The little red beetles are easy to spot and catch. The actual destruction of the plants is caused by their lavae. In my garden, these are appearing now (end of June/beginning of July).
The first symptoms are leaves like these...
Simply turning over the leaf, reveals the lavae...
I prefer to remove the lavae from the leaf by carefully wiping with a tissue.
It's worth getting down on hands and knees and inspecting the entire plant, from below. You'll almost certainly find some more...
This is what the lavae actually look like....