Relevant Fact Sheets from the CSU Extension:

Aspen and Poplar Leaf Spot: http://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/

Dwarf Mistletoe Management: http://extension.colostate.edu/topic-areas/yard-garden/mistletoes-in-colorado-conifers-2-925/

Mountain Pine Beetle:
http://extension.colostate.edu/topic-areas/insects/mountain-pine-beetle-5-528/

Ips Beetles: http://extension.colostate.edu/topic-areas/insects/ips-beetles-5-558/

Douglas Fir Tussock Moths: http://extension.colostate.edu/topic-areas/insects/douglas-fir-tussock-moths-5-542/

Cooley Spruce Galls: http://extension.colostate.edu/topic-areas/insects/cooley-spruce-galls-5-534/

Scale Insects Affecting Conifers: http://extension.colostate.edu/topic-areas/insects/scale-insects-affecting-conifers-5-514/

Leafminers: http://extension.colostate.edu/topic-areas/insects/leafmining-insects-5-548/

Managing Houseplant Pests: http://extension.colostate.edu/docs/pubs/insect/05595.pdf

Additional Resources:

Forest Pest Management by the Montana Department of Agriculture