I certainly would not put horticultural oil in the category of expensive. Spraying a dormant leafless tree is one thing, but spraying a tree in full leaf is quite another. I have used a horticultural oil (Brand name Ultra Pure) for control of mites, scale, aphids, thrips mealy bug, and white fly . It works wonderful. I apply in at the rate of 40 grams /gallon water. There is a huge difference in leaf surface evaporation time between oils, and this is important for the life of the leaf, as Brian wrote above.. Whatever, horticultural oil one uses be sure it is a lite high quality oil.