First time this yellowing has occurred, colour on north side is fine. These pines were native to property when we bought, so don't know what kind they are. Others in pics show no yellowing. Trees are about 15-18 feet tall. The trunk of this tree exhibits no more pitch than the other nearby trees. Would hate to lose any one of these beauties.
They look too young to be very liable to be attacked by bark beetles (which for preference go for slow-growing overmature trees), but that might be worth checking on if there has been a bark beetle epidemic in the region.
These trees are young. We've been here 30 years; at that time they were 3 feet tall. The entire region is heavily infested with mountain pine beetle. Would bark beetle damage be limited to one side of the tree (i.e. do beetles damage only where they land when in moth stage, or do they move around liberally and infest all sides)? Are this year's yellow needles then followed by the typical red of MPB damage?
I desperately hope you're wrong on this, Michael.