Emerald ash bore
Is it necessary to do any type of treatment for the Emerald Ash Bore in the Littleton area?
Denver County Colorado emerald ash borer
The emerald ash borer (EAB) has not been detected outside of Boulder County yet so it may be too early to treat trees in Littleton. However the decision whether to do so or not is up to each homeowner because just EAB hasn't been found in Littleton doesn't mean it's not there hiding in a tree somewhere.
It's definitely a great time to make a plan. Since pesticide treatment will have to be applied for the life of the tree, you might take this season to consider how healthy your tree is and how valuable it is to your landscape, then balance those against the expense and ongoing maintenance of treatment. Some homeowners with very new, young trees or very unhealthy, stressed ones may opt to simply get rid of their tree to avoid the headache involved and plant something else. On the other hand, if you opted to not treat a large, mature Ash and it dies sometime down the road, there will be expense involved in removing it. No one solution is right for everybody.
Some great information to help you make a plan is available from Denver's Be A Smart Ash site: https://beasmartash.org/