Although different species of scale can be quite different in appearance, most share the common features of being closely attached to the tree on which they feed, are covered with a waxy coating, and do not move around much. Most do not even look like insects. After all, as adults they don't possess the antennae, legs or wings of typical insects. However, scale insects can be devastating.
The method in which scale insects feed is quite different from leaf chewers. Scales are classified as one of the "piercing-sucking" insects. They insert long, threadlike mouthparts known as stylets into plant tissues and extract cell sap. As a result of this feeding, scales secrete a sweet, sticky substance called honeydew, which drops on the surfaces of plants below the infected tree (or cars and picnic tables). The honeydew is attractive to other insects, especially ants and wasps.
Damage from scales is twofold: the aforementioned loss of tissue in the tree canopy, as well as the weakness that they cause. Trees stressed from scale insects become more attractive to borers, spider mites and other insect pests as a result of the stress caused by their feeding.
Mature female scales are convex, oval, usually brown in color, and about 3/16 inch in diameter. Partially grown females overwinter attached to twigs and branches. In the spring they complete development and females give birth to living young. These crawlers migrate to the leaves and settle on the midribs or larger veins. The lecanium scale has only one generation per year. Insecticides should be applied when crawlers first appear in the spring.
Monitor your plants for damage-- adult and crawler stages-- and also for activity of biological control agents. If a scale has taken a shine to your plants, look for the tell-tale signs of honeydew and sooty mold. Look for ant activity. Ants, fond of honeydew, will fight off scale's natural enemies to protect the source. Look closely, with a hand lens: these insects conceal themselves near buds, along veins, in the dark cracks of bark, underneath the leaves.
Crawlers test the sight of even young eyes. Use double-sided sticky tape to capture crawler emergence. As they emerge from their protective cover, they stick to the adhesive tape encircling either side of the infestation. When crawlers are seen, apply soaps and oils, and insect growth regulators.
In addition to treatment at the crawler stage, some scales are vulnerable in their overwintering phase to horticultural oils used as dormant applications. These oils asphyxiate the scales. Oils should be applied in spring before plant bud break (March-April). There are temperature and host restrictions for applying these oils, so read all labels carefully. Thorough coverage is essential for achieving good control.
The timing of the dinotefuran treatment in the spring on pine needle scale doesn’t need to coincide with the crawler hatch or activity. It is preferable to treat the tree with product early, before they hatch, though. This prevents any damage on the new needles. Dinotefuran trans-locates in trees faster than imidacloprid (an alternative product) and has given more control on the hard scale insect species experimentally. Imidacloprid is slower to move up into a tree so it is usually recommended to be applied in the fall. Imidacloprid are often not very efficacious on hard scale insects but it does provide decent control on pine needle scale for some reason.
I think it is a very good use for this class of insecticide (neonicotinoid) as pollinators don’t use the conifer pollen and usually only grass or bare soil is found under conifers so there is little or perhaps no risk to them. In my opinion, the systemic insecticides are a very good choice for this pest. Systemic insecticides probably have much less impact on non-target fauna than spraying a tree with a broad spectrum insecticide in the spring when the scale crawlers are hatching. Plus, the drench and granule formulations of these insecticide products are within the capacity of most homeowners to apply correctly.
- Scale Insects on Ornamentals - Washington State University
- Lecanium Scale Insects - Vermont Forestry
- Scale Management Guidelines - UC Davis Extension
- Scale Insects on Trees and Shrubs - University of Minnesota Extension
- Armored Scales on Conifers - University of Minnesota Extension
- Oystershell Scale - Colorado State University Extension