I have had these butterfly bushes for 2 years and they have grown great. Presently, one of them has yellowing leaves but the veining of the leaf is still green. It’s hitting one really bad and the one next to it is beginning to show the same signs. Is it nutrients or insects?
Based on your description it could be either disease, environmental, or insect related. Is there any way you could contact your county Extension Office to arrange a sample drop off for closer inspection? Thanks for the question!