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For Insects, Spiders & Their Kin
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Photo#31491
Mystery Caterpillar - Schinia grandimedia

Mystery Caterpillar - Schinia grandimedia
Bluff spring Fen Nature preserve, Cook County, Illinois, USA
September 10, 2005
Caterpillar feeding on False Boneset.

Moved

Doug, do you have
Schinia grandimedia at Bluff Spring Fen. It has been pointed out that this looks just like the image on page 359 of the new Owlet caterpillar book.

Total Guess
It looks somewhat like a Euchloe species. See comparison shot here.

 
Food plant may be wrong
False Boneset is in the Asteraceae (Sunflower Family), but the link you gave gives food plants all in the Brassicaceae(Mustard Family). While some Brassicaceae specialists will feed on a wide variety of plants (Pieris rapae, the Cabbage White, is the best example), many don't.

 
Not Euchloe
It's not Euchloe. The butterfly fauna at Bluff Spring Fen is well know. Euchloe olympia, the only possibility in the genus, has never been recorded from Bluff Spring Fen, nor has Arabis lyrata, the local host. I'm pretty sure that it's moth rather than a butterfly. Not very specific, I'm afraid.

 
If not one of the Whites
maybe it could be one of the Sallows - Pyrrhia, Wagner page 398

 
This is a Pyrrhia exprimens
This is a Pyrrhia exprimens

 
Could be
This article describes it as "broadly polyphagous". Beyond that, I'm not qualified to say anything more.