Varroa and virus insights

Honeybee colonies around the world are dying at alarming rates. A variety of factors has been proposed to explain their decline, but the exact cause—and how bees can be saved—remains unclear. Varroa is thought to be one of the main stressors that reduce bee fitness. As they feed on the blood of pupae and adult bees, the mites can transmit several honeybee viruses with high efficiency.

Uncontrolled Varroa infestation can thereby cause an accelerating virus epidemic and so kill a bee colony within two to three years.

A new study by PhD student Fanny Mondet, from the University of Otago's Department of Zoology, and INRA, Avignon, France, examines the viral landscape in honeybee colonies in New Zealand after the recent arrival of the parasitic Varroa destructor mite.

Published in PLOS Pathogens, the study focuses on the complex interplay between bees, mites and viruses, taking advantage of a unique situation in New Zealand. The country was only recently invaded by Varroa, which was first detected on the North Island in 2001, and still had an active infestation expansion front traveling southward into Varroa-free areas of the country when the study took place.

The researchers monitored the first stages of the Varroa infestation and its consequences for bees and bee viruses.

The study reports that the arrival of Varroa dramatically changed the viral landscape within the honeybee colonies of New Zealand. Each of seven different virus species examined in detail responded in a unique way to the arrival, establishment, and persistence of the mite.

Consistent with observations in other countries, Deformed Wing Virus (DWV) is the virus most strongly affected by the spread of Varroa throughout New Zealand. DWV, which can multiply in the mites and is thought to be a direct cause of Varroa-induced colony collapse, was almost never seen in New Zealand bee colonies before the arrival of Varroa, or ahead of the expansion zone after 2001. Thereafter, DWV abundance gradually increased with Varroa infestation history, even when Varroa infestation rates declined.

Another highly virulent Varroa-transmitted virus, Kashmir Bee Virus (KBV), also showed a close association with Varroa. However, in contrast to DWV, KBV abundance peaks two years after an initial Varroa infestation and subsequently disappears from the colonies entirely, leaving DWV as the dominant honeybee virus in long-term Varroa-infested areas.

"The results of this study strengthen the idea that in Varroa infested bees multiple virus species interact to create a dynamic and turbulent pathological landscape, and that viruses play an important part in the survival or collapse of honey bee colonies infested by Varroa," says Ms Mondet, who is the lead author.

"For example, KBV could play a key role in the dramatic honeybee colony weakening observed during the first years of Varroa infestation."
The researchers hope that the results to date will be useful for the beekeeping industry by highlighting the importance of beekeeper awareness, of mite monitoring, and the timing and efficiency of Varroa control.

Otago zoologist Professor Alison Mercer adds that future research will focus on the mechanisms that form the evolutionary basis for the bee-Varroa-virus interaction.
This work was supported by a grant from the New Zealand Honey Trust Industry Fund.