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Extension > Garden > Insects > Scale insects on Minnesota trees and shrubs > Scale insects that feed on deciduous trees: Soft Scales

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Scale insects that feed on deciduous trees: Soft Scales

Dan Potter, University of Kentucky

Figure 16. Lecanium scale on the branches of a red oak.

Lecanium Scale

Parthenolecanium spp.

Appearance: Females are initially flattened and brown in appearance. As they mature, they become hardened and round. Lecanium scales, as a group, are often difficult to distinguish one from another. The European fruit lecanium scale (Parthenolecanium corni) is probably the most common lecanium scale found in Minnesota on deciduous plants (Figure 16).

Hosts: Oak, maple, crabapple and a wide variety of other trees and shrubs.

Damage: Dieback of twigs and branches and entire plants can occur in heavy infestations.

Life history: The scales overwinter as second instar nymphs on twigs. In spring, scales mature and females lay eggs in May and June. Crawlers hatch in June and July and migrate to leaves to feed. In late summer, crawlers migrate back to twigs to overwinter. There is one generation per year.

Management: Late fall and early spring (before bud break) applications of horticultural oils will suffocate overwintering nymphs. Scales can also be effectively managed by systemic treatments of imidacloprid applied in the fall or by spring treatments with dinotefuran. Insecticidal soaps, horticultural oils, insect growth regulators (pyriproxifen and buprofezin [available only to licensed applicators]), and residual insecticides (such as pyrethroids, acephate, and carbaryl) can be applied to treat the crawler stage. Timing of these applications when the crawlers are active is crucial for effective control.

Whitney Cranshaw, Colorado State University, Bugwood.org

Figure 17. European elm scale on the branch of an American elm.

European Elm Scale

Gossyparia spuria

Appearance: Mature females are up to 2/5 inch (10 mm), oval and brown with a white, waxy fringe. (Figure 17).

Hosts: Elm, especially American elm

Damage: Scale feeding can cause stunted, chlorotic foliage, premature leaf drop and branch dieback. Black sooty mold growing on honeydew secretions can blacken infested limbs.

Life history: The scales overwinter as second instar nymphs in bark cracks and crevices. Females mature in late May, mate and begin laying eggs. Crawlers begin appearing in late June and egg hatch may extend through the end of July. Crawlers feed on leaves throughout the summer, and then migrate to branches before leaves drop in the fall. There is one generation per year.

Management: Late fall and early spring (before bud break) applications of horticultural oils will smother overwintering nymphs. Scales can also be effectively managed by systemic treatments of imidacloprid applied in the fall or by spring treatments with dinotefuran. Insecticidal soaps, horticultural oils, insect growth regulators (pyriproxifen and buprofezin [available only to licensed applicators]), and residual insecticides (such as pyrethroids, acephate, and carbaryl) can be applied to treat the crawler stage. Timing of these applications when the crawlers are present is crucial for effective control. Imidacloprid is highly effective against this species.

Jeffrey Hahn, University of Minnesota Extension

Figure 18. Magnolia scale on the twigs and branches of a magnolia.

Magnolia Scale

Neolecanium cornuparvum

Appearance: Females are about 1/2 inch (12 mm) in diameter, smooth, elliptical, spherical, pinkish-orange to brown insects that are covered with a waxy coating. Overwintering nymphs are dark gray with a red-brown ridge down the middle (Figure 18).

Hosts: Magnolia

Damage: Infested branches and twigs can be weakened and growth slowed. Honeydew created by the scales attracts ants, wasps and other insects. Honeydew also gets on adjacent structures and plants causing the fungus sooty mold to grow on affected plant foliage. Heavy infestations completely encrust branches causing branches and entire plants to die.

Life History: Overwinter as nymphs on one- or two-year-old twigs. Nymphs mature in late July through early August. Crawlers hatch in late August or early September. Crawlers settle on the young twigs where they overwinter. There is only one generation per year.

Management: Magnolia scale populations can build rapidly within one generation. Treatments should be made the same season they are observed on the plant. Late fall and early spring (before bud break) applications of horticultural oils will smother overwintering nymphs. Scales can be effectively managed when fall and spring horticultural oil sprays are combined with a systemic insecticide in the spring. Insecticidal soaps, horticultural oils, insect growth regulators (pyriproxifen and buprofezin [available only to licensed applicators]), and residual insecticides (such as pyrethroids, acephate, carbaryl) can be applied to treat the crawler stage. Timing of these applications when the crawlers are active is crucial for effective control.

Deb Den Herder, University of Minnesota Extension

Figure 19. Cottony maple scale on a silver maple..

Cottony Maple Scale

Pulvinaria innumerabilis

Appearance: The most conspicuous stage is the brown adult female with a large cottony mass (egg sack) protruding from the rear. Females without egg sacs are 1/8 inch (2-3 mm) long, flat, pale to dark brown and soft (Figure 19).

Hosts: Maples (especially silver), honeylocust, linden and other hardwoods.

Damage: Infestations are rarely threatening to the health of the plant as natural enemies do an excellent job of regulating their numbers. Large amounts of honeydew produced by the crawlers feeding on the foliage support sooty mold growth. Heavy infestations are typically are not persistent from one year to the next.

Life history: Overwinter as immature, flat females on twigs. They begin growing in May, and by early June, the conspicuous, white egg sacks appear. Crawlers hatch in late June to early July and move to the undersides of leaves to feed. After spending the summer feeding on the leaves, mated females move back to twigs to overwinter. There is one generation per year.

Management: Cottony maple scale infestations are usually sufficiently managed by natural enemies and will not typically require management efforts. Late fall and early spring (before bud break) applications of horticultural oils will smother overwintering females. Scales can also be effectively managed by systemic treatments of imidacloprid applied in the fall or by spring treatments with dinotefuran. Insecticidal soaps, horticultural oils, insect growth regulators (pyriproxifen and buprofezin [available only to licensed applicators]), and residual insecticides (such as pyrethroids, acephate, and carbaryl) can be applied to treat the crawler stage. Timing of these applications when the crawlers are active is crucial for effective control.


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