Many pests are controlled naturally by other insects, “the good guys”, both predators and parasites. Consequently, if no control measures are taken, the problem will frequently regulate itself. Sometimes the good guys will completely eradicate the pest; other times they just keep them in check.
Yet biological control doesn’t always work. There are a couple of reasons for this. An application of a broad spectrum insecticide will not only kill the target pest, but the good guys as well. Ants also interfere with biological control by protecting the pest from its enemies. In addition, windy oceanfront locations and hot greenhouse environments are generally not conducive to biological control activity.
If, for whatever reason, biological control is not working in the garden, a pesticide application may be called for. The first choice to control scale insects would be horticultural oil and/or an insecticidal soap. In general, gardeners need to select insecticides that are least toxic to the beneficial insects.
The best time to spray is right after the scale eggs hatch. At this time the young “crawlers” are most susceptible as they move about looking for a place to feed.