The first thing you have to understand is that the damage that comes to the spruce is mainly from the weather, moths, and caterpillars. The caterpillars and moths dry out the tree slowly, sometimes it is not noticed until the tree is on the verge of dying. One way to make sure this doesn't happen is to look under the spruce branches and the underside of the plant. You will be able to notice when it is getting withered by caterpillar accumulation. If there is a lot of caterpillars of the grayish-white color, then there is a problem. BT (baccilis tharynginsis) is the best way to get rid of them, plus it is very safe. Another way is to keep it watered good especially during the winter, and you will have a healthy tree.
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