The San Jose Scale is probably a native of China, but is now present nearly everywhere in the United States. It feeds upon a large number of trees and shrubs, seriously checking their growth, and in many cases destroying them entirely. Besides fruit trees and currants, on which it is a most serious pest, it is often very abundant and injurious to the following shade trees and shrubs: Amelanchier, Cornus, Crataegus, Cydonia japonica, Populus, Prunus, Ptelea, Pyrus, Ribes, Rosa, Salix, Sorbus, Syringa, Tilia, Toxylon and Ulmus, of different species (see tabulated list of plants, page 142). It is also often found on other trees and shrubs, but the above are those most liable to be much injured.

The adult insect is smaller than a pin-head and covered by a hard shell or scale, circular in outline, and brown or grayish in color. It is usually most abundant on the smaller branches and twigs, but when very abundant may also be present on the leaves. When winter sets in all ages may be present, but the very young and the adult scales die during the winter, leaving only those from one-third to two-thirds grown to reach the adult condition in the spring. This is accomplished by the latter part of May or early June, and then the young appear, one or two every few days for a month or more. These young, which are born alive, are very tiny, lemon-yellow insects which escape from beneath the scale of the parent and crawl about for a day or two. Each has a long beak through which to suck the sap from the plant, and on finding satisfactory places the young settle down, force their beaks through the bark and begin to feed. White waxy threads now grow out of their backs and mat together, forming very small white specks as the first coverings of the insects. To these are added molted skins from the insects beneath, turning the scales brown or gray, and enlarging them, and thus the scale covering the adult insect is gradually formed.

The young become adult in about a month, and then they themselves produce young and there are three or four generations, according to the length of the season, before winter ends this process. During the summer enormous numbers of the insects are produced in this way. If all the young born survived, and themselves produced the normal number of young in each generation, it would be possible for the descendants of one female to number over three billions in a single season. Fortunately, death reduces this number greatly, but enough remain often to seriously injure or even kill the plant they are on, by removing the greater part of the sap from it, and thus drying it up.

This pest is very difficult to control, first, because being a sucking insect it must be actually hit by a contact poison sprayed onto it; second, because of its small size, which renders it very difficult to reach all of the individuals by spraying, and finally, because of the shell or scale which covers it, protecting it from the spray. For this reason very strong materials must be used, in order either to penetrate or work under the scale, and these are hardly safe to apply while the plant is growing. Spraying for this pest must accordingly be done during the period after the leaves drop in the fall and before the buds open in the spring. The lime-sulphur wash and miscible (often wrongly called" soluble ") oils are the most usual sprays to use for the purpose. These materials are on the market under various trade names, and only need dilution with water before applying. The spraying must be very thoroughly done, however, if satisfactory results are desired, and it seems desirable, if possible, to use an oil one winter and the lime-eulphur wash the next, thus alternating the two materials.