My neighbor’s lilacs, while showing signs of lilac blight earlier in the season, continue to experience massive dieback. Today, I decided to perform a closer examination, since the proximity of his plants puts my plants at risk.
To my surprise, his lilacs are covered with oystershell scale, and the infestation is so severe that it isn’t worth trying to save the shrubs. Certain types of scale, such as euonymous scale, are ubiquitous, but oystershell scale–especially when it has clearly been present for several years–is unusual among reasonably well tended suburban gardens. My guess is that he probably purchased inferior stock at one of the local big box stores. Combined with yearly bouts of powdery mildew, and a location with insufficient airflow, the stressed shrubs were doomed to an attack by opportunistic insects.
Oystershell scale is a member of the order Homoptera, family Diaspididae (armored scales) and genus Lepidosaphes ulmi. There are two races of oystershell scale, gray and brown. The gray race attacks lilac, ash, willow, poplar, and maple, among others. There is only one generation per year for the gray race.
If you’re not familiar with scale insects, there are two types: soft-bodied and armored. They can be dispersed by wind, tools, or people. Armored scales are more difficult to control because they are only susceptible as crawlers (juveniles). Once they mature, they develop a protective layer that insecticides can’t penetrate or smother, and, likewise, the eggs are protected by the armored casing of the adults. Huge populations can develop rapidly. In the picture at right (I recommend enlarging twice), notice the hundreds of small “oyster”-shaped, bumpy white scales along the branch. Underneath each of these adult female scales will be anywhere from 40 to 150 eggs, and this is how the insect overwinters.
Scale insects only feed during the crawler stage, when the six-legged, pale yellow juveniles emerge for a few days to a couple of weeks and use their sucking mouthparts to consume plant sap. Signs of damage include reduced vigor, foliage that appears smaller than normal, and, in severe cases–such as the photo of the attacked lilac shrub shown above–whole branches die, never to recover.
Other than pruning out affected branches (assuming limited damage), there are no natural controls. Biological predators emerge too late to be effective. If damage is still somewhat contained, control crawlers with a systemic, such as acephate (Orthene), and follow the label instructions. Make your first application of insecticide when Spiraea x vanhouttei (the old-fashioned, cascading variety) has just finished blooming. Remove old scale casings by gently rubbing the infested bark with a small, plastic kitchen scrub brush.