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There is an insect pest, in it’s larval stage, that causes extensive damage to Spruce trees in Edmonton. The Adult Yellow-headed Spruce Sawfly insect, is a small stingless wasp. The adult males and females look the same, in appearance: yellow in colour with black markings on their heads. This insect, generally produces one new generation per year. The destructive larva are approximately 18mm long, with yellowish-brown heads and green bodies with 6 visible stripes that go down the length of their bodies. Not to be confused with a “caterpillar”, the sawfly larva have 6 or more sets of pro-legs, which can be easily seen. A unique characteristic of these larva, are that when they are disturbed they will rear up the front part of their bodies, and release a green spruce- smelling fluid from their mouths.
The damage to spruce trees is primarily caused by the eating of the needles by the larva. Generally speaking, the larva will attack trees after the third season after planting, not earlier. The pest, usually gravitates to the upper part of the trees. Cleanly removing the whole needle, they will stop feeding mid- summer and drop to the soil, to cocoon. They pupate in late winter, and the adults emerge approximately the same time as the spruce bud swelling in the Spring. The females, which fly to a host tree, will lay a single egg in a slit at the base of each needle, all her eggs are usually laid on a single branch sprig.
Be aware that repeated removal of the needles of your tree, can lead to tree demise. Regular monitoring by your tree care professional, will be crucial. This insect pest, is a strong defoliator in the Edmonton area, and it’s damage is very visible. Often by the time its effects are seen, the insect has caused the majority of it’s damage for that season.
Contact your best certified arborist, in early Spring, to monitor and inspect your spruce trees for evidence of feeding larva.