Azalea bark scale (ABS, Eriococcus azalea, Figure 1) poses a serious threat to azaleas, rhododendron, and Pieris (Andromedas) in Georgia. ABS is a soft scale insect; unlike armored scale insects, the nymph and female soft scales secrete a protective coating of waxy crust on their body that cannot be separated from the scale insect. ABS also is found on blueberry, hawthorn, huckleberry, poplar, and willow. They often are mistaken for mealybugs because of their elongated, fuzzy white appearance.
Life cycle. Females of ABS lay purple-colored eggs in the white, felt-like eggmasses (ovisacs) during the spring (Fig. 2). The purple-red colored females reside inside the ovisac. An ovisac is about 3 mm long. The eggs hatch by mid-May, and may last for two to three weeks. The first nymphal stage of the ABS is referred to as a “crawler”. The red-colored crawlers are the only mobile stage of the ABS as they have legs and antennae. The crawlers also lack the white crusty covering. When they find a crack or crevice on the bark surface, they settle on those areas and begin feeding by inserting their long, thread- or needle-like mouthparts into the bark to reach the phloem tissue. Phloem tissue is where the nutrients are transported within the plant, and it runs along the stem beneath the bark. ABSs also found in areas where azalea branches fork. They shed their legs and antennae and grow into 2nd and 3rd nymphal stages. The scale can undergo two generations per year in Georgia. The females mature by September, and lay eggs in the ovisacs. The emerging, second-generation ABS nymphs overwinter within cracks and crevices in the stem (Fig. 3).
Damage. Symptoms of ABS infestation include yellow dropping leaves and dying branches (Fig. 4). Developing nymphs and females excrete sugary syrup-like liquid on the bark and leaves, often called honeydew. As time passes, the surface of the bark, and sometimes leaves, turn black because they get infested with a black sooty mold fungus (Fig. 5). The fungus feeds on the sugary secretions. The white crusts of scale insects are easily visible when the azalea bark completely turns black (Fig. 6). The ABS infestation rarely kills the plant, unless the infestation is extremely severe and not controlled.
Management. ABSs often attack weak, stressed plants. Keeping plants healthy with proper fertilization and irrigation will improve tolerance to ABS infestation. Pruning and removal of affected branches (leaves and stems) in winter will help reduce the severity of the ABS infestation. Several predators and parasitic wasps attack the ABS and reduce their populations. Avoiding broad spectrum contact insecticides conserves the activity of predators and parasitic wasps. If the infestation is not severe, the ABS can be physically removed by scaping the scale insects from the bark surface using a brush or fingernail.
A spray of horticultural or summer oil after azalea bloom in the late spring is advised for ABS control. The spray coverage is essential. If the infestation is severe, a second application might be necessary within the same season. The efficacy of the oil spray can be monitored on the second generation in September or the spring the following year. If the oil spray application was effective, the production of the new white crust will be reduced. The white crusts of old ovisacs do not peel off naturally, and they can still be attached to the bark regardless of whether the scale insect is dead or alive. The ovisac of dead scale insects will not be juicy when crushed. If the scale insects are still alive, the scale insects will be juicy when crushed, suggesting that the oil or pyrethroid insecticide spray application did not adequately control the ABS, and a follow-up application might be necessary. Avoid spraying when beneficial insects, such as pollinators, are active. For the most current insecticide recommendations, contact your local county Extension office by calling 1-800-ASK-UGA1.
Baker, J. 2019. Azalea bark scale. NC State Extension Publications, factsheet. https://content.ces.ncsu.edu/azalea-bark-scale
Gilrein, D. 2015. Matters of scale: Wax and azalea bark scales. Excerpted from Branching Out IPM Newsletter. Plant Pathology & Plant-Microbe Biology, Cornell University. Volume 22 No. 5. https://ecommons.cornell.edu/bitstream/handle/1813/60591/BranchingOut_MattersOfScale5-15.pdf?sequence=1&isAllowed=y [WSU] Washington State University Extension. 2021. Rhododendron: Azalea bark scale. Hortsense fact sheet. http://hortsense.cahnrs.wsu.edu/Search/MainMenuWithFactSheet.aspx?CategoryId=1&SubCatId=4&PlantDefId=32&ProblemId=537