Unfortunately, the flies in question are not in focus, unlike the surrounding leaves, which I can see clearly. Not sure what it is because can't see any detail

These are adult Large Rose Sawfly (Arge pagana), the females of which lay eggs into the tissue of new stems. They hatch into caterpillar-like larvae that in turn feed on the rose leaves and, as with other Sawfly species, can defoliate the plant in extreme cases.

The adults should be removed by hand, when seen. The larvae can also be removed by hand or, in severe infestations, controlled with any suitable garden pesticide, such as pyrethrum. Personally I've never found spray controls necessary with any sawflies - just get in there and squish 'em!