By Nate Royalty, Ph. D.
Scales (Superfamily Coccoidea) are a diverse group of sucking insects that are common pests of trees and shrubs. There are hundreds of species that attack ornamentals. Scale insects are unusual in that the adults secrete a shell-like layer that covers all or most of the entire body. This covering protects the insect from predators, parasites, and adverse environmental conditions.
Scale insects vary considerably in color – they can be black, brown, green, grey, orange, yellow, red, purple or white. The color often matches that of the host plant, making them difficult to detect, while the color of other species contrasts greatly with the host.
There are two major families of scale insects: soft and armored. The shell of soft scales typically is prominent, and is characterized by a thick waxy coating. Soft scales typically are round or oval, often hemispherical, and are 1/8 to 1/2 inch in diameter. Soft scales, like aphids and whiteflies, secrete honeydew; when infestations are heavy, infested plants will be covered with large amounts of this sticky substance. The honeydew is a food source for sooty mold. Foliage covered with this gray or black fungus is another indicator of a soft scale infestation.
In contrast, armored scales are covered with a thin, ‘hard’ shell that doesn’t have the thick wax characteristic of the shell of soft scales. Armored scales range in size from 1/16 to 1/8 inch in length; females typically are larger than males. Armored scales can be circular, oval, oblong, thread-like or pear-shaped. Armored scales do not secrete honeydew.
Host material and range
Scales attack hundreds of ornamental varieties, and are found everywhere ornamental plants are grown. While some scales infest a wide variety of plants, others are host-specific. Many ornamentals are attacked by a single scale species, while others are attacked by numerous species.
Armored scalesScales damage plants by sucking the juices from the plants. Heavily-infested plants appear unhealthy and produce little new growth. Scales that feed on leaves can cause leaf spotting; spots progressively increase in size as the scales continue to feed. With serious infestations, leaves drop prematurely, and twigs and branches may be killed. The sooty mold that develops on the honeydew produced by soft scales inhibits the plant’s ability to harvest light energy, reducing photosynthesis and further weakening the host plant. Scales that feed on twigs or bark generally do not produce leaf discoloration, but long-term feeding weakens the host plant. A chronic infestation can kill the host.
Periodically inspect host plants, particularly plants that have been previously infested. Since scale insects may occur on all plant parts, check the trunk and branches, as well as the twigs and leaves. The midvein of the underside of the leaf is common site for leaf-feeding scales to inhabit. Stems should be checked around buds, leaf petioles and lenticles or other depressions which may offer a place for the scales to hide and feed. The color contrast between the bark and the scale can be increased by wetting the bark with water before inspecting. Crawlers, the immature mobile life stage, may be monitored for by placing double-stick tape on some branches of the plant.
Many cooperative extension services have excellent references on scales; information on the most commonly occurring scale problems that attack specific ornamentals can be found on their Web sites.
Stressed plants are more susceptible to scale attack. Maintaining healthy plants will minimize the likelihood of a plant being attacked.
Scales, especially armored scales, are difficult to control. The life-stage most susceptible to insecticides is the crawler stage, which typically appears in spring. Crawlers can easily be controlled by oils and contact insecticides, but successful control depends on precise application timing. If the armored adults are present, control is much more difficult. Foliar sprays with synthetic pyrethroids or neonicotinoids can be effective against the armored adults, particularly when mixed with horticultural oil. Soil-applied systemic insecticides such as imidacloprid are effective against a wide variety of scales, but it is important to remember that no one product is effective against all species. Cooperative extension Web sites and product labels provide useful information on the efficacy of products against a particular species.
It also is important to remember that it may take weeks or even months before complete control is observed. Control is delayed if the host plant is under stress, as uptake of the systemic insecticide is slowed. Judging efficacy is complicated by the fact that the scale shells often remain on the plant long after the insecticide has killed the insect underneath. After application, test for efficacy by scraping the shell off the plant and looking for the soft, pulpy insect underneath. If only dry shells are scraped off, then the treatment probably was effective.
What can you do?
Keep susceptible host plants healthy and well watered; scales nearly always attack stressed plants. Inspect plants periodically for presence of scales. If an infestation is detected, treat before the infestation becomes too severe, and monitor the progress of the treatment in the weeks and months after application. Be aware of the most commonly occurring scale problems in your area. Annual applications of soil-systemic insecticides to susceptible ornamentals are a good idea, particularly if plants are under stress.
Nate Royalty, Ph.D., is product development manager — insecticides for Bayer Environmental Science