When you are out and about in Bend, Oregon in the fall, you will notice some of the ponderosa pine needles turning yellow. If the needles are on the interior branch of the tree, this is normal. Ponderosa pines and other evergreen trees lose some of their needles in the fall. Unlike the trees themselves, evergreen needles have a shorter life span. Pine trees have needles with an average life span of 3 – 4 years and Mugo pine needles will live 5 years or more.
The amount of needles that drop will vary from year to year. Drought, heat, diseases, pests, or herbicides are some of the factors that may affect the amount of needles that fall and the time of year they fall.
If you see a tree that is turning yellow from the top down, than that is an indication that the tree is in serious trouble. According to the USDA NRCS National Plants Data Center there are approximately 200 insect species that affect ponderosa pine from cone stage to maturity. Pine beetles are a common cause of death for older trees, drought stressed trees or trees that have been damaged by construction or lightning. Several species of adult bark beetles bore into the tree and tunnel to lay eggs in the soft inner bark. When the eggs hatch, the larvae feed on the tissues of the inner bark, restricting the flow of nutrients to the top of the tree and some transmit fungi that stain the bark blue.