This has been a year when there have been a lot of slow flying jet-black flies around, no doubt producing the “ugh” reaction from most people. But these are not flies that want to enter your house and land on your food, they are flying around with the sole aim of mating to produce the next generation. These are St Marks Flies (Bibio marci) so called because they tend to appear around St Mark’s Day each year. Their slow flight and resting in full view on tree leaves makes them tempting targets for some birds. If you see a Starling dashing around in the trees frantically going from one branch to another it is most probable that it is these flies they are seeking to catch. Other birds may also take the flies and having Tree Sparrows nesting in boxes on the house means that you sometimes get a chance to see what they are bringing for their young to eat.
This one, waiting to feed its young clearly has, among the beak full of flies, one of the St Marks Flies. So, they play a useful role in feeding the birds of our woodlands and gardens. These are the adults, but where are the larvae and what do they eat?
You may find them in your garden if you are clearing matted leaves in a damper area during late autumn and winter. There are often several of them together underneath the decaying leaves. Their principle food is decaying vegetation and besides dead tree leaves they can also be found in the thatch of dead leaves around grasses where they also play a part in recycling dead vegetation. This however has led them into conflict with anyone who is trying to establish a pristine lawn surface either in the garden or golf course as they are also said to eat grass roots. Most sites found on searches say they damage the roots of grasses and some other commercial crops, but it is hard to find a scientific study verifying this. I can image that they would damage roots under some circumstances for example when lawns have had thatch mechanically removed thus depriving them of their preferred food. Despite this, and their blundering flight that can be annoying when out in the garden, it is hoped that they can be appreciated and tolerated for their important role as recyclers and potential live food for birds, especially when feeding their young.