Can you post a picture? There aren't usually many insects that bother Monarda
, powdery mildew is probably the more common problem. If the damage look anything like this picture of damage to vetch, look for the bug that is also in the picture. They're quite elusive and quick to duck out of sight - look for them in the cool of the early morning (if it is still cool in early morning there !) as they'll be more sluggish then. The juveniles (nymphs) look different, but if the damage looks the same then I can show you a picture of those too.
Another possibility, if there are holes right through the leaves, is flea beetles. Other than that I can't find many pests listed for beebalm and I don't usually see significant damage on them here. It sounds like your plants are still doing OK, if so I wouldn't do anything unless it is the four-lined plant bug. First thing is to identify the culprit, even a picture of the leaf may help if it isn't round spots like in this picture.