Pest 411 Azalea bark scale

Prune now to reduce infestation

Scout for scale insects on azalea bark

by Shimat V. Joseph, Department of Entomology, University of Georgia

Azalea bark scale (ABS) (Eriococcus azaleae) (Fig. 1) poses a serious threat to azaleas, rhododendron, and pieris or andromedas in Georgia.

They are also reported on blueberry, hawthorn, huckleberry, poplar, and willow. Often, ABSs are mistaken for mealybugs because of their elongated, white-fussy appearance. ABS is a soft scale, meaning the nymphs and females secrete waxy crust (protective coating) on their body and cannot be separated from the scale insect, unlike armored scale insects.

Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org Fig. 1. Females of azalea bark scale.

Lifecycle

Females of ABS lay purple-colored eggs in white, feltlike eggmasses or ovisacs during the spring (Fig. 2). The purple-red colored females reside inside the ovisac, which is about 3 mm long. The eggs hatch by midMay. The egg hatch may last for two to three weeks in May. The first nymphal stage of the ABS is referred to as "crawlers," which emerge from the eggs. The redcolored crawlers are the only mobile stage of the ABS as they have legs and antennae. The crawlers also lack the white crust covering. They walk around searching for a suitable spot to settle. 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, reaching phloem tissue. Phloem tissue is where the food is transported within the plant, and it runs along the stem beneath the bark. They are also found in areas where the azalea branches fork. They shed their legs and antennae and grow into the 2nd and 3rd nymphal stages. The scale can undergo two generations per year in Georgia. The females mature by September, and they lay eggs in the ovisacs. The emerging, secondgeneration ABS nymphs overwinter within the cracks and crevices in the stem (Fig. 3).

Damage

When azaleas lose leaves or experience branch dieback, ABS could be the reason. Sometimes the leaves will turn yellow, especially on the dying branches (Fig. 4). The white crusts of the scale insects will be visible on branches.

The developing nymphs and females excrete sugary, syrupy liquid on the bark and leaves, often called honeydew. As time passes, the surface of the bark and sometimes the leaves turn black because they get infected with a black sooty mold fungus (Fig. 5). This fungus feeds on the sugary secretions on the surface of the bark and leaves.

At this point, we usually notice that the azalea plants look different and have some serious problems. The white crusts of scale insects are easily visible when the azalea bark completely turns black (Figs. 6 and 7). The ABS infestation rarely kills the plant, but can kill if the infestation is extremely severe and not controlled.

Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org Fig. 2. White, felt-like ovisacs of azalea bark scale.

Management

The scales often attack weak, non-vigorous plants. Keeping plants healthy with proper fertilization and vigor will improve tolerance to the ABS infestation. In the spring, adding a high dose of nitrogen fertilizer could enhance the aphid problem; thus, applying slowrelease fertilizer would help to improve plant vigor and reduce aphid problems. Pruning and removal of affected branches (leaves and stems) in winter will help reduce the severity of the ABS infestation. Several predators and parasitic tiny wasps attack the ABS and reduce their populations. Delaying the spray of contact insecticides will enhance the activity of predators and parasitic wasps of the ABS. If the infestation is not severe, the ABS can be physically removed by scraping the scale insects from the bark surface using a brush or figurenail. A spray of horticultural or summer oil after the azalea blooms in the late spring is advised for the ABS control. 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 in the following year. If the oil spray application was effective, the production of the new white crust would be reduced. Contact insecticides, such as pyrethroids, can also be used. The contact insecticides (oils or pyrethroids) target the crawlers and young, developing nymphs.

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.

Chris Hardin, University of Georgia Fig. 3. Azalea bark scales on azalea stems in November.

Chris Hardin, University of Georgia Fig. 4. Thinning or loss of leaves due to azalea bark scales infestation on azalea.

Chris Hardin, University of Georgia Fig. 5. Infection of sooty mold fungus on honeydew excreted from azalea bark scales on azalea.

Chris Hardin, University of Georgia Fig. 6. The bark of the azalea stem completely turns black due to sooty mold fungal infection.

Chris Hardin, University of Georgia Fig. 7. Sooty mold fungi on leaf and stem. 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. The use of systemic imidacloprid is effective when applied as a soil drench. The drenched imidacloprid solution in the soil is picked up by the roots and intoxicates the ABS feeding on the bark. Because imidacloprid can affect the pollinators and other beneficials, label directions must be carefully followed to minimize nontarget exposure. Application of insecticides should be avoided when the azaleas are in bloom. Any direct spray exposure from the insecticide residues can negatively impact the beneficials. Insect growth regulators, such as pyriproxyfen, are effective against the ABS as they affect the development of nymphs, egg production, and viability of eggs.

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