I finally found the culprit that’s been chewing on the pigweed plants. It’s a small (~1/4″) creature that looks a little like a Mexican bean beetle larva but it doesn’t have the yellow color or the branched spines on its body. I’m pretty sure this is some sort of beetle larva but I don’t know what kind it is.
If there’s anyone that can ID this insect, please let me know. It’s not causing much of a problem in the garden (except for the pigweeds!) but I’d love to know what it is.