Garner Exteriors performs dormant oil services and we found this article which explains many of the details of this process. Follow the link if you would like to read the whole article or see below for just an excerpt.
“This time of year we see the recommendation to spray dormant oil to control insects on everything from fruit trees to lilacs. But is it just any oil sprayed when the plants are dormant? Most commercial dormant oil sprays are refined from petroleum oil. A few are made from cottonseed oil. Unlike home remedies, commercial spray oils have an emulsifier added to allow the oil to mix with water. Many of the newer commercial oils are more highly refined than past dormant oil products. The new formulations are labeled to also be used, usually at a reduced rate, during the growing season. These are generally listed as horticultural, ultra fine or summer oils. Read and follow all label directions for proper timing and rates.
Oils kill exposed insects and mites by either suffocating them (covering up breathing tubes) or by directly penetrating the outside cuticle and destroying internal cells. Spraying trees with dormant oil after bud break and leaves have emerged will still control the pests, but it may kill the young leaves or cause leaf edges to turn black if the correct oil is not used at the proper rate.”