Management: Horticultural oils can be used to treat the overwintering stage when trees are dormant. Insecticides are more effective in mid-April to early May before the candles open. Cut off and destroy galls before they open. A soil application of insecticide may also work well in fall. Contact your local extension agent for current recommendations. Avoid planting Colorado spruce and Douglas-fir together to reduce the severity of Cooley spruce gall.