I think you are correct. That looks like a GST to me. Though I've never seen one, except here in pictures. I'm jealous. My rue is just tiny seedlings.

Martha

It will be my first, Martha. It's also my first year to plant Rue. I'm excited!

That is most definitely a GST! I have been lucky enough to find some on my rue too, and then this year on my wafer ash.
Just so you know, they eat very slowly, and rest a lot. Unlike BSTs who are voracious little piggies.

You are in for a treat when he or she finally ecloses, they are stunning!

Hee, congrats, I've had them a couple of times in my life on various plants. I find the adult to look awkward and bulky, but still glad to see them.

Your so lucky that you found him/her I wish they came this far north.
Jacob

I'm so glad to find out they eat slowly, weed30. I have been worried about him. Does anyone know how long it takes to pupate? I want to make sure I have enough rue.

They spend two days in each instar and at least five days in the fifth instar. Keep that Rue comin'...

Interesting. So pretty. I planted my first rue plants too as soon as I saw the GST. And got 8 cats. Sadly 7 died from wasps eating them I was able to save one and raise it in butterfly
House. To my surprise, it wasn't a GST. It was eastern black swallowtail because it changed color when it got older. But as a baby looked like a bird poop heh

A beautiful caterpillar photo.