"My lilac is not growing well and some stems are even dying. What is wrong?"
Oystershell scale is a hardshell scale, meaning that insects develop a hard, protective covering over themselves that is difficult to penetrate with insecticides. The insect overwinters as an egg under its mother's shell.
Some races of oystershell scale have one generation per year, and others have two generations per year.
In single generation races, a new group of insects hatch out in late May and are called "crawlers". Those races with two generations hatch in late May and again in late July to early August.
"Crawlers" are the only mobile stage in the life cycle of oystershell scale. They are pale in color and smaller than a pinhead in size. After a few hours, the crawlers find a suitable location, usually on a shaded area of the tree. They insert their mouthparts into the plant, begin to suck sap and soon molt. From this point on, they will remain in the same spot for the rest of their lives. Within a week they are covered with a waxy scale covering that provides protection from most insecticides. Old scale shells will stay attached to the tree for several years before falling off.
Oytershell scale can be found on ash, maple, lilac, cotoneaster, dogwood, poplar, willow; fruits such as apple, pear, apricot, plum, raspberry, currant and grape; and many other hardwood trees and shrubs. A heavy infestation will weaken the plant and heavily infested plants may drop leaves. The insects are not obvious and can build up large numbers on trunks and twigs before being noticed. Branch and tree death may result from large scale infestations.
To control these insects, spray twigs, branches and stems with horticultural oil in the spring before the buds expand, March to early April. Some plants may be sensitive to oil sprays, so check to make sure your plant is listed on the product label before spraying.
Prune out heavily infested stems.
During the winter scrape off the parent scales, which are filled with overwintering eggs, from infested plants by lightly rubbing the branches with a plastic dish pad.
Spray plants when insects are in the crawler stage, late May to early June. To determine when the crawler stage is emerging on your plants, place double-sided tape on several plant stems, and monitor for the presence of the tiny new insects. Insecticides labeled for control include horticultural oil, insecticidal soap, acephate, bifenthrin or malathion. When making applications to fruit plants, be sure the insecticide is labeled for food producing plants. Repeat application in 7 to 10 days. Repeat again at August egg hatch if necessary. Read and follow are pesticide label directions.
After scale insects form their 'shell', try using imidacloprid applied around the base of the plant.