One of Nature’s Mysteries to Solve

Hey there!

What is this unexpected accumulation?

Mystery #160

I’ll check back later for your guesses!


We had a guess of ‘worm poop’ folks, and that is so close! These are caterpillar fecal pellets–poop–better known to scientists as ‘frass’. The word frass is used for any type of insect poop, which we squirrels didn’t know. We’ve just heard it used for butterfly and moth poop. And so you know, the size of the frass does vary with the size of the caterpillar, and grows as they grow, but it always has those little grooves down the sides.

This is really what Nutmeg and I saw when we bounded to one of our favorite decks to visit a water dish:

Caterpillar frass on deck

Frass everywhere! We looked up, and sure enough, the black oak leaves were being chewed to the veins.

black oak leaves defoliated by orange-striped oakworm

A closer inspection upon climbing the tree revealed dozens of orange-striped oak worm caterpillars at work.

orange-striped oakworm in black oak tree

If you forget that name, you can just search for orange and black caterpillar on oak trees and easily find it.

Orange-striped oakworm caterpillars feedign on black oak leaves

orange-striped oarkworm

They are pretty common in oak forests, and turn into a plain brown moth. A photo and more information can be found on this Michigan State Extension bulletin.

Hope more of you humans aren’t sharing your outdoor space with these critters…but we’re sure that’s not a problem considering how hot it’s been!


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