Coordinator notes: I presume the submitter identified this on the internet. A google search for "American Dagger Moth" shows "Acronicta americana." The larva matches the photo submitted. I am not sure of the nomenclatural standing of the name americana but this name is not included in the book Moths of Western North America by Powell and Opler. Instead, the description of the larva they give on p. 283 also matches the photo only they call this the "Cottonwood Dagger Moth" in Colorado and list the scientific name as "Acronicta lepusculina" to which is the basis for having confirmed the sighting under this name.