Hazardous neighbours

By Athayde Tonhasca

Since their discovery in the 1800s, viruses have confounded scientists and philosophers because they raise questions about the very nature of life. Viruses consist of genetic material (DNA or RNA) coated with protein, and that’s about it. They draw a blank on six of the seven fundamental indicators of a living organism: movement, respiration, response to stimuli, feeding, excretion, and growth. They do better on the seventh – reproduction – but to a point. They do multiply, although only by seizing the cell machinery of a host to make copies of themselves. But viruses have one essential asset: genetic heredity, which allows them to evolve. So viruses are considered living or non-living, depending on who you ask. That’s why they have been defined as ‘biological entities’, ‘at the edge of life’, and ‘existing at the border between chemistry and life’. Considering the viral upheaval that hit us in 2019, we could settle for the definition offered by immunologist and writer Peter Medawar (1915-1987): ‘a virus is a piece of bad news wrapped up in protein’ (although you may be surprised to hear that viruses are fundamental to life on Earth). 

Bacteriophages (viruses that infect bacteria) mobbing a bacterium © Graham Beards, Wikipedia Creative Commons

Living or non-living, viruses act as intracellular parasites, and insects certainly are not immune to them. Some viruses are used as biological control agents against agricultural pests and vectors of diseases, but others are harmful to insects that benefit us, such as the European honey bee (Apis mellifera).

Beekeepers have to deal with a number of pests and diseases, including RNA viruses with ominous names such as acute bee paralysis virus, black queen cell virus, Israeli acute paralysis virus and slow bee paralysis virus. Among this disagreeable family, the deformed wing virus (DWV) is particularly damaging to apiaries in Europe and other temperate zones; DWV variants are among the world’s most widely distributed and contagious insect viruses. Infections are generally detected in workers with shortened abdomens and deformed or missing wings.

A honey bee with deformed wings © Shawn Caza, Wikipedia Creative Commons

The virulence (i.e., the capacity for causing disease) of DWV and other viruses is dramatically heightened by another honey bee scourge: the varroa mite (Varroa destructor). This pest transferred – ‘host jumped’ – from the Asian honey bee (Apis cerana) to the European honey bee in the early 1900s, and since then it has caused havoc to the beekeeping industry around the world. The mite depletes bees’ reserves by sucking up their fluids, and it also injects virus particles into its hosts. The synergistic combination of varroa mite and DWV has had a devastating impact on managed honey bees, with sharp increases of overwintering mortality and colony losses.

A honey bee pupa infested with the varroa mite, a vector of DWV. Image in the public domain

Harm to honey bees is bad enough, but DWV can do much worse. It has been found in bumble bees, mason bees, mining bees, wasps and at least one species of ant and one species of beetle. We don’t know how badly DWV affects most of these non-Apis hosts because infections are typically asymptomatic. Hosts’ immune system may suppress viral replication, or the virus may just be picked up accidentally by feeding on pollen or nectar. But some bumble bees are not so lucky: they die or develop wing deformities when infected by DWV. 

Bumble bees are not parasitized by varroa mites, so they get infected by DWV some other way. The most likely route is through feeding; bumble bees pick up the virus when collecting nectar or pollen from a flower that was visited by a diseased insect – probably a honey bee. Direct virus transmission from honey bees to bumble bees has not been demonstrated, but there is plenty of circumstantial evidence to suggest it: honey bees deposit viruses on the flowers they visit, and apparently viruses occur mostly on flowers near apiaries; bumble bee rates of infection are higher when honey bees are present and almost non-existent when honey bees are not around. 

A dangerous encounter: a bumble bee and a honey bee sharing a flower © Uroš Novina, Wikipedia Creative Commons

Putting all the clues together, flower sharing seems to be responsible for honey bee to bumble bee ‘pathogen spillover’, which happens when a pathogen transfers – or ‘spills over’ – from a reservoir species to another receptive species. There is much to be discovered about bee viruses and hosts’ responses, but what we know suggests we should keep honey bees apart from bumble bees and other pollinators, especially when rare or endangered species are involved. That’s one of the reasons why some countries such as Australia and the United States restrict beekeeping in national parks and other conservation areas.  

Honey bees are incredibly important and valuable: but they can also be a health hazard to fellow pollinators. Awareness of this risk can help us manage bee hives and pollinators’ habitats, for example by planting more flowers throughout the growing season so that the likelihood of spillover is reduced. Which is good for honey bees, for wild pollinators, and for us.