Those are the larvae of the Lysimachia sawfly. Evil little things, aren't they. The good thing is that they are very easy to control with insecticidal soap. Just mix up a little in your hand sprayer and direct the mist to the under side of the leaves, where they hang out. The spray needs to be applied directly to the insects.

Of course, with just one plant you can collect them by hand....you've already done a fine job at that!

The key is to keep an eye on your lysimachia next year. You need to catch these pests BEFORE they can completely destroy your plant. That takes some hands on care, as the larvae are extremely tiny when they hatch and as I said, they will hide on the lower surface of the leaves.