Hi
My curly kale are riddled with them. Leaves look very healthy on top, but look underneath and there are what appear to be whitlefly eggs, millions of them [well, lots anyway].
Cabbage also has clouds of whitefly, but they don't seem to like laying their eggs on that, and I don't think they actually do any harm.
Going to try ultra-fine netting [won't even let whitelfly through] next year.
:)