Nursery, Greenhouse, and Landscape News

Conserving Natural Enemies of Armored and Soft Scales

Most armored and soft scale problems in landscapes occur when conditions become hostile to natural enemies, and favorable to scales. Maximize the use of local natural enemies by adopting strategies that consider plant health, and the sensitivity of scales and their natural enemies to pesticides. First, improve plant health by adopting proper watering and fertilization practices. Then, start monitoring trees for live scales and natural enemies, starting in the dormant season.

Check twigs, branches and trunks for the presence of live soft and armored scales by scraping your thumb across a scale infested twig. When crushed, live soft and armored scales will be full of fluid.
Check for parasitoid activity by examining scale covers for round holes from where adult parasitoids have emerged. Note that holes left by emerging male scales are often ragged.
Maggot-like parasitoid larvae may also be found within scales.
Check for presence of lady beetle adults feeding on scales.
Check for larvae of predators feeding on scale bodies or in egg masses. Some larvae are covered with wax and resemble a mealybug, like the mealybug destroyer or are naked like Chilocorus kuwanae.

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, it only kills the insects it covers. 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, are not killed in the dormant season by oils. Look for live scales on plants when crawlers are supposed to emerge. If crawlers are abundant, then apply a verdant spray of 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 significantly more crawlers have emerged and a second application of soap or oil is needed. 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 initiating the crawler or dormant spray. 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 local numbers of natural enemies, they can return in future years if use of these materials is avoided.

- Cliff Sadof, Purdue University

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