You could try the Bonide systemic granules.

I also used to buy these systemic capsules that I used with my hibiscus that came back inside for winter.

Thanks for the suggestion, however I neglected to mention that I am in Canada. Unfortunately Bonide systemic granules are banned in Canada because they contain imidacloprid.

I recently read that worm castings will get rid of whiteflies! here’s an article: Worm castings for whiteflies

Ahhh, hibiscus, I have six, all of them of newer developed breeding with extra long lasting individual blooms, some up to five days each! I'm always struggling with both aphids and spider mites and plants requiring semi regular kitchen sink spray downs and sprayings of Safer's End All concentrate. BUT, your plants are much larger and some type of systemic would make things easier IF possibly it could be obtained.

BTW, your plants are very nice healthy vigorous, what is your fertilizer routine?

I'm sorry. As soon as I typed up that information, I wondered if perhaps you may be in Canada. It may be worthwhile to contact a local garden center to ask what they recommend since the active ingredient in the products I mentioned is banned there.

With that being said, one of the best sources of information regarding hobby hibiscus growing is here: https://www.hiddenvalleyhibiscus.com/care/whiteflies.htm

I use this. Maybe it is okay in Canada.

https://www.lowes.com/pd/Garden-Safe-Brand-Insecticidal-Soap-24-fl-oz-Insect-Killer/4382973

Unfortunately I can't take them under the shower as both plants are at least 4 ft tall and wide and heavy, not on the same floor level as the shower. After severe snowstorms we are having a break. I was hoping for a milder day to take them outside under a rain for 10 minutes, but so far 8 degrees has been the mildest. I have never had them outdoors below 10 degrees, nor exposed to such cold water...so I don't dare to do that.

So far I have only sprayed them regularly with insecticidal soap and neem oil, and I also soaked the pots once in insecticidal soap and neem oil before bringing indoors. So resistance is probably an issue...Because of this I assume a neem oil drench may not work either ?

I must check out other products incl. sticky traps.

If they were not so good looking I would have cut them back already,...but they are spreading out their blooming and each bloom lasts about 4 days.....At Christmas we had 9 open flowers, so pretty.

I give them fresh potting soil once a year and use Plant Prod 15 -3- 15 water soluble fertilizer roughly once a week throughout the warmer months. Last year I started rotating this with liquid seaweed fertilizer.

Just hang in until spring. don't give up. You probably will not win, but keep on keeping on.

they look fantastic! the whitefly is very difficult to get rid of without systemic. make sure you spray underleaves thouroughly: that’s where the eggs are. i would cut some leaves : they are too thick , so very hard to reach all underleaf surfaces. you need to lift ths branches to get everywhere: best in the shower of course.

I've used a systemic (granules) to eradicate persistent mealybugs on potted plants - but only outdoors, not liking the idea of long-term exposure in the house. The key to getting rid of or at least controlling whiteflies is to spray something relatively non-toxic (insecticidal soap, Neem etc.) frequently enough to take care of newly hatched juveniles. Effective leaf coverage is more difficult with large plants.