Garden Pests and Diseases: Some kind of borer ruined my shastas, 1 by winging

winging wrote:
Three of my shasta daisies died this year. I assumed it was due to poor drainage and root rot, since they are planted in the lowest spot of a bed sloping down to the retaining wall that lines my driveway. I didn't worry too much about them since I have to divide the healthy ones anyway and was just going to use the divisions to replace the others.

But today I finally looked at the roots and crown and found these. Any ideas what kind of borer? Strategies for getting rid of it?

Thanks in advance,
w