Today I encountered a heavily populated and damaged mugo pine in Gloucester. The photo shows just one group of Red Headed pine sawflies, but the plant has been defoliated by roughly 25% and the damage will likely continue until we receive a hard frost. The stem to the right side of the photo also leads me to believe that they were a significant problem last year, as there are no 2 year old needles on it.
There are sawflies of different species found on the hard pines throughout the growing season, from May until now, so always be on the lookout for these voracious and gregarious feeders. They can be managed in a few ways- if populations are small, they can easily be pruned out of the shrub. If populations are heavy, or you don’t want to put your fingers near them, they can be sprayed with a product that contains spinosid. If this is a persistent problem, consider replacing the shrub. If these pests are left to feed until the plant is defoliated, the plant will likely die, as evergreens generally do not recover when they are defoliated.