I have 3 identical plants that are all planted on the Northeast side (front) of my home. About a month ago they all began to drop their needles and turn brown. One of them was removed when it became bare. Please help. There are many thriving plants adjacent to these plants. They were planted approx. 4 years ago
The denuded branches were probably eaten by the larvae of the redheaded pine sawfly. This caterpillar-looking larvae eat in groups and, if you look closely at the tips of some of the shoots in the left hand photo, you will see half eaten needles.
These larvae have long since crawled off, turned into sawflies, and flown away. New needles should grow from these shoots next spring, because the shoots are not dead.
Next year keep an eye on your mugos in summer. If you see a gang of these larvae munching on your pines, it is easy to knock them into a container of soapy water where they will quickly drown.
It's possible that the mugo on the left also has a needle disease. Look closely at the healthier needles for discoloration or streaking. If you see anything suspicious, send us a close photo. Otherwise, clean up all the fallen needles of both pines in case there is infectious material there.
Mugos should be planted where they get good air circulation and sun in order to stay healthy.