The yellow-headed spruce sawfly (YHSS) affects young spruce trees in Calgary. The sawfly infests and feeds on spruce trees that are growing singly or on the edge of a group of trees. Infestations can begin when a tree is only three to five years old and if left untreated, can kill that tree within three years.
The larval form of the insect looks like a caterpillar and is a voracious feeder on spruce needles. This causes complete defoliation of branches and twigs with only a few chewed brown needle stubs remaining. The damage is likely to be noticed first on lower branches close to the ground.
In late spring/early summer, homeowners should regularly inspect trees for signs of damage or colonies of feeding larvae. You can remove the crawlers by hand if there are only a few spruce trees and a low level of infestation. However, in situations where there are many trees, a high pressure blast of water is an effective non-chemical method of control.