FRASS: noun. A term meaning debris or excrement left by the larval stage of insects especially caterpillars and wood boring insects. It can be visible to the naked eye depending on the size of the insect leaving its' debris.
Entomologists initially used this term to name the debris left by wood boring insects. It was identified by the combination of excrement and wood shavings that were not digested by the boring insect. It could be found on the exterior of the wood, on the ground or packed densely in the hole to block other prey insects. The definition broadened to include the wood dust not ingested by the insect but pushed out of the bored hole to give the insect room to move through the wood.
The meaning of the term broadened further to include the excrement of all munching insects. Seeing the dots of solid waste on the leaves of susceptible plants is the best clue that you have caterpillars devouring your herbaceous plant. Stop and look closely at the top and bottom of every leaf of the plant to find the offending caterpillar. They are often camouflaged and hard to find. Some of the villains in the garden who leave their frass calling card include tomato hornworms, cucumber beetle larva, and tobacco budworms (petunia and geranium destroyers).
Researchers have found the presence of frass is a useful sign to identify early damage/disease in plants from small to large. Certain insects damage plants in ways that increase their vulnerability to mortal diseases. The presence of frass allows growers and home gardeners to recognize early infestations of the offending insect and intervene in the most environmentally responsible manner possible. Entrepreneurs have even figured out how to harvest frass for fertilizer: big bucks, even, low nutrient profile (2-2-2). You would need a lot to make much difference in the garden.
**For fun access the Word of the Week article referenced above. There is a link to a video of caterpillars catapulting their frass a distance in order to fool predators about their location.