Answer: Summer weight horticultural oil (sometimes called dormant or summer oil) is usually recommended for control of spidermites on spruces. However, it should not be used on blue spruces. The "blue" on the needles is a waxy "bloom". Oils will remove the bloom and your blue spruces will no longer be blue. You can treat the pests with insecticidal soaps but these, too, should be used with caution. They have fatty acids which will temporarily discolor the needles of your spruces. This will eventually weather away, but until then the trees may look unsighltly. If you're growing 300 trees, you might be a commercial grower. If so, your best course of action is to talk to a forester or contact your local university for their recommendations.
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