We have been receiving reports of infestation of pine trees and pine shrubs such as Mugo Pine by Pine Sawfly caterpillars. They eat the needles and can be veracious feeders and strip the plants quickly. This is also the time of the year when Spruce Sawfly are actively eating the tender new needles of several varieties of spruce trees such as Colorado, Black Hills and Norway Spruce.
If you notice new shoots without needles, or just stubs of needles, sawfly probably are or have been actively feeding on the new growth. To stop the damage quickly, spray with an insecticide. To kill the sawfly caterpillars more slowly, but with an organic product, try Bt spray, which is safe for humans, but infects caterpillars with a bacteria which eventually kills them. This may not take effect quickly enough to prevent the loss of a large percentage of the needles.
The picture here shows healthy new growth that is just getting started. These needles will soon be as long as the needles from last year’s growth. If you notice the new growth with few needles or no needles – this is damage from the sawfly.