Horticultural oils are nothing new. They’ve been around practically since humans have been gardening… and encountering insect pests.
One of the best known is dormant oil which is applied to the plant in the spring, after the snow melts but before bud break (hence the name, since it’s applied to dormant plants). However, there are also lighter oils that can be used while plants are growing. The latter may be called horticultural oils, summer oils or superior oils. All are considered biological pesticides.
Oil acts by covering the insect (or its pupae or eggs) and thus plugging the pores through which it breathes, killing it by asphyxiation. It also melts the waxy coating that protects many insects, such as mealybugs, leaving them exposed to the drying effect of air. Essentially, horticultural oils operate in the same way as do insecticidal soaps.
The biggest advantage of horticultural oils is that they are not poisonous; their action is strictly physical. Thus they are harmless to humans, mammals, birds and fish. To be effective, however, they must touch the insect. Also, their residual effect is minimal: they degrade after a few days and disappear.
In the eyes of a laidback gardener, their greatest fault is that they are a very broad spectrum pesticide: they kill indiscriminately both insect pests and beneficial insects. (To be fair, the same could be said about most insecticides.) They should therefore be used carefully, only directly on the insect you’re trying to control. Dormant oils, of course, are used when few if any beneficial insects are present. If you apply a horticultural oil in the summer, however, its probably best to do so early in the morning before beneficials such as bees are present.
Horticultural oils are used to control adelgides, aphids, caterpillar eggs, leafhoppers, mealybugs, spider mites, thrips, and whiteflies, among others. Also, they can help to prevent certain diseases, such as powdery mildew and, indirectly, viruses, because the latter are transmitted by aphids and leafhoppers.