By Boone Holladay, County Extension Agent-Horticulture
You may have noticed this while driving around. As you look down a row of young oak trees, one of them at random looks light yellow, while the others are a nice dark green. Well, if you haven’t, I have. Upon close inspection, these sickly trees are increasingly covered with Obscure Scale insect.
As the name implies infestations are obscure and difficult to spot. Infestations, especially heavy infestations will appear as if someone has sprinkled ash on the limbs. When checking for this type of infestation, it is best to check on 3 or 4 year old wood.
In the life cycle of this scale, females will excrete a waxy covering about 1/8 inch in diameter under which they feed and lay eggs. When immature emerge from eggs they are referred to as crawlers, which is the only mobile stage of this insect. Once a crawler has moved and settled in a new location it begins to feed, excretes the waxy covering and it will never move again. Fortunately, there is only one generation per year. Unfortunately, if not treated, they will exponentially multiply on the plant.
These insects feed by inserting their mouthparts or styles into the conductive tissue of the tree and feed on the plant sap. This shows up as yellowish or stunted new growth. Heavy infestations can cause limb dieback and will reduce overall tree vigor.
There are some beneficial lady beetles and mites that feed on scale and a few parasites which help, but if control is needed that control will have to come from the application of a dormant oil. Al-though we now have some highly refined light summer oils, they may burn foliage during high summer temperatures.
It is the general thought that these oils kill insects by suffocation where the oil covering prevents respiration. Although the exact mode of action can be debated, the bottom line is that there has to be good coverage.
Scale control can be difficult and with heavy infestations it could take a couple seasons for management. Some of the difficulties for control include: the scale’s waxy cover provides protection against pesticide exposure; some crawlers tend to settle under the protection of older scale covers, thus producing a layering effect; and crawler activity extends over a long period of time.
During the growing season, when dormant oil cannot be used, insecticide treatments can be used to control crawlers but these treatments must be correctly timed for crawler activity. Prior to purchasing any control products, read labels thoroughly.
For additional information on Obscure Scale, or any pest insect issues you may have, contact us at http://fortbend.agrilife.org for more information.