The larval form which resembles a caterpillar is a voracious feeder on spruce needles. A severe infestation over a three year period can kill trees. Trees from the ages of 3-5 years in an open growing situation are the most susceptible to infestation. The infestation can continue until the tree reaches a height of 8 meters.
Damage appears as complete defoliation with a few chewed brown needle stubs.
The larvae feed first on new foliage and continue on to consume the older needles. The damage is likely to be noticed first in the top most growth. The adult sawfly is not a fly at all, but a stingless wasp. The adults are reddish brown in colour and 8-10 mm long. The larva are caterpillar like.