I suspect it is the scent of the virburnum blossoms. Some of them can be quite overwhelming. What kind of flies are they? It would be helpful to know what their natural food sources are. Sandy

Some plants are pollinated by flies and a second possibility may be if you used manure or compost around this plant flies might be hatching out and hanging around. Flies like plants that don't exactly have a sweet fragrance. Hope this helps until others weigh in.

I had a similar situation with a holly bush in my garden. Flies everywhere and then what looked like eggs appeared on the leaves. I tried spraying the flies with a non-toxic spray like horticultural soap or hot pepper spray, but nothing seemed to faze them. I finally shovel pruned the bush, it grossed me out.