Soft Scales

Soft scales (Coccidae) infest numerous species of broadleaved perennials. These phloem-sucking insects develop through three life stages: eggs, nymphs, and adults. They do not resemble most other insects. Adult females are 1/5 inch long or less and circular to oval. Adult females and settled nymphs are wingless and lack a separate head or other easily recognizable body parts. Mature females can be raised in profile, domelike or hump shaped. The surface of soft scales is part of their body, unlike the cover of armored scales that can be removed to reveal the insect beneath.

Adult males are rarely seen and apparently do not exist in certain species. They are tiny, delicate, whitish to yellow insects with one pair of wings and long antennae. The elongate, pale cases of male puparia of certain species are sometimes abundant in groups on bark. Most soft scales have one generation per year, although brown soft scale has several.

Eggs are produced beneath the body of the female. The mobile, first instars (crawlers) walk to new sites or disperse with wind, moving water, and as contaminants on propagation tools and workers. After settling to feed, soft scales remain attached, feeding at the same spot for much of their life. On deciduous hosts they move to bark to overwinter, generally during the second instar. If the quality of the host plant declines, nymphs and pre-ovipositional females may be able to walk slowly to settle and feed elsewhere.

Black scale nymphs are brown to yellowish. Adult females are 1/12 to 1/5 inch long and dark brown to blackish. During the late second instar, nymphs develop a raised ridge on top that expands to form an H-shape. Older nymphs become mottled dark and gray and develop a leathery texture. Once egg laying starts, black scale adult females become globular and hardened and the H-shape ridge on the back disappears.

Brown soft scale adult females are 1/16 to 1/5 inch long and slightly raised in profile. Adult females and nymphs vary in color from yellowish green to dark or pale brown, and can be mottled in coloration or uniformly colored.

Hemispherical scale adult females are 1/12 to 1/6 inch long and dark brown, hard, smooth, rounded domes. Nymphs are oval and a paler brown.

Green shield scale nymphs and females are light brownish to yellow-green. Mature females are about 1/6 inch long, excluding their egg mass. Each mature, female green shield scale produces an egg mass of flocculent, pale wax that becomes apparent around the margin of her body. Even after green shield scales stop feeding and have died, this pale wax may persist on plant parts.

Soft scales suck and feed on phloem sap and excrete copious sticky honeydew, followed by growth of blackish sooty mold, which fouls plants. Honeydew attracts ants that protect scales from their parasites and predators, and carry scales between plants. Green shield scale ovisac wax persists a long time on plant surfaces.

Biological and cultural controls and insecticide application can manage soft scales.

Most soft scale species can be well controlled by various predators and parasites. Especially important are parasitic wasps (Aphytis, Coccophagus, Encarsia, Metaphycus, and others) that feed as larvae inside scales. Parasitism is evident in scale covers from emergence holes of adult wasps. Immature parasites are sometimes visible through the surface of scales' body. Scales can be differently colored when parasitized, such as scale nymphs that normally are brown to yellowish may turn black when parasitized.

Scales are preyed upon by brown lacewings, green lacewings, predatory bugs, and various lady beetles including Chilocorus, Hyperaspis, and Rhyzobius spp. Lady beetles can easily be overlooked because many species are relatively small, the adults are commonly colored and shaped like scales, and the larvae often feed beneath scales. The mealybug destroyer, Cryptolaemus montrouzieri, is an effective predator of green shield scale because this scale produces waxy ovisacs that attract the lady beetle and induce it to lay eggs.

To take advantage of scale biological control, effectively control ants, reduce dustiness around growing areas, and avoid or minimize the application of persistent, broad-spectrum insecticides for all pests. See Relative Toxicities of Pesticides Used in Floriculture and Nurseries to Natural Enemies and Honey Bees to learn which insecticides are most compatible with biological control.

Start production with pest-free stock. Rogue and dispose of plants with abundant scales to avoid the risk of spreading throughout production areas. Control weeds and promptly remove and dispose of weeds and crop residue in covered containers. Exclude windblown crawlers of scales and other pests by covering openings to the greenhouse with fine mesh screens of sufficient surface area to provide adequate ventilation. Prune out and discard heavily infested plant parts, then apply insecticide if pesticide-susceptible life stages (generally crawlers and settled nymphs) are present.

Biological and cultural controls are organically acceptable management methods. The botanical neem oil and certain narrow-range oils (Organic JMS Stylet Oil) are organically acceptable.

Visually inspect plants for scales, honeydew and sooty mold to locate infestations, which may indicate applications are needed only to spots where scales occur. Watch for ants on and around plants, which may be attracted to and tending soft scales or other phloem-sucking insects. Insecticide application generally is warranted when pesticide-susceptible scale stages are present.

Crawlers and early instars are most susceptible to insecticides. For soft scales with only one generation annually, most crawlers will emerge within a period of several weeks during spring. To monitor crawler activity to time applications use traps of double-sided sticky tape wrapped around twigs in late winter near where overwintering scales are observed. Since not all crawlers emerge at the same time,multiple applications can be warranted depending on the duration of crawler emergence and the pesticide used. See Establishing Action Thresholds and Sticky Tape Traps for more information.

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