Frass: The excrement of larvæ; also, the refuse left behind by boring insects.*
This year’s huge populations of caterpillars are producing large amounts of caterpillar frass. While the caterpillars may be a nuisance defoliating certain plants, their frass is a great addition to garden soil, containing many nutrients and beneficial microorganisms.
*”frass, n.” OED Online. Oxford University Press, June 2017.