Looking at the photo, it appears that an insect is eating the leaves of the holly.  However, a full photo of the plant would be helpful.  How long has the holly been in your landscape?  Is it crowded by other plants?  Have you been able to see any insects on the plant?   Could you bring a sample into the Denver Extension Center at 888 East Illiff for insect identification under a microscope or bring the sample to the Denver Botanic Gardens for insect identification?  More information is needed to help make a definite identification of the insect that is eating this Holly.