Spruce-Gall Aphis
(Chermes Abietis). The maker of curious galls on the young twigs of Sprucetrees, especially in crowded plantations. In such situations, Spruces are often loaded with the galls of this insect. They resemble young cones or miniature pine-apples, whence they are often called Spruce Pine-apple Galls. When situated at the tips of young branches, they are usually quite regular in form; but when the base of the twig is affected, the Gall, though generally larger than when near the tip, is often confined to one side of the twig, which becomes much swollen, while the other side remains stunted. When full-grown, the Galls vary from 3/4in. to 1 1/2in. in length, by about two-thirds as much in breadth, and consist of the swollen, overlapping bases of the leaves of the twig, which assume the appearance of scales, frequently prolonged into short, green tips. The galls are often red, but may be yellowish or pale green, according to exposure to, or absence of, light. While fresh, they are fleshy in texture; but, after a time, the scales gape apart, exposing numerous cavities between their bases, and become woody and brown. In this condition they frequently remain for months, or even for years, on the trees, the branches of which are often loaded with such dry galls, the growth of several previous years.
The gall-makers are Aphides, belonging to the genus Chermes, distinguished from most of the Aphides by the short, stout antennae, the absence of honey-tubes, and the simple, unbranched veins in the fore wings. The species is C. Abietis, sometimes known also as Adelges Abietis. The insects vary a good deal, according to their stage of development and their sex. The originators of the galls are wingless, viviparous females, which are very small, ochreous-yellow (with green or purple shades and green legs), woolly, and oval. These females hybernate on the branches; and in spring each fixes herself near the base of a bud when it begins to grow, and, pushing in her beak, she thus at the same time obtains her own food, and, by the irritation she causes, gives rise to the swelling of the young leaves that form the gall. Between the swollen leaves are cavities, in one of which she is sheltered; and here she lays numerous eggs, which are covered with the woolly coating of her body. The larvae soon emerge from the eggs. They spread over the gall, and, by the irritation their suction causes, increase the size of the gall, the scale-leaves overlapping and covering them, in the opinion of some observers; whilst others believe that the larvae creep, through minute slits, into pre-existing cavities between the scales. In the cavities they become pupae, and about June and July winged females are numerous. These are much larger than the wingless females, and are yellowish-green or golden-brown, with short, green legs, and horizontally-folded, broad wings. The males are very rare; they are very small, and wingless. The winged females soon scatter themselves over the trees, and lay eggs, from which, in due time, the wingless females are again developed.
Remedies. The best, where applicable, is to pick off and destroy the galls while young, before the insects have spread from them over the plants. Badly-infested trees or branches should be burned without delay. The galls should not merely be thrown on the ground, as they contain enough sap to permit of the insects coming to maturity in them after their removal from the trees. Overcrowding, deficiency of air and light, and bad drainage, greatly increase the risk to the trees of injury from these galls; hence, judicious thinning, draining damp soils, and, in fact, whatever tends to strengthen the trees, are all useful preventives of injury.