This small caterpillar was on a wild parsley (Pastinaca sativa) plant. There were perhaps 6 of them, each in their own little web cases made in the flower heads. According to information I read, this species was accidentally introduced from Europe to NA and first noticed about 1869. Apparently, "they feed exclusively on a handful of furanocoumarin-containing hosts, including wild parsnip and several species of Heracleum."

Wouldn't it be interesting if they could be used to control the Giant hogweed, a Heracleum species, that is so worrying people at the moment!