Oh dear, a virus ‘jumps’ from plants to bees

The bane of the bee is varroa. We warn new beekeepers that varroa will kill their bees faster than they can say “varroosis five times.  Varroa kills. Thirty years ago, the mites weren’t as bad as they are now. In those days, they sucked a bit of bee innards, slowly weakening and killing the bees. But over the years, peripatetic mites began to carry viruses from bee to bee. Some researchers suggest that varroa’s viral accomplices cause more damage than the mites themselves. Because of the attached viruses, the effect of varroa is more harmful than it used to be. And the problem may get worse with time as mites encounter new viruses and spread them.

Tobacco leaf with ringspot virus

A new viral culprit was recently identified by researchers at the USDA and the Chinese Academy of Sciences.  Tobacco ringspot virus is incredibly nasty but was thought to just injure plants, not animals. Ringspot almost wiped out the tobacco business (must try harder next time) and causes such grief that farmers may abandon infected crops, plowing them under, and returning only years later when they hope the virus population is low and won’t be a problem for a while. Despite the name, the tobacco virus also affects dozens of different sorts of plants. Now, it seems, it can hurt animals, too. This is new to me – until now, I never heard of a virus jumping from plants to animals. Avian flu jumped to humans and HIV came to humans from chimps – but I didn’t know that a plant virus could hurt some animals.

Normally, a plant virus is benign to insects. Plant and animal cell structure is fundamentally different. Injury shouldn’t occur, but viruses ride in pollen and travel from plant to plant, spreading plant infections like uncovered coughs. In the case of honey bees, the virus is picked up in pollen and brought to the hive where it’s ingested. It’s not uncommon to find a variety of viruses in bee guts and saliva. Typically, the virus does no harm to the insect carrier. It hangs out in the bee, then gets excreted onto some unlucky plant during a bee’s cleansing flight. Liberated, the virus attaches to a new host plant and starts making ringspots again.

In the journal mBio, American and Chinese scientists reported that tobacco ringspot virus affects bee guts – and wings, antennae, blood and all other body parts of bees. The virus is believed to shorten a bee’s life. The virus travels when a varroa mite sucks out the innards of honey bees. As a mite passes from bee to bee, she (phoretic varroa mites are girls) injects victims with the virus as she eats.

You can read the entire paper online, but here’s part of the abstract from “Systemic Spread and Propagation of a Plant-Pathogenic Virus in European Honeybees, Apis mellifera”:

“…In the present study, we showed that a plant-pathogenic RNA virus, tobacco ringspot virus (TRSV), could replicate and produce virions in honeybees, Apis mellifera, resulting in infections that were found throughout the entire body. Additionally, we showed that TRSV-infected individuals were continually present in some monitored colonies. While intracellular life cycle, species-level genetic variation, and pathogenesis of the virus in honeybee hosts remain to be determined, the increasing prevalence of TRSV in conjunction with other bee viruses from spring toward winter in infected colonies was associated with gradual decline of host populations and winter colony collapse, suggesting the negative impact of the virus on colony survival. Furthermore, we showed that TRSV was also found in ectoparasitic Varroa mites that feed on bee hemolymph, but in those instances the virus was restricted to the gastric cecum of Varroa mites, suggesting that Varroa mites may facilitate the spread of TRSV in bees but do not experience systemic invasion.”

From the abstract, above, you’ll note that varroa mites spread the virus but the mites don’t “experience systemic invasion.” Wouldn’t it be great if it were the other way round – a virus carried by bees that doesn’t hurt bees, but kills mites?  I’ll bet someone is working on that right now.

Spreading a virus?

The idea that a plant virus can spread within an animal is an uncomfortable surprise. It reminds us of the original movement of varroa itself from Apis cerana, where it didn’t cause much mischief, to Apis mellifera, where it is devastating. Once again, we have a pest jumping species (actually, in this case, jumping from the plant kingdom to the animal kingdom).

Ringspot now gets added to the growing list of other viruses spread by varroa mites: deformed wing virus, acute bee paralysis virus, varroa destructor virus-1,  the Israeli acute bee paralysis virus and the Kashmir bee virus. The novelty with the tobacco virus is that it shouldn’t reproduce inside honey bees and hurt them, but it does.

What’s this got to do with you and me? Well, two things. If you give up smoking, there will be fewer tobacco plants and that means fewer ringspot viruses and that means healthier bees. (And a healthier you.)

Secondly, if the big problem is unpredictable new mite-carried viruses, control of mites becomes more and more urgent.  The ‘new’ virus warns us that unexpected varieties of these tiny creatures will continue to invade bees and make them sick. We can’t anticipate what sort of virus will be next nor can we create inoculants. (Heck, we can’t even tame the virus that causes human colds.)  So, be prepared – this problem is going viral.

How do you prepare to fight viruses? Rest, drink lots of fluids (chicken soup!), stay warm, and reduce stress. On a deeper level, white blood cells and the hormone interferon help you fight viruses. Similarly, honey bee colonies may shake off some viral infections if the bees are otherwise healthy, have prolific queens (the source of healthy young replacement bees), plenty of nutritious pollen, and strong populations. Spring can be a particularly vulnerable time – bee population is low, queens are aging, fresh pollen is scarce.  Life-cycle stresses weaken the hive. You want strong hives. Strong colonies are more resistant to afflictions of all sorts.

Do everything you can to keep healthy colonies and kill those blasted virus-toting mites. You’ll give your bees a good chance to survive the spring and grow into honey-making hives.

There was a time when tobacco and bees mixed freely.
This is from a 1950s Virginia tobacco festival parade.

About Ron Miksha

Ron Miksha is a bee ecologist working at the University of Calgary. He is also a geophysicist and does a bit of science writing and blogging. Ron has worked as a radio broadcaster, a beekeeper, and Earth scientist. (Ask him about seismic waves.) He's based in Calgary, Alberta, Canada.
This entry was posted in Bee Biology, Diseases and Pests, Ecology, Science and tagged , , . Bookmark the permalink.

3 Responses to Oh dear, a virus ‘jumps’ from plants to bees

  1. Pingback: Oh dear, a virus ‘jumps’ from plants to bees | Raising Honey Bees

  2. Pingback: Oh dear, a virus ‘jumps’ from plants to bees | Beginner Beekeeper

  3. Anonymous says:

    Thanks!And thanks for sharing your great posts every week!


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