Answer: Bassd on your description it sounds like your hibiscus are infested with whitefly. This is a common pest on hibiscus and very difficult to control. They multiply quickly in the warm and sheltered indoor environment. Although the hibiscus would enjoy higher humidity, the warmer temperatures can contribute to the whitefly multiplying extra fast.
You can control them using commercially formulated insecticidal soap or horticultural oil according to the label directions. (Neither of these would require using a plastic bag or extreme measures.) Be sure to spray tops and bottoms of all the leaves and stems thoroughly. You will have to reapply it at least once or twice per the label directions to kill emerging generations of the pest.
Often these plants are lightly infested while outside for the summer but it is not noticeable as the pests are somewhat controlled naturally by wind and weather and natural predators. Next fall, treat the plants again before you bring them indoors just to be sure there are no hitch-hikers coming inside with them.
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