This week’s column features an aquatic moth. 

That’s right. The Petrophila confusalis moth has an amazing life story, and it starts under the water. Petrophila, by the way, means “rock lover.” Female moths lay eggs in fast-moving water and the larvae eat the algae on submerged rocks. They form pupal cases (waterproof, silk cocoons, of course) and after metamorphosis, the adults float or swim(!) to the surface.

Dozens of these small moths are fluttering around near the river right now. They are sometimes attracted to land on T-shirts and shorts, so you’re likely to find one if you’re so inclined. Upon macro-photo inspection, they have an attractive hind wing pattern, like beads on a string. 

If you aren’t already awed by these insects, get this: Female Petrophila moths dive into the water to lay eggs. The Coast Fork of the Willamette isn’t that deep, but BugGuide says they can go up to 13 feet underwater. To do so, they trap air against their bodies, for use as a scuba tank for up to 12 hours. They lay 200 to 300 eggs, and they don’t resurface. Yes, the females take a one-way trip under the water to incept the next generation.

The Bug Lady says there’s a wasp that parasitizes the larvae of our western Petrophila moths. You can probably guess what that means: Aquatic wasps! Her column (linked below) says if the two species co-exist, up to half of Petrophila Confusalis larvae may grow up to be wasps, not moths, because Tanychela pilosa wasps laid eggs in the (submerged) caterpillars of the moths.

Stay curious!

See more of Karen’s work here.


The BugLady’s column: Accessed 8/29/23.

BugGuide info page: Accessed 8/29/23.