Update on checkerspot caterpillars on Echinacea:
When I went to check on them around noon today, I could only find 4 caterpillars total (out of a rough estimate of 2-3 dozen last night). Oh, and the crumpled remains of a fifth caterpillar being carried by a wheelbug nymph as it sauntered down the Echinacea stem.
When I looked this evening, I could only find 1 caterpillar on all 3 Echinacea plants.
Not knowing how large the caterpillars get before they either pupate or quit eating "gregariously", I can't say for sure whether the caterpillars moved on of their own accord or whether they became breakfast, lunch, or dinner for a little wheel bug, a cardinal, or some other animal. Whatever actually occurred, it appears that my Echinacea are safe for now.
Once more, doing nothing proved to be a highly successful strategy. Being "lazy" isn't always bad.