Sawfly larvae “appear from nowhere” around late April or early May in the Twin Cities’ area. They look like caterpillars but are actually the larval form of a wasp-like fly Neodiprion sertifer (Geoffroy) which was accidentally introduced from Europe. The larvae are about an inch long when fully mature, greenish-grey in color with light black stripes along their backs and black heads. The larvae occur in mass colonies situating themselves along the needles.

Mugo pine are typically the most affected plant in the urban forest, but Jack pine, Scotch pine, red (Norway) pine and can also be attacked. Sawfly larvae rarely affect Austrian or Eastern white pine. The insect is species specific. This means it will not affect landscape plants other than pine.

The adult sawfly lays its eggs in late autumn. The eggs hatch in spring and the young larvae begin their voracious feeding. If noticed early on, these tiny larvae can be controlled easily with an application of almost any insecticidal soap or chemical containing acephate or malathionon as the active ingredient. They can sometimes be sprayed off the plant with a jet of water from the garden hose; they can also be hand-picked. This for the organic gardener: I’m told they are excellent fishing bait.

An excellent alternative treatment to spraying an insecticide is the application of a systemic control. Merit (active ingredient imidacloprid) can be applied in fall for control the following spring. In this case, the insecticide is absorbed by the roots of the plant; when the larvae start feeding in spring, they ingest imidacloprid and die.

Sawfly larvae do considerable damage if left unchecked. If the previous years’ growth is insufficient they will also eat new needles. These needles will not grow back. While this doesn’t typically kill the plant, severe infestations may cause tip dieback and the aesthetic appeal is greatly reduced. The masses of larvae moving in unison as one approaches can be unnerving, especially as a dwarf mugo pine is often planted near one’s front walk.

Majestic can help you manage your sawfly problem. Call today!