Emerald Ash Borer
An occurrence of Emerald Ash Borer was confirmed in NE Boulder in October 2013. Currently, there is no need to preventively treat ash trees in the Denver metro area. Applying any kind of preventive insecticide to ash trees at this time would introduce insecticides unnecessarily into the environment which could affect honeybee and other beneficial insect populations.

Lilac Ash Borer
High-risk ash trees may benefit from control of Lilac Ash borer. These could include recently transplanted trees, trees in poor sites, trees receiving limited water, damaged or stressed trees, or trees that show significant effects of previous damage. More information can be found on this Fact Sheet:
http://www.ext.colostate.edu/pubs/insect/05614.pdf

Cherry Tree
Common problems that affect cherry trees include peach tree borers, cherry slugs, Coryneum blight, cytospora canker, as well as aphids and mites. Here is some information that may be helpful to you on fruit trees and proper insect and disease control.
http://www.ext.colostate.edu/pubs/garden/02804.pdf
 

IPS Beetles
Ips beetles are a common problem on pine and spruce trees in Colorado, and they usually prefer stressed trees. Ips beetles can only be treated preventatively, so once they infest a tree, the tree cannot be cured. Here is more information on the beetles:
http://www.ext.colostate.edu/pubs/insect/05558.html

No matter what tree you have in your landscape, it is important to keep your trees healthy and growing, with regular water, winter watering, proper pruning, mulching and other cultural practices. Healthy trees are more resistant to insects and disease:
http://www.colostate.edu/Dept/CoopExt/4DMG/Trees/caring.htm
http://www.cmg.colostate.edu/pubs/Trees.html
http://www.treesaregood.com/treecare/resources/ProperMulching.pdf