroa adapted to this new host. Subsequently, it could spread globally due to human transport of European honeybees over long distances. It was discovered in Sweden in the late 1980s.
"Varroa is a major problem for beekeeping due to the viruses it spreads. The best known of these is Deformed Wing Virus, which causes deformed wings. Colonies that are not treated for Varroa die within a few years," says Joachim de Miranda, a researcher at SLU.
In the study, researchers analysed 14 viruses in historical bee samples collected around 2010 from just before and after the Varroa expansion front in different parts of the world. In Scandinavia, these samples were available thanks to previous monitoring projects.
Varroa has caused dramatic changes in the honeybees "virome", the viruses they carry. Deformed Wing Virus is overwhelmingly the most affected by Varroa, but several other viruses also increase in prevalence and/or abundance when Varroa arrives, while some viruses decrease.
One virus that has increased in both prevalence and abundance is the Black Queen Cell Virus (BQCV), which can kill queens in the pupal stage. Sacbrood Virus (SBV), which affects larvae, has only increased in prevalence. However, the Acute Bee Paralysis Virus (ABPV), which causes sudden death in adult bees, has decreased.
The reasons for increases or decreases in viruses depend on factors such as their impact on different stages and their virulence (how much disease and death they cause). One explanation for the decline in ABPV is that it is too virulent when transmitted by Varroa to pupae. These pupae die too early to hatch into adult bees and are removed from the colony, together with ABPV and the transmitting Varroa mite. This is a strong selection against mites that transmit ABPV, and the reason for the gradual disappearance of ABPV from beekeeping over the years.
The results are like a time capsule, a baseline, showing how the situation was 10 years ago. The reason why 10 years ago is relevant, is because at that time there were sufficient ‘before-varroa’ and ‘after-varroa’ samples across the geographic regions for meaningful analyses and conclusions to be made. The results can be used as a reference for current and future changes. Viruses that are increasing will be more relevant in the future, and those that are disappearing will be less relevant for monitoring and research.
"Research has been very focused on Deformed Wing Virus, perhaps too much so, while other viruses have been overshadowed. So maybe the most important purpose of the article has been to restore some balance, so that other viruses also get the attention that is needed," says Joachim de Miranda.
Varroa and the viruses it spreads are not only a threat to the health of honeybees. There is a risk that increased virus levels in honeybees can spill over to wild bees and other pollinating insects. “This is where a lot of the future research is heading”, says Joachim de Miranda.
The study has recently been published in Royal Society Open Science.