Big Poplar Sphinx Caterpillar

green caterpillar
Bugman,
We found this caterpillar in the yard today and would like to know what type it is. We live in Lafayette, Colorado, near Boulder. We’d like to try to keep this guy in a terrarium for a while, any suggestions?
Thanks, the Heggestads.

Dear Heggestads,
I was unsure exactly what your caterpillar was, but I thought I would try searching what I was assuming was the host plant, the poplar tree. I concluded that you have a Big Poplar Sphinx Caterpillar, Pachysphinx modesta. Here is a site entitled Caterpillars of the Eastern Forests, with a pretty good photo. I would recommend keeping several inches of damp, not wet, soil in the bottom of the terrarium for the caterpillar to dig into when pupation time occurs.