The European pine sawfly is found in large numbers and it often attacks the Mugo pine as well as many other pines in the home landscape.
Sawflies are non-stinging wasps that have their ovipositor serrated like a saw. It is used in making slits in the plant. Eggs are then deposited in the slit. These insects attack pines. The sawfly larvae feed on the foliage in the spring. At first the small larvae eat only the outer layer of the needles. As they grow larger, the entire needle is eaten. After four weeks the sawflies fall to the ground to spin a cocoon. There is one generation per year. Mice (eat the pupae) and diseases often cause the populations of this sawfly to crash. The European pine sawfly"waves" in mass as a means of scaring off predators.They feed on older mature leaves, leaving the emerging needles alone.
The European pine sawfly overwinters as an egg in needles of the tree. In the spring, the eggs hatch into larvae that feed on the foliage. At first the small larvae eat only the outer layer of the needles.Later in the spring, the larvae drop to the ground to form pre-pupae. These pupate later in the summer with adults emerging later in the growing season. The adults lay eggs in slits in the edge of needles.
Failure to control the European Pine Sawfly insect is not usually fatal to the plant. However, a more sparse plant will be the result. If an insecticide is applied, it is best done when larvae are hatching or very small to minimize damage. Time to treat often corresponds to saucer magnolia petal drop. Control as soon after egg hatch as possible.