Frass

Frass is bug poop.

Tiny black caterpillar poop on Romaine lettuce

​Frass can indicate a caterpillar or other insect infestation. It adds nutrients to the soil, but can sometimes prevent certain plants from growing. For example, insects that feed on eucalyptus leaves poop out a substance that prevents plants in the mustard family from growing nearby. Weird, huh?