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.
What you can do
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.
What we do
The City inspects individual spruce trees in parks and green spaces and applies low environmental impact controls as necessary. We don’t spray spruce trees along residential streets or on private land. For trees along residential streets we recommended spraying the trees with high pressure water soon after the larvae hatch or removing the sawflies by hand.