Dear Tom,

In Colorado, ponderosa pines most commonly shed their older, interior needles from August through November. 

As you have had your trees checked for borers, the answer to your needle browning may not have a simple answer.

Winter Desiccation is a common stress for pine trees. Monthly winter watering is recommended.

Check for needles for discoloration. Needle cast, for instance, typically results in a severe loss of inner foliage and a very bare, sickly-looking tree. If you suspect your tree to have a disease then a sample should be sent to the Plant Clinic at the Extension Office.

Check for the presence of insects feeding on the sap from twigs, branches and trunks.  Again, the Plant Clinic can help with diagnosis and treatment options.

Sometimes, the tree can be damaged by hail.  Check for signs of damage.

The following articles provide additional information.