I use this spray for my jasmines.I think it will work on the hibiscus too.

White flies get stuck on the sticky leaves and die. mix 2 tablespoon of black seed oil with 500ml water and a drop of dishwashing liquid. Shake well before spray.

Ideally spray in the evening and keep in shade for 2 days. Slowly introduce to the sun.

You can use neem oil too. hope this helps, Good luck
Do a search,you will get more ideas.

Thanx.
We have some cloudy/rainy days coming up, and the greenhouse is covered, so I'm going to strike then while the sun's hiding.
Plus, I've been doing some research into using lacewing larvae a week or so later as a cleanup squad.
My understanding is that whitefly is no wimpy foe and is going to take a lot to control it.
Thx for the Blackseed suggestion. I've also heard good things about Neem.