Virus-carrying hoverflies – are they really a threat to wild bees?

A recent item in The Conversation opened an interesting line of debate. The article was heavily slanted towards bees. To me, it implied that there was a potential problem of hoverflies carrying bee viruses and acting as the source for further infection of bees. Surely this is the wrong way of presenting the issue at this stage? After all, the only thing that can be said with any certainty is that some bee viruses have also been found in some Diptera.

Let us start with the knowns:

• Bee populations and especially honeybees have suffered a series of blows from pathogens that include viruses and parasites.
• Wild bee populations have declined significantly over the past 30 years.
• Bee pathogens are present in both wild bee and honeybee populations.
• Hoverflies and a wide range of other insects have also declined by similar or even greater levels than wild bees over the same time-frame.
• Hoverflies (or at least some hoverflies) also carry the viruses that have been detected in bees.

But, there are also unknowns:

• The article states that the bee pathogens have been found in two species of donefly: which ones?
• If they affect droneflies does that equate to all hoverflies?
• And, if these pathogens affect hoverflies, do they also affect other flies? Hoverflies are by no means the only flies to come into contact with bees – what about Calliphoridae, Muscidae, Sarcophagidae and Anthomyiidae, all of which are regular flower visitors and will often be found in association wild bees!
• How long have those pathogens been present in some hoverflies?
• Can pathogens be detected in long-dead specimens – in which case there is a case for investigating museum specimens to determine whether this is a new jump or a long-existing situation?
• Moreover, is the threat to bees, or is it that the pathogens have jumped into a new host and that in turn threatens those new hosts?

And then there are the unknown, unknowns!

The results of this study certainly open up a much wider set of questions. From a wider ecological perspective, it seems to me that the regular movement of pathogen-carrying hive bees may be a factor in the wider decline of invertebrates; especially if those pathogens can jump into the bigger pool of invertebrates. At this stage, the blame game should not be pointing at hoverflies. If anything I would be pointing the finger at commercial use of mobile honeybee hives.

I’m also unconvinced by the argument that migratory hoverflies pose a threat to wild bees! Firstly, the viruses have reportedly been found in two species of dronefly i.e. Eristalis sp. I am far from sure there is much evidence that Eristalis are major migrants on the scale of Episyrphus balteatus. True, Eristalis similis does move about and sporadically turns up in the UK. If others do, I cannot recall ever seeing any evidence in the field. Furthermore, if the genus as a whole is migratory I would expect some of the more obscure European Eristalis to turn up in the UK. So far, we have no evidence of this! It would, however, be very useful to get somebody to work through all collections and check that we are not overlooking other European species. It would need to be somebody who is familiar with these additional species, so perhaps we need to organise a study tour for one of the European specialists?

So, what is undoubtedly an interesting scientific finding has already been sensationalised and nuanced. This is a great shame because there are so many important questions to look at. Of course, if you want research money then you have to point towards the ‘sexy’ subject. Pathogens in bees equates to a threat to bees, which in turn get the kudos as pollinators. BUT, as a field naturalist I would argue that bees only make up a tiny fraction of the full range of ‘pollinators’ and we focus our attention on one very much to the disadvantage of the others; especially if the others are presented as possible vectors of disease. Who is to be sure that the bees are not the vectors and it is the commercial use of honeybees that is threatening a much wider assemblage of pollinators!