I have three mugo pines that have been doing great. Last year they developed a frost like appearance. It looked pretty at first but I am afraid it is some type of scale insect.
You're right. It is a type of scale insect. The adult pine needle scale forms a hard white shell that looks like someone spilled paint on the needles. Continued attacks by this insect can weaken and ruin the appearance of your mugo pine. Timing is critical for successful control. Their hard shell protects the adults and overwintering eggs from predators, the environment and pesticides. Apply insecticidal soap, Neem, light weight horticulture oil or another eco-friendly insecticide labeled for controlling this pest, when the immature shell-less scales are active. This coincides with the bloom time of vanhouette spirea (often called bridal wreath) in the spring and hydrangeas in the summer. Make two applications 7 to 10 days apart for both the spring and summer treatment. Or kill the overwintering eggs by applying lime sulphur in late winter when the plants are still dormant.
Photo: Mark Brothen