How2Day: Gray Scale on Shrubs

Azalea shrubs can be affected by scale. ULTRA.F/Photodisc/Getty Images
Gray scale on shrubs may appear as simple bumps on leaves and twigs, but home gardeners should heed their presence as a sign of trouble. Scale are sucking insects that remove inner tissue fluid, or sap, from shrubs. Feeding causes cosmetic to severe damage that can be avoided through proper care and early diagnosis.

Scurfy scale, Chionaspis furfura, are pests with dirty white/gray color that attack deciduous shrubs. Causing both twig and branch death and overall diminished health, scurfy scale insects are present on most plant parts including trunks, twigs, branches and leaves. Home gardeners can expect feeding during June from immature crawlers. Scurfy scales are tiny, with females measuring approximately 0.1 inch in length, with males even smaller. For control, horticultural oil offers management during shrub dormancy or on crawlers, according to the University of Minnesota Extension. For more severe problems, gardeners should apply a chemical pesticide appropriate for the particular shrub with the active ingredient carbaryl or acephate just after egg hatch. Contact your local county extension agent or licensed professional to verify proper control or get assistance.

Oystershell scale, Lepidosaphes ulmi, are scale insects that measure approximately 0.1 inches in length. Oystershell bodies, as the name implies, are shaped like oysters. Attacking deciduous shrubs as their host plants, oystershell scale display bodies in gray with a purple cast. During the active crawler stage of May through June, oystershell scale insects seek out trunks and branches where their feeding results in fading and poor growth of foliage, and cracks in bark as well as discoloration. Extreme cases can result in plant death, so control is essential, according to the University of Minnesota Extension. To manage oystershell scale on shrubs, gardeners should follow the same management method as for scurfy scale by applying a insecticide labeled for treating scale.

Gloomy scale, Melanaspis tenebricosa, attacks shrubs like mulberries. Displaying gray to near-black armor that covers their bodies, gloomy scale insects often blend into the color of their host shrub's bark, according to the North Carolina State University Cooperative Extension Service. Crawlers, or immature scales, are microscopic and mature into female adults measuring 1/16 inch in width and males that measure up to 1/32 inch. With crawlers appearing in May, gloomy scales are most active in all life stages during August to September. Feeding leads to twig die back and diminished health. For control, maintain vigorous shrubs since stressed plants are more vulnerable to attack. Gardeners should release predatory insects like lady beetles that hunt and kill scales, or parasitic wasps that lay eggs within scale bodies, leading to death. These natural enemies are available in garden supply stores, catalogs or online. Additionally, low-toxicity insecticide horticultural oil is effective when applied to shrubs just before bud break.

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