In m Shiva wrote:
Sounds like whiteflies. They are tiny, almost like a bit of lint, and
fly off en masse when disturbed. I squish them like aphids.

This is them, I found some photos. Lint is exactly what they look
like! What exactly do they do, do you know?

Like aphids, only more mobile. Funny, I've never heard of them
attacking roses. They like tomato plants though.

Whiteflies are easy-beats, Shiva. Anything systemic will wipe them out.