Look at center of the leaf on the left side of the photograph:
That small green ball is a Red Admiral Butterfly egg; I watched her lay it. I'm planning to add caterpillar photos when it hatches.
Unfortunate update... it got away. I found the open egg, but absolutely no sign of the caterpillar despite endless amounts of time spent searching every Pellitory leaf on the plants numerous times. I hoped that I'd find the Red Admiral Caterpillar by finding chewed leaves when the caterpillar grew up a little, but that didn't work this time either. It crawled off to a different plant, or something ate it, and I don't really want to think about which. I'll keep my eyes out for another egg.