Hi Kurt,

You are correct, Japanese beetle do not occur in that part of CO. Those are some sort of scarab beetle larvae that also feed on plant roots and look nearly identical to Japanese beetle larvae. The various larvae can be distinguished by the rastral pattern, the pattern of hairs on the tip of their abdomen. I think those are the larvae of the ten-lined chafer (Polyphylla decemlineata), but I'd need to see them under a microscope to confirm. 

https://www.ecolandscaping.org/10/landscape-challenges/disease-identification-and-control/commonly-encountered-invertebrate-lawn-and-garden-pests/

But it's not important for our purposes. Scarab beetle larvae are all managed the same, more or less. 

1. These larvae are prone to desiccation. Simply allowing the soil to dry out between waterings kills the larvae.

2. Physical removal: you can sift through the soil, probably at the end of the growing season, for the larvae and pupae. They can be crushed or fed to animals. 

3. Insecticides like Sevin (active ingredient Carbaryl) can provide control. But using insecticides for aesthetic purposes is not worth the damage they cause to the environment.