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Azalea Bark Scale

The 'bumps' you see are actually insects!

By: Sandy Feather ©2009
Penn State Extension


Q.  I have a large azalea that is approximately 50 years old.  Over the past several years, it has developed a white scale on the branches, usually at the joints.  It kind of looks like pinesap that has dried up and turned a crusty white color. This has spread considerably over the past year. The azalea barely bloomed last year, but the leaves seemed normal. I have some smaller azaleas that I started from cuttings of the original plant, and now they are starting to show signs of the same thing. Can you tell me what it is and how to get rid of it?

A. Your description of the problem actually contains the correct answer – scale! It sounds as though your azaleas (Rhododendron spp.) are infested with azalea bark scale, Eriococcus azaleae. It can be difficult to realize that those immobile white bumps are actually insects since they do not move once they are mature. What you are seeing are female insects and their egg sacs, covered with a white, waxy coating. That coating makes the adults fairly impervious to insecticide applications.

Azalea scale insects overwinter as immature adults on the bark of azaleas and rhododendroms. They mature in spring, then lay eggs that will hatch from late June through mid-July. We have one generation annually in the Pittsburgh area.
 


SCALE CRAWLERS

The immature insects that hatch are known as crawlers because that is the only time in this insect’s life cycle they are mobile. Crawlers tend to settle in bark crevices and at the crotch of branches, as you describe. They feed with piercing-sucking mouthparts, which is somewhat similar to a person drinking through a straw, but on a very tiny scale. Once female azalea bark scale crawlers insert their mouthparts to feed, they stay in that place for life.

purple azalea
Large purple azalea in full bloom

Scale insects are broken down into three categories: armored scale, soft scale and mealybugs. Armored scales are generalist feeders; they insert their mouthparts into parenchyma tissue and do not excrete “honeydew,” which is a polite term for insect excrement. Euonymus scale and pine needle scale are examples of armored scale insects. They tend to be more difficult to control than soft scales.

 


SOFT SCALE INSECTS

Soft scale insects, such as azalea bark scale or magnolia scale, insert their mouthparts right into the phloem. Phloem is the part of a plant’s vascular system that transports the carbohydrates produced by photosynthesis in the leaves to the lower trunk and roots for storage. Insects that feed in the phloem excrete large amounts of honeydew. This very sweet, sticky honeydew drips onto anything underneath – plant stems and foliage, or plants growing under infested shrubs – and can attract bees, yellow jackets and ants that feed on it. Plant foliage and bark can take on a very dark – even black – appearance as a secondary fungus known as sooty mold colonizes the honeydew. Sooty mold does not harm plants, but it can be unsightly. Since soft scale insects feed in the phloem, systemic insecticides that move through the plant in the phloem control them pretty easily.


CONTROL OPTIONS FOR AZALEA BARK SCALE

There are a number of control options for azalea bark scale. A dormant application of horticultural oil (not dormant oil or Volck oil) helps suffocate some of the overwintering, immature adults. It may be getting too late for a dormant application of horticultural oil, but if your azaleas have not started to leaf out or bloom, go ahead and spray them with horticultural oil.
  
You can mechanically remove the insects by picking them off the stems. If you do this religiously, every time you notice one of the adults, you can eventually get rid of the population. You can also monitor the plant, starting in late June, for the very tiny, reddish crawlers. A magnifying glass is helpful to see them clearly, but they are visible to the naked eye.

Scale crawlers
Scale crawlers on Magnolia

Horticultural oil works very well to control crawlers, although you will have to make repeated applications, according to label directions. Horticultural oil has no residual once the spray dries, which makes it safer for pollinators and beneficial insects that visit the plant or feed on the scale. Other crawler sprays that have a longer residual include Orthene (acephate), Sevin (carbaryl) and malathion. Bayer Advanced Tree and Shrub Insect Control (imidacloprid) provides season-long, systemic control of azalea bark scale. It is applied as soil drench around the base of affected plants in late spring. Be sure to pay attention to label directions when using Bayer Advanced Tree and Shrub Insect Control. You have to pull any mulch back and apply the product to bare soil, water it in, and then replace the mulch. It is also important that the soil around affected plants is moist prior to treatment. If not, water them thoroughly prior to application.


MORE

Photos of garden insects

Tree and Shrub Pesticides

Lawn plantings in Fall

 

       


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