Early spring watch for the European pine sawfly. If branches on short needled pines such as scotch and mugo pine appear to move when you walk past, it’s probably a mass of European pine sawfly larva. They will feed on last year's needles, so will disfigure pines but most likely won't kill them.
Control pine sawfly larva with most general purpose insecticides except Bacillus thuringiensis as soon as they appear. Instead of spraying insecticides, another possible option is to knock the larva off the branches with a stream of water or remove them by hand.
Don't cut off infested branches, as the terminal bud will be removed and new growth will not appear on that branch. Pines should only be pruned by cutting back the new candle growth, which typically appears in June.
Usually only one generation occurs and the winter is spent as an egg inserted into slits along the edge of needles. The eggs hatched in April through mid-May and the larvae may feed until mid-June. The caterpillar-like larvae are grayish-green and have a light stripe down the back, a light stripe along each side followed by a dark green stripe. Full grown larvae are about one inch long. The larvae feed in groups or colonies, often with three or four feeding together on a single needle. Distributed larvae raise their heads and tails in a threatening manner. Mature larvae drop to the ground and spin tough, brown cocoons in the duff. A few larvae may pupate on the tree. The adults emerge in late August through September to mate and lay eggs. Each female lays six to eight eggs in a single needle and 10 to 12 needles are used. These eggs can be located after a hard frost turns the egg laying scar yellow.
Healthy Mugo Pine with new growth
Damage after Pine Sawfly infestation.