Flies Under Vine - What Are They?
 

I have a long pergola thingy covered with a vine, and now that it's
got into gear, it's full of hovering flies (not hover flies!).
They are black, a bit like houseflies but smaller, but they just hover
around and only under the vine.
Anyone tell me what they are, how I might get rid of them or reduce
their numbers?
Thanks!
NB -never had a vine before - just moved to this house where it was
already established!