Japanese maple scale is an exotic armored scale species that infests a wide range of host plants (45 genera in 27 families) in the eastern United States. It infests some of the most common nursery and landscape plants in North Carolina including red maple (Acer rubrum), Japanese maple (Acer palmatum), dogwood, and broadleaf evergreens like holly, Japanese holly, and boxwood. These armored scale insects have a hard cover, called a test that protects them from their external environment. Adult scales are oyster shell shaped and 1 – 1.8 mm in length. Male and female adults look similar in shape but the females have a thicker, dark brown cover, which is the remnant of the second instar exoskeleton. The adult tests are often covered with white wax which can be wiped off to expose the brown cover. Beneath the test, scale bodies and eggs are usually purple.
Japanese maple scales typically infest the trunk and branches of trees but have been observed on leaves in heavy infestations. They use piercing-sucking mouthparts to penetrate the tree tissue and feed on parenchyma cells just beneath the bark. This damages the tree by robbing it of nutrients and energy necessary to grow. Heavily infested trees will exhibit twig and branch dieback, thinning canopy, and a gradual decline in health. The bark of heavily infested trees will have a bumpy white texture due to scale insect covers. Once scales die, the cover will often remain attached.
Japanese maple scale has one to several generations per year depending the location in the United States. They have two generations per year in Maryland, so they likely have two to four in North Carolina. They develop eggs in the spring, which emerge as crawlers over a several week period from late spring through early summer. The second generation emerges in August. Eggs and crawlers are purple in color. Adult females produce purple egg masses under their armored covering. Once crawlers emerge, they will settle on the tree bark within hours and begin producing their waxy cover.
In lightly infested trees, look for Japanese maple scales near twig crotches and buds. Once scale populations increase, whole stem surfaces or trunk may become covered in multiple layers of scales. Scales are often more common and dense on interior areas of plants where tissue is older and insecticide coverage is more difficult. Scout anytime of year to identify infested plants. Flip the test up to look for live females or mash scale covers to see if hemolymph is emitted. Beginning in April, check the tree bark for crawlers and nymphs or use tape (sticky side out) to determine when crawlers are active.
There are no established aesthetic or economic thresholds for when Japanese maple scale intervention is necessary. Consider site characteristics to help predict if scales will become abundant. Trees in hot sites surrounded by impervious surfaces (buildings, roads, parking lots) are likely to become heavily infested because the heat increases scale insect reproduction and tree stress. Scales may persist at low densities on trees in more vegetationally complex, cooler landscapes and never need intervention.
Research has shown that other armored scale insect species are more abundant and damaging in urban than rural or natural areas. In addition, the amount of surrounding vegetation and impervious surface affect the temperature and water availability to the trees. Therefore, consider the characteristics of the landscape when planting or managing a tree. Reducing plant stress by proper planting and watering may reduce susceptibility to infestation by Japanese maple scale. However, excessive fertilizer may increase scale abundance by making the tree more nutritious to scales and reducing the trees natural defenses.
Prune out infested branches. Light to medium infestations may be reduced 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 removing or damaging tree bark.
Parasitoid wasps and other natural enemies such as lacewings, lady beetles, and predacious flies may provide sufficient control to maintain a low population of Japanese maple scale. At least four species of parasitoid wasps are known to attack this scale along the eastern United States. However, in habitats that are not hospitable to natural enemies or where broad-spectrum pesticides are used, natural enemy control may not be sufficient.
Insecticide applications should coincide with crawler emergence for best control. This is challenging for Japanese maple scale because crawlers emerge over an extended period. Therefore, broad-spectrum contact insecticides such as pyrethroids may not be effective and can contribute to the problem by killing natural enemies. Avoid foliar applications of broad-spectrum insecticides.
Horticultural oils and dormant oils kill insects by smothering them and breaking down cell membranes. Dormant oil applications should be made in the fall after leaf drop or spring prior to bud break. These may be more practical when treating trees that are smaller in size. There is additional information on horticultural oils in Horticultural Oils as Insecticides.
Trunk sprays or soil drenches of systemic insecticides such as dinotefuran, clothianidin, 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 as soon as crawlers begin to emerge. See current product availability for armored scale management in the North Carolina Agricultural Chemicals Manual.
Publication date: July 3, 2015
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