Fly of the month – bog hoverfly

The bog hoverfly is elusive, easily disturbed and listed as Critically Endangered. Catherine Mitson explains why she has been studying it.

The bog hoverfly.

Many of us enjoy watching hoverflies as they dart around our gardens amongst the bees and wasps. Sometimes, it is hard to tell the difference as many hoverfly species mimic honeybees and bumblebees (there’s even a hoverfly that mimics a hornet!).

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Hoverflies belong to the family Syrphidae which consists of over 250 species in the UK, some of which are incredibly rare. One of these remarkable rare species is the bog hoverfly Eristalis cryptarum.

Those who have been fortunate enough to see the bog hoverfly will be quick to tell you how difficult they are to find. Theoretically, they are an easy species to spot as they have bright orange legs, red gingery hairs on its thorax and yellow banding on their abdomen.

But the bog hoverfly is famously elusive and incredibly easy to disturb. I had to remain in the same position for at least three hours before I got to see a bog hoverfly for the first time. But they truly are a beautiful fly and it was well worth the wait.

As the name suggests, the bog hoverfly is found in wetter habitats, specifically along valley mires of Rhôs pastures (marshy grasslands that are grazed by wandering livestock).

It has a few favourite flowers that they feed on too, such as devil’s-bit scabious, bog pimpernel, bog asphodel and marsh St John’s wort. And similar to its drone fly relatives in the Eristalis genus, the adults have a long flight season between May to late September.

The boggy valley mires of Rhôs pastures are home to the aptly named bog hoverfly. © Catherine Mitson
The boggy valley mires of Rhôs pastures are home to the aptly named bog hoverfly. © Catherine Mitson

The bog hoverfly has always been a scarce species, with previous records scattered across Devon, Cornwall, the heaths of Dorset and the New Forest. Sadly however, by the 1970’s it was only found on Dartmoor National Park in Devon, where it was last seen in 1978.

For a long time, the bog hoverfly wasn’t recorded at all, and it wasn’t until a Dipterists Forum field meeting in 1993 that it was found again on Dartmoor.

By 1999, the bog hoverfly was listed as a priority species on the UK Biodiversity Action Plan because of this obvious decline. Not only this, it has also been given a Critically Endangered status because of the restricted number of sites on Dartmoor we are able to find the bog hoverfly.

As with any species of conservation concern, an understanding of a species ecology is vital. Previous projects have brought to light specific locations on Dartmoor we are likely to find the bog hoverfly, the types of habitat that it prefers as well as its favourite wildflowers to feed on.

However, the biggest hole in our knowledge surrounds its larval ecology. The larvae of other Eristalis species are known as rat-tailed maggots, due to the long tail like extension they use as a snorkel so that they can munch on decaying matter whilst submerged in stagnant water.

The rat-tailed maggots are so named because of the long “snorkel” tail extension. © John Walters
The rat-tailed maggots are so named because of the long “snorkel” tail extension. © John Walters

It is widely assumed by experts that the larvae of the bog hoverfly would be very similar to other Eristalis species and would also be of the rat-tailed variety, but we haven’t yet found bog hoverfly larvae to confirm this.

Finding the larval stage of the bog hoverfly would be a huge step forward in our understanding of the ecology of this species and could help to tailor conservation efforts effectively.

My research project with the University of Exeter attempted to develop a molecular survey tool for the bog hoverfly using environmental DNA (eDNA). eDNA is ‘free’ DNA released by an organism into the environment, and this eDNA can be screened for as a non-invasive surveying technique.

This is a handy tool for particularly elusive species and so, based on the assumption that the bog hoverfly larvae are aquatic, I collected water samples from various sites on Dartmoor in order to screen for bog hoverfly larval eDNA, therefore helping us to determine its presence or absence at a particular location.

Unfortunately, I was unable to detect any bog hoverfly eDNA, but I was able to detect hoverfly DNA from water samples where I was housing a known number of larvae of a closely related hoverfly species. This leaves me feeling optimistic that using molecular methods could still have some potential in the search for the bog hoverfly.

If you ever find yourself knee deep in a sphagnum moss covered bog, keep your eyes peeled for a quick flash of orange. They may be tricky to find but each record is valuable.

The Hoverfly Recording Scheme Facebook page is a great place to share your hoverfly photos to be identified and verified by experts, as well as the Hoverfly Recording Scheme website


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Catherine Mitson is an active volunteer for the invertebrate conservation charity Buglife and has recently completed a Master’s by Research with the University of Exeter studying the ecology and phylogeny of the bog hoverfly.