Q. Should you spray fruit trees with horticultural oil if you are planting them for the first time? New bare root fruit tree ready for planting A. Horticultural oils are made from refined mineral oil and used during cooler times of the year to smother insects that might cause problems to a number of different plants. Even though horticultural oils are “natural” they are considered an insecticide since they kill insects by “smothering” them. They are most effective on aphids, scale insects, whiteflies and spider mites. Horticultural oils are indiscriminate killers of insects, similar in that regard to soaps used as insecticides. They are indiscriminate because they kill any insect, bad or good, that is sprayed. But these types of sprays are safer to apply and for the environment than most conventional insecticides. Aphids causing leaf curling and stickiness on Plum No, you don’t need to spray newly planted trees with horticultural oil when planting them unless you think they were infested when you bought them. However, it is not a bad idea and probably a good safety precaution since these trees are coming from places where there are usually a lot of new insects that can be brought into your garden.