Q: Is it a good practice to apply horticultural oil on shrubs and trees when they are dormant in order to prevent bugs this spring?

A: Yes, applying plant-safe oil is a great way to kill insects in winter. At one time, thicker oils were recommended for winter use but lighter oils were better for use in summer. Most oils nowadays have no temperature restrictions. The best time to apply an oil is on a warm winter afternoon when dormant insects are respiring; the oil easily suffocates them.