It's not a beetle, but the nymph of the Four Lined Plant Bug, one of what are called 'true bugs'. The damage is distinctive to those insects.

Attached are some images so that you can see what the adult looks like.

Rhizo, many thanks for identifying the critter.

My remedy: When I noticed that bugs were hanging out in the mint leaves, I harvested all mint with damage (removed stems to the ground). The well-behaved bugs stayed put on the leaves (did not drop off), as mint was removed and put into bags for disposal. About one hundred bugs were collected. Usually one or two bugs per stem of mint.

I feel better already.....

Again many thanks for the i.d.; it was a great help!

Here are links to other images, including pics of adults, plus life-cycle information.
images of adults and nymph
Four-lined plant bug, adult + life cycle
More images
Bug Guide