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Notes from the Lab

Notes from the Lab – September 2019

- September 1, 2019 - Scott McArt - (excerpt)

wild bees and varroa

If you don’t control varroa, you’re probably making wild bees sick and reducing overall pollinator health in your community.

Do you know the Varroa levels in your colonies right now? If you’re like many beekeepers who I interact with, you don’t.

And that’s surprising since varroa and its associated viruses are the major driver of honey bee colony losses throughout the world. This is especially important in September (i.e., right now), since we’re entering peak varroa season in many parts of the United States.

Because varroa only parasitizes one bee species in the U.S. — the western honey bee, Apis mellifera — you might think that any varroa problems in your colonies will be restricted to your bees. But is that the case? This is the topic for our twenty-second “Notes from the Lab,” where we highlight “RNA virus spillover from managed honeybees (Apis mellifera) to wild bumblebees (Bombus spp.),” written by Samantha Alger and colleagues and published in the journal PLoS One [14(6):e0217822 (2019)].

As I write this, I’m on the plane home from the International Conference for Pollinator Biology, Health and Policy, which was held at the University of California at Davis Honey and Pollination Center ( During the conference, I counted eight (8!) talks with data linking varroa, RNA viruses, and honey bee colony losses. Many of you know about the virus-mediated link between varroa and colony losses, but for those who don’t, here’s an analogy:

Do you know someone who’s had Lyme disease, Rocky Mountain Spotted Fever, or malaria? Each of those diseases are transmitted by a relatively benign pest — a tick or mosquito that bites you and collects a tiny fraction of your blood. But, in doing so, that tick or mosquito transfers a harmful microbe into your body — the bacteria Borrelia burgdorferi or Rickettsia rickettsii in the case of Lyme disease or Rocky Mountain Spotted Fever, or the protozoan Plasmodium spp. in the case of malaria. It’s these microbes that do the real harm to your health, not the tiny bit of blood taken by the tick or mosquito.

Similarly, it’s not varroa that kills your bees, it’s the highly virulent viruses it transmits that kill your bees. Several RNA viruses are known to be vectored by varroa, and researchers have found links between many of them and honey bee survival. From these studies, three viruses stand out. Black Queen Cell Virus (BQCV), Israeli Acute Paralysis Virus (IAPV) and Deformed Wing Virus (DWV) are commonly associated with varroa, and DWV especially has been linked to colony losses.

Because a growing number of recent studies have found these viruses and other pathogens in wild bees, Alger and colleagues decided to set out and test whether honey bees could potentially be an important source of RNA viruses in wild bees. In other words, if the colony in your apiary gets sick, could your bees spread viruses to wild bees?

For their study, the authors sampled honey bees, bumble bees (Bombus vagans and B. bimaculatus) and flowers from sixteen sites, seven of which were within 300 m of a commercial apiary, and twelve of which were at least 1 km from any apiary. Bees were collected while they foraged at flowers across a large area at each site. At sites with apiaries, honey bee foragers were also collected directly from hive entrances. Four of the seven sites without an apiary nearby did not have honey bees present, so only bumble bees and flowers were collected at those sites.

With nearly 400 bee and flower samples in hand, Alger and colleagues then went back to the lab and tested each sample for levels of BQCV, IAPV and DWV. In addition, they sequenced the ….