Dormant season application of horticultural oil is advisable when natural enemy activity is low and there are many live scales present. Although oil may kill some parasitoids that overwinter in the scales, oil only kills the insects when sprayed directly on them; oil does not affect natural enemies that arrive after it dries. Note that oils are most effective on scales that overwinter as immatures or fertilized females. Those which overwinter as eggs, such as oystershell scale and pine needle scale are not killed in the dormant season by oils.
Inspect plants when scale crawlers are supposed to emerge. If crawlers are abundant, apply a verdant spray of soap or oil to kill crawlers during the peak of crawler emergence. As with the dormant spray, this action will help to reduce scale numbers while minimizing the impact on beneficials. The inspection process should be repeated in two weeks to determine if enough additional crawlers have emerged to require a second application of soap or oil. Repeat this inspection and treatment decision making process for each generation of scale crawlers.
When heavy infestations have caused significant branch dieback, more drastic rescue measures are needed. If the tree is a vigorous grower, consider pruning out and destroying the most heavily infested branches before spraying with oil. As a last resort, apply a half rate of broad spectrum insecticide with 1% insecticidal soap during one crawler period to lower the pest population and reduce tree stress. Although this can reduce numbers of natural enemies, they can return in future years if use of these materials is avoided.
- Cliff Sadof, Purdue University