Tipping the scale

Published July 12, 2022

Magnolia scale
Photos : Raymond Cloyd. Top Left: Brian Kunkel, University of Delaware

Scale insects are notorious pests of trees and shrubs. It is essential to distinguish armored scales (family Diaspididae) from scales in other families such as soft scales (family Coccidae). This is because armored scales are less vulnerable to contact insecticides than soft scales.

Some of the most common armored scale species in nurseries are euonymus scale, tea scale, Japanese maple scale, false oleander scale, and juniper scale but the list will vary based on your region and crop plants.

Source: Greenhouse Management magazine, North Carolina State University

facts about scale insects

Identification: Soft scales are usually more convex or “helmet-shaped,” while hard scales, also known as armored scales, tend to be flatter. Indirect damage can also help growers determine which type of scale they have. Presence of honeydew or ants or black sooty mold means you have soft scale; absence of these signs means you likely have hard scale.

In addition to determining whether they have soft scales or hard scales, growers need to determine whether their scales are alive. When scales die, they remain stuck to plants. Because of this, some growers may think they have a current problem with scales, when those scales are already dead. Use tweezers or a blade to flip them over to see if the legs are moving.

Brown soft scale and ants

Because scales don’t fly, growers must do visual inspections. To identify specific scales, growers can send samples to a diagnostic lab, entomologist, or refer to extension publications.

Life cycle: Outdoors, scales need to overwinter. Compared to hard scales, soft scales have fewer generations but lay more eggs per generation. Every armored scale species has a different life cycle. However, all armored scales are immobile beneath a waxy cover, called a test, for most of their life.

Control: The parasitoids Aphytis melinus and Metaphycus helvolus, and the ladybug Lindorus lophanthae, are a few beneficial insects that attack scale insects. But not every beneficial insect will attack every type of scale.

Hemispherical scale

Aphytis melinus attacks California red scale in floriculture and citrus, and San Jose scale in prune, plum and other trees, according to the University of California Division of Agriculture & Natural Resources’ Integrated Pest Management Program. Lindorus lophanthae, also known as Rhyzobius lophanthae, is “the most important armored scale predator,” according to “Integrated Pest Management for Citrus, Third Edition,” by Steve H. Dreistadt.

Contact insecticides, such as soaps, oils and insect growth regulators, can also control hard and soft scale larvae or nymphs, or “crawlers.” However, they won’t work once the pests reach adulthood.

Once scales reach adulthood, things get more complicated for growers. Systemic insecticides will not be effective on hard scales when they’re applied to the soil or the growing media as a drench or granular application. However, insecticides with contact activity will work, if they are labeled for scales, depending on the mode of action and the compound. Applicators must read the label.

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