These caterpillars (6 legs, 8 prolegs, brown head, cream body with shiny black spots, not hairy) have been feeding on the flower spikes of my salvia all summer. As a result, I have had few flowers for my hummingbirds. I notice the frass when the caterpillars are about the size of a grain of rice. In the current heat wave, they move rapidly from from one flower spike to the next, drilling into the bud so many flowers don't open at all and the ones that do open have holes. They do not appear to feed on the foliage, just on the flowers. I isolated some plants, hoping to have at least one pot with flowers, but they were quickly infected, too. The caterpillars spin silk and can hang from the plants if disturbed. They appear to be in their second generation now, since the plants were able to send up new lateral buds after the first flowers spikes were destroyed six weeks ago and I had not seen any frass in the interim.