Bee health – interconnected between species and populations
In my work, I discuss bee health mainly related to honey bees. But of course, if we talk “bees” it’s all about the whole 20,000 species worldwide. And as all organisms, bees – each of these species – has a variety of parasites and diseases to cope with. Some of them are specific for certain species, but many are not and may spread from one to the other. Managed bees, mainly honey bees but also bumblebees, play an important role in this.
Earlier this week, there was a discussion on Twitter around some photos posted by @SuffolkNature. They showed a female Colletes bee, with deformed wings.
As the honey bee pathologist I am (I always say my patients have at least six legs), I immediately thought of Deformed Wing Virus (DWV), which produces these symptoms. Especially as we already know that this virus spreads between species; I reviewed this in an earlier post. It became special attention since it is clear that it’s associated with Varroa mites. DWV was present already before this parasite was introduced, but didn’t make any “clinical symptoms”. The crippled wings and shortened abdomens appeared only after Varroa mites arrived.
Spreading viruses to wild pollinators
Now, this virus made its way on bumblebees and solitary bees, too. What confuses me is the presence of symptoms – the deformed wings – even without the presence of Varroa mites. I knew this from bumblebees, there are observations on solitary bees like the Colletes above and maybe it spreads over also to other insects. We know concerningly little about the development of the infection in bumble bees and solitary bees. If in honey bees the symptoms appeared only after the Varroa mite made its appeareance – why do we see this also in the other bee species? Varroa mites are honey bee parasites, at least until now there is no sign they switched also to bee species outside the genus Apis. So, are there other factors that induce the crippled wings or does the virus cause more harm in other species?
You may say that it doesn’t matter how the disease develops. That the pure fact that a disease spreads between species is concerning enough. But, if we want to go further than just acknowledging a fact, if we want to prevent or stop the spread, we need to know more. What we know is that honey bees are the drivers for the spread of DWV. In 2014, Fürst and colleagues wrote about the shared diseases between honey bees and bumblebees in UK. They found that honey bees and bumblebees shared the same viral strains and that there was a local transmission between these bees. The driver, however, were honey bees. Wilfert and colleagues confirmed this last year: the virus originates from European Honey Bees and the spread follows the spread of Varroa mites.
Condition-dependent consequences of infections
There are lots of viruses that have “jumped over” from honey bees to wild bee populations. I won’t make a list of them, you can find a very good review here. What is important in the context of bee health is not only the presence of a pathogen. It’s also the conditions in which it makes its host sick. Only since a few months there is some evidence for another honey bee virus which switched over to bumblebees: the Slow Bee Paralysis Virus (SBPV). Also this pathogen is associated with Varroa, I will explain later why I insist on this so much. Now however, the point is that this virus affects bumblebees. They live for a shorter period when they carry SBPV. But – only if they starve. If the bumblebees got enough food, they didn’t show any symptoms and lived as long as non-infected bees.
Although these were lab experiments (it is difficult to do experimental infections in the field…), these are highly relevant data. It shows that bee health is much more than the presence or absence of pathogens. Enough and good quality nutrition helps to keep bees healthy. I wonder how deep we are in the spiral of emerging diseases, habitat loss and degradation, pesticides, climate change and all the factors causing pollinator decline. This paper shows the interaction not only of different bee species “sharing” their diseases, but also the influence of nutrition. If you consider last week’s post, this may even get further importance. Bees need flowers not only for food but also for self-medication.
Bee health in relation to managed pollinators
I promised that I would explain why I mentioned Varroa mites and their association with viral diseases so often. Well, it’s quite simple. It’s because we can treat against these mites. But, it has to occur in time and with the right methods. Many beekeepers are responsible and do so. But others don’t. And they don’t put at risk only their own colonies, which is bad enough, but also their neighbours’ and wild pollinator populations. Of course, there are still knowledge gaps and also nutrition has an influence. But, if the spread of DWV follows the spread of Varroa, this may be also the case for other viruses associated with the parasite. Another point is that it’s not only the presence of the viruses, but also their “quantity” increasing the risk of transmission. Varroa treatments in time keep the DWV level low and, therefore, reduce their spread.
It’s a bit more complicated than that, of course. But as there is increasing evidence that managed bees (honey bees and bumble bees) are drivers for disease spillover, we could at least start with the most practicable step we already can do. In parallel, more research has to be done to increase the measures to prevent the spread. We have to understand the dynamics of disease, how and under which conditions it develops to encounter it properly. As I already discussed some weeks ago: taking action and research are complements, not opposites for helping pollinators.