The term “sucking insect” is applied to all insects that have the mouth parts modified as delicate stylets, by means of which the plant tissues are punctured and the nutrient sap sucked up. Not only may such insects weaken the infested plants, but they also cause the destruction of chlorophyll, interfere with the normal functioning of the stomata, and have a toxic effect upon the tissues; further, many serious plant diseases are carried and spread by sucking insects, whilst the punctures made when feeding may allow the entry of disease spores.
Among sap-sucking insects are scale insects, mealy-bugs, aphids, leaf-hoppers, white-flies, thrips, etc. Infestation by most of these insects (especially in the case of scale insects, mealy-bugs, and aphids) is very often detected by the sticky nature and blackened appearance of the plants; this is due to the fact that the insects excrete a sweet, sticky substance known as “honey-dew,” which collects on the foliage and branches, whilst upon it grows a black, sooty mould.
Scale Insects and Mealy-bugs.
Scale insects and mealy-bugs, collectively known as coccids, are of very great economic importance on account, not only of their widespread depredations upon plants, few being free from infestation, but also of the commercial value of some species—e.g., in the production of lac, cochineal, Chinese wax, etc.; it is with the injurious forms that the New Zealand horticulturist is concerned. The term “scale insects” is derived from the appearance of many of the species that are protected by a scale-like covering, which forms a conspicuous scaly incrustation when a plant is heavily infested.
Of the several kinds of insects injurious to vegetation, the coccids as a family are undoubtedly of major importance, because they infest not one group, or allied group, of plants, as do so many other injurious insects, but an extensive range of widely different plants. Some coccids are much more injurious than others, the San José Scale, for example, having a very virulent toxic influence, while the Greedy Scale may cause but little damage, even when the plant is completely encrusted by it; further, some plants may be more susceptible to injury than others by the same species of coccid.
Coccids, as a whole, are highly specialised insects, and among themselves exhibit a great variety of forms. Throughout the group the sexes differ to a marked degree. The adult males, which vary but little in all the coccids, are usually minute, and, with few exceptions, two-winged ([Fig. 5]); none has mouth parts, these appendages having become atrophied during metamorphosis, which is complete, while many have one or more hair-like tail appendages. On the other hand, females are never winged; some are comparatively large; all have well-developed mouth parts throughout life, and undergo incomplete metamorphosis, while in many forms the legs and antennæ are lost before maturity.
In all cases coccids secrete a protective covering, which assumes different forms; this fact, together with the chief methods of female development, is utilised for the purpose of this work to arrange the coccids under three main types as follows:—
1. Less Specialised Forms.—Examples are the mealy-bugs and cottony-cushion scale, which belong to the more generalised or least specialised representatives. The protective body covering is in the form of a powdery or mealy secretion; the legs and antennæ are retained throughout life, and the insect remains freely mobile.
A typical-form life-cycle may be studied in that of the cottony-cushion scale ([Figs. 5] and [6a]). During development the female insect passes through three larval stages; each of these stages is, on the whole, similar, except for size and minor structural changes, and the white powdery secretion that covers the reddish body of the adult.
2. Intermediate Forms.—An example is the olive scale ([Fig. 5]). In such forms there is a tendency to specialisation, owing to more or less sedentary habits in later life, and protection is afforded by a thickening and toughening of the cuticle on the upper surface of the body. Unlike the cottony-cushion scale, the female olive scale passes through two larval stages; the minute first stage larva is active and very flat; it soon settles upon a leaf and commences to feed, when it becomes much flatter and a little larger; the second stage differs from the first in size and in the development of a dorsal longitudinal ridge, which eventually forms the cross-bar of the two transverse ridges that are characteristic of the third or adult stage, when the insect swells and assumes the shape of the mature form. After settling in the first larval stage, the insect becomes very sluggish, and does not move, except to migrate, as most do, from the leaves to the twigs, there to take up a permanent position. The legs and antennæ are retained throughout life, but in the adult are functionless, being folded against the body; in some species of intermediate forms the appendages become atrophied during development. In the olive scale, and related forms, the toughened cuticle not only serves as a protection to the insect, but also as a receptacle for the eggs ([Fig. 5]); as these are laid and increase in numbers, the body of the parent diminishes and is crowded against the dome-shaped cuticle.