I have on my rooftop terrace two purpleleaf sand cherries, or Prunus x cistena. They have been on the terrace for well over 10 years and have held up very well. This year I have discovered tons of small dark bumps and a clear sappy liquid all over the branches and stems of these trees. There is also some dieback on the upper branches, but I know the containers are an adequate size. I rubbed the bark and realized that I can dislodge these bumps with my fingernail. What is this issue and how best do I treat it?
Prunus x cistena is one of my favorite small trees for its rich reddish purple foliage and fragrant spring flower. From my experience with this tree, I know that they are victim to a number of insect pests. In this case, the tree is infested with an insect called scale. There are both “armored” and “soft” types of scale insects. Typically the adult forms of scale insects are surrounded by a hemispherical oval covering that protects them. They insert their piercing, sucking mouthparts into the phloem sap of the host tree and feed off of it, thus producing large amounts of clear, sticky honeydew on the stems. Eventually the adults lay up to hundreds of eggs. These eggs hatch and the “crawlers” emerge, eventually finding sensitive parts of the host plant on which to feed. There they mature into adults, overwinter on the stems, and repeat the cycle.
To best control scale, you need to know the type of scale as well as the timing of its different life cycles. Because many scale are very host specific, you can usually narrow down the list my knowing what kind of plant they are feeding on. In this case I am guessing that the scale in question is called a European Fruit Lecanium (Parthenolecanium corni). I have found that these are a soft-bodied scale, apparently native to North America despite their name, and enjoy feeding on a wide range of ornamental fruit trees. The best time to treat scale is when the adults give birth to their many fragile crawlers. In this case I have researched to discover that the females’ eggs hatch in late May or early June. As crawlers emerge and spread out in June treat the entire tree with an application of Horticultural Oil. Horticultural oil coats and suffocates soft bodied insects and helps to control infestations. Before treating you can confirm the presence of the crawlers by shaking the limbs over a clean white sheet or piece of paper and looking for the mobile young. Treatment of horticultural oil is not considered effective on adults because of their protective covering, so the best option to remove adults is to do so by physically removing them. A coworker passed on to me that using a tooth brush and a mixture of baking soda and water usually works to scrub away the adults, even though the task can be rather tedious.