Scale Insects
(Coccidae). These form one of the most destructive families among Insects. They belong to the class Homoptera, in which are also included the Frog-hoppers and the very numerous Aphides or Green-flies, both very hurtful to many plants. The Scale Insects have been studied by several entomologists, and of late years very important advances have been made in working out the life-histories of many species; but there are very wide gaps still to be filled before satisfactory results can be arrived at with regard to the transformations and life-histories of most of the species. Among those who have done most in this field for some time past are M. Signoret, in France, and Professor Comstock, in the United States of America. Our British species have been worked out, and the results have been published in the March and April numbers of the "Entomologists' Monthly Magazine" for 1886, by Mr. J. W. Douglas. He has succeeded in identifying a large number of species of these insects in our islands. Most of them are found on wild plants; but in greenhouses some species infest many of the shrubs and other plants to such a degree that, by the continued suction of the sap, and the consequent tax on their strength, the plants are much weakened, and ultimately die. The females and larvae are the hurtful members of the family; while the males, when fully developed, do no harm to plants. The females are by far the more numerous, and the more conspicuous; in fact, the males of even the commonest species are seldom seen, and those of a good many are still unknown. They are most successfully procured by collecting the larvae and pupae, which somewhat resemble scales, very early in spring, upon branches, which, with these scales on them, should be put into some secure vessel, e.g., a glass jar. The males are very unlike the females. They usually emerge early in the year, in the form of minute flies, with the head, thorax, and abdomen distinct and well formed--two delicate wings, six legs, and usually two slender filaments at the hinder end of the body. They have no mouth, and therefore cannot take food in this state. They seldom live more than a few days, their sole function, as perfect insects, being to fertilise the females. In certain species, both wingless and winged males have been found. Female Scale Insects are much larger than the males, and they alone fully deserve the name of Scale Insects. They are developed from larvae of an oval form, which have three pairs of small legs (by the aid of which they can crawl about), small antennae, and a beak with which to bore into leaves and young stems of plants, in order to suck up the sap. The larvae are protected by a scale-like covering on the back. This scale is formed either by an excretion from the body (in Coccinae and Lecaninae), which increases in size as the larva grows, or also by the cast skins (in Diaspinae). After the larvae have fixed themselves in a suitable location on a plant, by pushing in the beak, they never leave the spot. As they increase in size, they undergo great alterations in structure, whereby the body becomes more rounded, and the beak becomes situated in the middle of the lower surface of the body. The legs and antennae, and the rings of the body, almost or altogether disappear. The females thus at last lose all power of movement. After being fertilised by the winged males, or, in some species, without the action of the male being necessary, the females produce eggs, which are packed away below the scale-like bodies, and are thus protected by them, even after the death of the mothers. The females of some Scale Insects cover the eggs also with a white coat of felted threads. In certain species, the females are viviparous. The number of species of Scale Insects already named is very large; but of many the males, as before observed, are still unknown. A large proportion live on the woody kinds of wild plants, fixed to the bark or to evergreen leaves, but many others select hardy cultivated plants; while others are confined, in this country, to greenhouse plants, to which they are often very destructive. Some broods consist wholly of females, and others of both sexes. Their rate of increase is much below that of the Aphides, there being, in most of the species, only one brood in the year.
The number of species already recorded as injurious to cultivated plants, especially in the warm zones, is too large to permit of more than a brief mention here of some of the more important kinds. They have been divide, by means of comparatively minute characters, into numerous genera; some of the species live upon several kinds of plants, and, on the other hand, many plants support various kinds of these insects. The latter are so much alike as to render it vain to attempt here to state clearly the differences between the species, as these are found in minute structural characters; nor, indeed, is this necessary, since the same remedies are serviceable against all of them, and are most successful when directed against the larvae. In Great Britain, Scale Insects are far more numerous, and are usually more injurious, in glass houses than in the open air. But among the "outdoor" kinds the following must be noted: 1. Apple Mussel Scale, or Oyster-shell Bark Louse (Aspidiotus conchiformis or Mytilaspis pomorum), like single valves of very small mussel-shells adherent to the branches of Apple-trees; they occur on both sides of the Atlantic. 2. Pear Oyster Scale (Diaspis ostreoeformis) very like the Apple Mussel Scale but of smaller size. 3. Rose Scale (D. Rosoe), like a white, scurfy coat on twigs and stems of Roses, especially of cultivated kinds. 4. Camellia Scale (Aspidiotus Camelliae), on buds and bracts of Camellias. 5. Spindle-tree Scale (Chionaspis Euonymi), so abundant on Euonymus japonicus, near Montpellier, in France, as to threaten the existence of the shrub. Several species are found in Britain in greenhouse, where they frequently do very great harm. Among the more hurtful are: Aspidiotus Nerii, on Acacias, Lemons, Oleanders, &c.; A. palmarum, on Palms and Cycads; Lecanium Hesperidum on Orange leaves, or on other food-plants (e.g., Myrtaceae); and Dactylopius adonidum, on most greenhouse plants.
Many others have been described as very hurtful, especially by Professor Comstock in his "Report on Scale Insects"; and probably a number of these will be found in English greenhouse, e.g., Dactylopius longifils, on Ferns and Euphorbiaceae; D. destructor, on Coffee, Oranges, and, in fact, almost every greenhouse plant; Ceroplastes floridensis, on Oranges, &c.; but for a full account of these the reader is referred to the above-mentioned work. Remedies are two-fold, viz., natural and artificial. The best natural remedy is to encourage the multiplication of certain minute insects belonging to the great division Hymenoptera, which are parasitic in the Scale Insects, and destroy large numbers of them. It has been found useful to carry branches bearing Scale Insects infested with parasites to localities where the parasites did not previously exist, inasmuch as they soon multiply, and produce a marked effect on the number of Scale Insects. Artificial remedies are numerous. Among the most useful are the following: Soap solution (1/4 1b. of soap in one gallon of water) or kerosene solution (about one gill in five gallons of water), syringed or sprayed over the plants every second day; phenyle, in a strength of from three to six teaspoonfuls to four gallons of water, applied at intervals of eight days; alkaline washes, such as concentrated lye of wood-ashes or of coarse potash, which, used with a brush, frees the branches from the insects; strong solution of tobacco; and animal oils, e.g., whale oil. The last-named suffocate the insects by closing the breathing pores along the sides of their body.