From the MCOFS website:
http://www.mcofs.org.uk/assets/pdfs/...lls%202008.pdf
Black (Birch) Flies
These flies belong to the genus Simulium, a part of the black fly
family and, like the smaller Highland midge (Culicoides
impunctatus), the females require a blood meal in order to
reproduce. Most of these blood-sucking insects attack livestock
but, according to entomologists, several species are known to
bite humans: Simulium posticatum, found along the River Stour
in Dorset where it is known as the Blandford Fly, and those on
Speyside where they are known as Birch Flies. The predominant
species on Speyside is Simulium reptans, with lesser numbers of
S. tuberosum and Prosimulium hirtipes.
The worst affected areas appear to be localised, being confined
to woodland around Loch Insh near Kincraig and along Speyside
around Aviemore.
The Birch Fly appears when Birch trees start to leaf in May and black swarms of them congregate in
shady areas under the trees, normally in the morning and particularly near running water where they
breed.
The bite of the Birch Fly is extremely unpleasant and can cause extremely irritant sores, much worse
than mosquito bites. The usual insect repellents do not seem to work. Locals are aware of the problem
and resist picnicking or fishing under birch trees for a couple weeks in that area. Warm and very wet
weather early in the spring provides ideal breeding conditions leading to a surge in the population of
Birch Flies. Local businesses often place posters warning tourists of the danger. So taking an interest
in the weather and reading notices may be prudent if you’re staying down in the woods!
(with thanks to Jeremy Watson of Scotland on Sunday, Rory Post of the Natural History Museum, London, and
entomologists Jon Bass, Roger Crosskey and John Davies)

Our garden has about half a dozen spindly birch trees in it, might get the chainsaw out tomorrow....
The Cairngorms National Park Authority insisted on a replanting scheme of native species including birch when our houses were completed but in their infinite ignorance they probably haven't considered the birch fly problem we have. Might rip them out too plenty of alder/oak/rowan saplings left over!!!