Hello Aldona,

Thank you for contacting the Ask Extension.  Since I did not see the actual insect, one of the types of insects in Colorado that affect leaves are leaf-cutter bees. They are native bees that are also pollinators.  As the article states: "They use cut leaf fragments to construct their nest cells. They often are essential pollinators of wild plants. Some leafcutter bees are even semidomesticated to help produce alfalfa seed. However, their habit of leaf cutting, as well as their nesting in soft wood or plant stems, often attracts attention and concern."  In addition: "The only known control of leaf injuries is to cover susceptible plants with cheesecloth or other loose netting during periods when leafcutter bees are most active:   Here is a link to the article. https://extension.colostate.edu/resource/leafcutter-bees/

Another possible cause is the root weevil. Adult root weaves chew distinctive notches along the edges of leaves at night.  This article also explains possible ways of controlling this insect. https://extension.colostate.edu/resource/root-weevils/

There are other insects that skeletonize leaves, but that does not appear to be the case here. 

Without seeing the actual insect, it is hard to know for sure what insect is causing the problem. If during the spring  you can detect the insect, take a photo of it so we would know for sure what it is and how to mitigate the problem.

I hope you find this information helpful.

Regards,

Marion C. 

El Paso Master Gardener