Oh my! It might be some sort of caterpillar. If so, I'd spray it with Bt, the natural bacteria that is parasitic on caterpillars (and won't hurt bees). If not, spraying with Bt won't hurt! I buy the brand Thuricide, but there are lots of other brands out there.

Good luck and keep us posted!
Happy gardening.

Based on the shape of the holes could be a leafcutter bee.

Lenering,

Wow! Someone else is growing comfrey here!

I found a green caterpillar eating mine, he was the same as the ones that eat cruciferous leaf vegs. Can't remember the name of the bug. Cabbage worms?

Which type of comfrey are you growing? I tried both Bockings and some true comfrey seeds that never came up. I got some BIG generic comfrey roots from ebay and they are doing OK. MUST water daily or they will to the ground I finally seem to have found the best spot for them, shady, but I don't think enough sun for winter. No matter what I do, they are struggling.

Where do you live? Have you had the comfrey long? What are your comfrey secrets?

The BT works great on those green caterpillars.

Guys, thanks for you answers. I really appreciate your input.
Don't know about using BT, because I don't want any pesticide in my garden.
I am growing Bocking #4 and Bocking #14 here in Phoenix.
I had done the same in NE for long time, and I never had any problem growing Comfrem there.
I noticed that Comfrey likes a lot of water. I also noticed that my Comfrey that is located in the east side of my house is doing better than the one in the west side. I guess it doesn't like the evenning sun.

It's 3 years later but lenering, are you still growing comfrey in Phoenix? I'm interested in an update.