If you can deal with the sawfly and a few others I call Calgary's most wanted, then nothing will be able to take down your tree.
A spruce defoliator, once the sawfly gets set up in a spruce, that tree is in serious trouble and is usually removed. This can happen within two years. You can cut a branch on a deciduous tree almost anywhere, anytime (except late in the season) and it will grow a sucker and leaf out again. Deciduous trees are much better equipped for this and can much more readily generate new buds. Spruce trees have a more limited ability to do this. Each leaf on a deciduous tree has a new bud forming in the axis of the petiole, where the leaf’s stem is attached to the twig. Spruce needles do not have this feature. The needle grows once and, once gone, it never comes back. So when a needle-eating insect—or several thousand—finds a suitable weak and usually neglected spruce, bad things will happen.
The yellow-headed sawfly is known to our entomologist friends as Pikonema alaskensis. It attacks Engelmann, black, white, and blue spruce. Our concern is our white and blue spruce. The sawfly has one generation per year. The larvae overwinter in cocoons under the tree. The adults emerge from the soil from mid-May to mid-June. Eggs are laid singly, at the base of the needles. The little emerging caterpillars first eat the new growth and then, as they grow, move into older branches.
Nature is good at this. Whenever you see the new growth on spruce, you will see the small caterpillars. They can attain a length of close to an inch. With a yellowish-orange head and a body of light and dark green stripes. Once identified, they are hard to forget.
During a heavy infestation, most of the tree’s foliage will be eaten. Since the adults will emerge each spring from under the same tree, once marked, that tree will be a target until there are no needles left. Most trees get a few; this is quite common. If you notice high populations and significant bare patches on your spruce branch tips, you should act. Call a professional spray service. Today. Although picking them off by hand or spraying with a blast of water can be effective if you have only a few, you will need to seriously fight back a heavy infestation.