Australasian Science Magazine

Honeybee colonies around the world are dying at alarming rates, and the Varroa mite is thought to be one of the main stressors reducing bee fitness.

As they feed on the blood of pupae and adult bees, Varroa can transmit several honeybee viruses with high efficiency. Uncontrolled Varroa infestation can thereby cause an accelerating virus epidemic that can destroy a bee colony within 2–3 years.

Now a study by University of Otago PhD student Fanny Mondet, published in PLOS Pathogens, has examined the recent invasion of New Zealand by Varroa, which was first detected on the North Island in 2001 and was still expanding south into Varroa-free areas of the country when the study took place.

Mondet monitored the first stages of the Varroa infestation and its consequences for bees and bee viruses, and found that the arrival of Varroa dramatically changed the viral landscape within New Zealand’s honeybee colonies. Each of seven different virus species examined in detail responded in a unique way to the arrival, establishment and persistence of the mite.

Deformed wing virus (DWV) was the virus most strongly affected by the spread of Varroa throughout New Zealand. DWV is thought to be a direct cause of Varroa-induced colony collapse, and was almost never seen in New Zealand’s bee colonies before the arrival of Varroa, or ahead of the expansion zone after 2001. However, DWV abundance gradually increased with Varroa infestation, even when infestation rates declined.

Another highly virulent Varroa-transmitted virus, Kashmir bee virus (KBV), also showed a close association with Varroa. However, KBV abundance peaked 2 years after Varroa infestation and subsequently disappeared 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,” Mondet says. “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 will highlight the importance of mite monitoring and the timing and efficiency of Varroa control by apiarists.