Blog updated on 16th October 2014 with confirmation of the mites as being Poecilochirus carabi.
On Sunday 10th August, I spent the day at Dundreggan, leading one of the series of biodiversity skills training courses we’ve scheduled for this summer, as part of the programme of work funded by the £50,000 grant we received from the People’s Millions scheme late last year. This particular course lasted for a day and was focused on one of my special interests – plant galls. Although we spent most of the day outside (in the rain!) locating galls on various trees and plants, we began with a Powerpoint presentation I’d prepared about plant galls.
I showed this in the lodge at Dundreggan, and we all had a cup of tea or coffee while I gave this illustrated introduction to galls. While the tea was being prepared, one of the participants made an unexpected discovery – they found a large beetle in one of the mugs in the kitchen! When they brought it out to show me, I recognised it as a burying beetle, although I couldn’t say which species it was at the time. I also had no idea how a beetle like this, which is normally found on the forest floor, ended up being in a mug in the kitchen of the lodge at Dundreggan!
As soon as I saw the beetle, I noticed something unusual about it – there were a number of small mites on it swarming all over its head and the underside of its abdomen. These were a pale brown in colour, and my immediate thought was that they must be parasites of some sort. Indeed, the beetle seemed quite affected by their presence, as it was moving about quite frantically, and appeared to be irritated by these small invertebrates that were scurrying all over it.
I didn’t have an opportunity to photograph the beetle during the training, so I brought it home with me and took some photos the next day. From searching on the Internet, I was able to identify it as being the banded sexton beetle (Nicrophorus investigator), a species which is widely distributed in Britain, and which we have previous records for from Dundreggan.
By doing some online research I also discovered that the mites were in the genus Poecilochirus, and were most likely to be Poecilochirus carabi, and this has subsequently been confirmed by a mite expert at the Natural History Museum in London. What’s more, I found that far from being parasitic on the beetle, these mites actually have a mutualistic, symbiotic relationship with it. They belong to a group known as phoretic mites, meaning that they utilise phoresis, ie they rely on another organism to transport them to their food sources.
What this means in practice is that the mites hitchhike a ride on the beetle when it flies off to find a new carcass to lay its eggs on. As a burying beetle, the banded sexton beetle locates fresh corpses of small mammals etc in the forest, and then excavates the soil from underneath a carcass, so that it falls into the resulting small hole, whereupon the beetle will cover it with earth and lay its eggs there. Upon hatching, the beetle larvae feed on the carcass, until they are fully grown, when they undergo metamorphosis and the adults emerge. Mites have no wings and are very small, so finding a new carcass would be difficult for them alone. By hitchhiking on the beetle, which can fly, they have increased opportunities of finding new food sources.
The mites themselves apparently eat the eggs and larvae of dungflies and other invertebrates that feed on carcasses. Those are competitors to the sexton beetle’s larvae for the rich food that a carcass represents, so the mites are actually benefitting the beetle, by feeding on and removing some of these other species and thereby reducing the competition. Thus, in this mutualistic relationship, the mites benefit by getting transported by the beetle to an ideal food source, and they in turn benefit the beetle by reducing the competition that the beetle’s larvae face for the food contained in the carcass.
It’s one of those remarkable relationships in Nature, which renew and deepen my sense of awe and wonder at the diversity of species on the planet, and the astonishing connections that develop between some of them. It also adds to my excitement about, and interest in, the tiny things in the forest – there’s a whole world of miniature life and relationships there that I’m continuing to explore and discover.
This left me very satisfied and fulfilled at the end of the day, but I never did discover how the beetle came to be in the mug in the first place! That remains a mystery …
As for the beetle, I released it into my garden after I’d finished photographing it, complete with most of its mite passengers – I kept two of those behind to see if I can get them identified to species level by an expert.