Gloomy scale is primarily a pest of red maples but has been observed feeding on other tree species including sugar maple, elm, tulip poplar, hackberry, boxelder, buckthorn, sweet gum, gallberry, mulberry, native hollies, and soapberry. Gloomy scale is found throughout the southeastern United States as far north as Maryland, south to Florida, and west to Texas. They are more abundant on trees in cities than in natural areas. Adult female covers, called tests, are up to 2 millimeters wide with a central pale ring. The test is convex and can be grey to brown and blends in with the bark. Beneath the test the soft-bodied scale insect is pink, legless, and wingless. Males are smaller, have an oval shaped armored covering, and develop legs and wings as an adult. The nymphs, called crawlers, are less than 1mm and orange. The young female nymphs resemble adults but have significantly smaller bodies and armor.
Gloomy scales use their piercing-sucking mouthparts to penetrate the tree bark and feed on parenchyma cells, which synthesize and store organic products within the tree. This damages the tree by robbing it of nutrients and energy necessary to grow. Damage may also be caused by toxins in saliva that are injected while feeding. Heavily infested trees will exhibit twig and branch dieback, thinning canopy, and eventual death. The bark of heavily infested trees will darken and have a bumpy texture due to scale insect covers.
Prevent gloomy scale infestations and long-term management by selecting correct planting sites for red maples. Trees planted near large amounts of impervious surfaces are more prone to gloomy scale infestation and subsequent damage. Impervious surfaces reduce water availability and increase soil and air temperatures. Gloomy scale becomes more abundant as tree canopy temperature and drought stress increase. When planting red maples, be conscious of the surrounding landscape and avoid planting in areas with greater than 60% impervious surface. Research has shown that reducing plant stress by proper planting and watering can reduce susceptibility to infestation and damage by gloomy scale. Equipment such as Tree Gator® slow-release watering bags can reduce drought stress. However, excessive nitrogen fertilizer may increase scale abundance by making the tree more nutritious to scales and reducing the trees natural defenses.
The most common parasitoid wasps known to attack gloomy scale are in the genera Signiphora, Encarsia, and Ablerus. Other natural enemies such as lacewings, lady beetles, and predacious midges may also provide supplemental control of gloomy scale populations. However, natural enemy control is minimal in urban landscapes either due to warmer temperatures, lack of alternative resources, or lack of vegetation refuges. To maximize potential biological control, provide habitat for natural enemies and reduce temperatures by increasing vegetation cover and complexity around trees.
Although there have not been any reported trials with gloomy scale, light to medium infestations of scale insects may be effectively treated with pressure wash applications. High-pressure water sprays can wash scales and scale covers off bark and reduce populations without the need for chemical controls. Make applications when trees are dormant for the winter and make sure the water pressure is not damaging tree bark.
Chemical control of gloomy scale can take several years to see results and is often expensive. When trees are very heavily infested, consider costs and benefits of treatment compared to tree replacement with another species.
Foliar insecticide applications should coincide with crawler emergence for best control. This is challenging for gloomy scale because crawlers gradually emerge over 6-8 weeks. Therefore, broad-spectrum contact insecticides such as pyrethroids may not be effective and can contribute to the problem by killing natural enemies.
Horticultural oils and dormant oils kill insects by smothering them and breaking down cell membranes. Horticultural oils can also penetrate waxing scale covers. They can be applied during crawler emergence or when trees are dormant to kill overwintering adults. These may be more practical when treating trees that are smaller in size. There is additional information on horticultural oils in Horticultural Oils for Ornmental Plants.
Trunk sprays or soil drenches of systemic insecticides such as dinotefuran and acephate may provide effective, season-long control of many armored scale insects. Acetamiprid is a systemic insecticide that can be applied to foliage. Insect growth regulators such as pyriproxyfen and buprofezin can also provide effective control and are applied to foliage. See current product availability for armored scale management in the North Carolina Agricultural Chemicals Manual.
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