For most of their lives these pests are nothing but an alimentary canal with two ends -- one for sucking plant juices and the other for excreting excess sugars. These sugars rain from the tree as a sticky substance called honeydew and fall on everything underneath including hosta leaves, grass, cars and deck flooring. 'Sooty mold' then grows on/off the honeydew, and the sticky honeydew turns black.
At a specific time of the year, varying with species, the body mass of each female scale transforms into a mass of tiny eggs. From the eggs hatch pollen-sized 'crawlers' which can be seen by turning over the remaining shell of the female body, or by shaking them off a branch onto a piece of white paper and inspecting for walking dust-motes. One year we were informed about an early emergence of magnolia scale by a homeowner whose daughter played under the tree and became covered with the tiny reddish insects.
The time of crawler emergence is the best time to spray for scale. The newly-hatched insects are easy to kill. Once they have settled, grown a shell, and hooked into the sap-stream of the host plant, sprays are not effective. Nor is much else. Horticultural oil applied during the dormant season might be of some benefit, depending on species, but we have never seen it used to successfully rid a plant of scale. With oil products, complete coverage of plant parts is necessary.
This year GreenStreet is experimenting with a combination of trunk injection (with acephate) and soil injection (with dinotefuran) to control adult scale.
Magnolia scale: one of the largest scales on the planet, magnolia scale (left) can kill a large specimen tree in just a few years. Eggs hatch and crawlers emerge starting in mid-August.
European fruit lecanium scale: this species is devastating to purple-leaved ornamental plum trees. By the time the insect's appearance is recognized, it is usually too late and/or too costly to control it. Crawlers usually appear early July.
Lecanium species on other deciduous plants: for several years, many species of woody plants in western Wayne County have been plagued by lecanium scale. Prominent hosts include honeylocust, oak, maple and redbud. State entomologists report that this problem flairs up periodically then recedes as the populations of beneficial parasitizing insects and pathogens build up. In the meantime, residents are decidedly not happy with the street-side honeylocusts that are dripping honeydew on parked cars. Some of the trees are so badly affected by this that they may not survive. A second type of scale, known as calico scale, is also being a nuisance on a variety of trees in the same area.
Pine needle scale: this is a familiar pest on mugo pines, sometimes giving specimens the appearance of being covered with snow. It may also be found on scotch pine.
Euonymus scale: this scale can be devastating on evergreen euonymus cultivars, particularly when grown against a building. Since euonymus is a vigorous grower, sometimes the best thing to do is to severely prune stems back to clean tissue.