Answer: When a plant defoliates in response to a spray application, it is a sign of foliar damage. The plant should be able to grow replacement leaves if it is in good health. If however the whitefly infestation has been severe, it may be less stressful for the plant if you trim it back somewhat. This has the second advantage of probably removing some of the whiteflies as well. Since butterfly bush blooms on new growth, this will reduce the size of the plant but will not impede blooming unduly.
Whitefly can be a difficult pest to control for sure and the warmer the temperatures the faster it reproduces. However, mixing different chemicals is almost never a good idea as the combination can have a bad effect on the plant. It is also important to read and follow all of the label instructions. Also pay attention to timing and reapplication of the sprays per the label so that you catch both the adults at the beginning and then subsequent hatches with the follow up sprays.
Make sure you are contacting all surfaces of the plant including especially the undersides of the leaves. Lastly, no matter what you are spraying, it is usually a good idea to test spray a few leaves and wait and see for a few days to make sure there is no adverse reaction.
I apologize for the delay in ansering your question. I hope the plant has done well for you.
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