This is not the first time I have come across this problem on my blackfoot daisies. Today I noticed that this plant in the sunken garden had died. I pulled it out and saw these insects on the root. I am almost positive that they are root aphids. Last year I saw the problem in the front gravel garden; decline of the plant and a few ants around the base of the plant. They were busy farming the aphids for honeydew. It has been a bad year for my blackfoot daisies and I think I now know why. The only thing I can think to do is to drench the base of the plant with spinosad and see if that gets rid of them. Fingers crossed.