Various types of oils have been used for centuries to control insects and mites on plants. In the 1800s, mixtures of kerosene and soap were used to control pests on plants. For most of that time the oils had to be used when the plants were in a dormant state, as the viscosity of the earlier oils tended to damage most foliage. Recently refinements have been made so that horticultural oils may be used all year 'round. The early oils were petroleum-based. However most modern oils are either plant- or mineral-based. In fact recent studies have shown that plant based oils such as soy bean are just as effective as petroleum or mineral oils and much less phytotoxic.

Horticultural oils are an excellent alternative to chemical pesticides. They are also an excellent choice when used in conjunction with other alternative pesticides, including Bt (Bacillus thuringiensis) and NPV (nucleopolyhedrovirus) insecticide.

The way in which most oils work to control insects is that they block the insect's spiracles (the air holes from which the insect breathes) thus asphyxiating them. Oils may also affect how an insect feeds thus starving them. Though not effective on all garden pests, horticultural oils can successfully combat common nuisances including aphids, scale, whiteflies (insects), and mites (arachnids). All are controlled by relatively low concentrations of oil (usually 1-2%) that generally are not phytotoxic.

Dor,ant Season ApplicationSummer/Foliar Applications
Aphids that curl leaves in spring Adelgids
Caterpillars that winter it's eggs on leaves. (Leaf Rollers, Tent Caterpillars) Aphids
Mites, that winter on the plant (e.g.conifer infesting species) Eriophyid mites
Kernes Scale Leaf Hoppers
Cottony Maple Scale Scale Insects

Powerdy Mildew can also be controlled by use of summer oils.

I know many of you are wondering about the effect of these products on beneficial insects. Most beneficial insects, such as green lacewings and ladybird beetles, scatter before the spray comes and aren't bothered by the residue when they return. However, small, soft-bodied beneficial insects such as predatory mites can't move out of the way fast enough and are killed. If you rely on beneficial mites to deter other pests, think twice before using oil (or any other pesticide). Better still, release beneficial mites several days after you treat with the oil spray.

Some precautions to consider when using horticultural oils.

1. Do not use oils in a spray tank that has previously contained a sulfur-based fungicide.
2. Do not mix oils with a fungicide, or spray within 2 weeks of a fungicide.
3. Treat small areas first if phytotoxicity is a concern
4. Do not mix oils with carbaryl (Sevin).
5. Do not apply if rain is predicted, or foliage is wet.
6. Do not apply if humidity is expected to remain above 90% for longer than 36 hours.
7. Do not apply to drought-stressed plants.
8. Do not apply to sensitive plants.
9. Do not apply when buds are fully open.

A Horticultural Oil Glossary

Dormant oil: An oil used on woody plants during the dormant season. This term originally referred to heavier weight, less well-refined oils that were unsafe to use on plants after they broke dormancy. However, these older oils have been replaced with more refined, light-weight oils that have potential application to plant foliage. Dormant oil now refers to the time of application rather than to any characteristic type of oil.

Horticultural oils: An oil used to control a pest on plants.

Mineral oil: petroleum-derived oil (as opposed to vegetable oils).

Narrow-range oil: A highly refined oil that has a narrow range of distillation. Narrow-range oils fall in the superior oil classification. The terms may be used nearly interchangeably. Spray oil: An oil designed to be mixed with water and applied to plants as a spray for pest control.

Summer oil: An oil used on plants when foliage is present (also called foliar oils). As with dormant oil, the term now refers to the time an application is made rather than to the properties of the oil.

Supreme oil: A term used to categorize highly refined oils that distill at slightly higher temperatures and over a wider range than the narrow-range oils. Most supreme oils meet the characteristics of superior oil.

Superior oil: A term originated by P.J. Chapman in 1947 to categorize summer-use oils that met certain specifications. This included a high proportion of paraffinic hydrocarbons and purification that allowed year-round use without phytotoxicity. Since then, further developments have resulted in oils that distill over a narrow temperature range. Most superior oils are now better referred to as narrow-range oils.

Vegetable oil: An oil derived from the seeds of some oil seed crop (e.g., soybeans, canola, and cottonseed).

For those of you not familar with the term phytotoxicity, it is defined here.