Horticultural oil sprays have often been recommended as treatment for scale and other insect pests.Homeowners, however, don't always find them reliable. Are they not effective or are other factors involved? The following information should be helpful.
Many pesticide labels, including horticultural oil, will give a time range in which to reapply the material, for example, in 5 -7 days. The reason for repeat applications is that certain stages of growth, like the egg and pupae stage, are not susceptible to insecticides as are the other stages of development. Consequently, eradication of a scale population, whitefly as well, may require as many as four applications. The 5 -7 day interval is to give time for the non-susceptible stage of the insect to develop to a susceptible stage.
Another factor is that horticultural oil is most effective when sprayed soon after the young scales, called crawlers, have emerged. This stage is susceptible to oil and other insecticidal sprays whereas older, usually darker colored adults are much less vulnerable. In fact, if the plant is sprayed when the majority of the insects are full grown adults, the results will be poor. Many of these adults may actually be dead, with eggs underneath the shell.
Be sure to apply a thorough coverage of the oil solution to both the top and underside of the leaves. In addition, it is prudent to make an application as soon as the insect pest is detected and populations are low. Do not apply oil when it is over 90°F. Drought stressed plants can be injured when sprayed with oil.
Concerning horticultural oils themselves, they are specially refined petroleum products, often called narrow-range or supreme oils. Some plant-derived oils are also available. One study has shown that the harsher organophosphate insecticides, like malathion, were no more effective than a thorough spray of oil or insecticidal soap alone, when properly timed. Another study found spraying oil on foliage once during the late spring was as effective as using a systemic organophosphate (acephate).
