Unfortunately, once they numbers are out of control, as you said, it's pretty difficult to manage. The problem with using insecticidal a is that you're knocking out the biological control that is already in place. What kind of damage are you seeing? If there's no obvious damage, I would try to maybe introduce plants that attract their predators. Aphid Midge happened to manage my aphid issue much better than anything I could have done about them. I had fairly dense aphid population, too. From what I remember about whitefly, they're in the aphid family, so I would think that they have the same predator enemies!
I don't think lady beetles would hurt, but the larva would be much better. My bolting and blooming cilantro and dill were very good attractors!