Azalea bark scale, or ABS, is a non-native scale that poses a serious threat to azaleas, rhododendrons and pieris. While ABS usually won't kill your shrub, it can rob the plant of its aesthetic value, leaving it a sickly yellow with stunted growth. Unlike armored scale insects, ABS is a soft bodied scale and in order to protect itself, it forms a waxy coating over itself that is white in color. Inside the safety of the waxy shell, the female inserts her straw-like mouth parts into the plant and begins sucking, robbing the plant of its vital fluids. As the insect feeds, it secretes a sugary substance known as honeydew, which drips down and accumulates on anything and everything beneath the plant, leaving it coated with a sticky film. A fungus called sooty mold, then grows in the honeydew, turning it black. The presence of sooty mold on leaves, severely limits the plants ability to photosynthesize, resulting in chlorotic shrunken foliage.
While beneficial insects such as parasitic wasps, can usually help control minor infestations, intervention may be required in cases of higher pest populations. Cultural practices such as irrigation and mulch may help, however the proper use of chemicals may be called for. Non-toxic pesticides such as soaps and horticultural oil, can often give you adequate control when used alone or with systemic insecticides. As with all pesticides, timing of application and an understanding of the target pests' life cycle are essential to reducing their numbers and restoring your plant to good health.