[D] A small quantity was submitted to Mr. Skey, Colonial Museum Laboratory, for analysis, no more being available at the time. Mr. Skey considered it as a new substance, probably of the nature of a gum, not resinous; but further examination of larger quantities is necessary.
There is, however, one substance produced by these insects which has an injurious effect upon the plants they grow on. This is a transparent glutinous fluid, apparently analogous to that exuding from Aphides, and which may receive the name of "honeydew," as in that family. In fact, this fluid would seem to be produced by most of the Rhynchota, for the Psyllidæ and Aleurodidæ also excrete it. The quantity issuing from Coccids seems to vary greatly. In some cases-e.g., Lecanium hesperidum, Ctenochiton viridis or perforatus, Fiorinia asteliæ-the insects appear to discharge "honeydew" freely; in others-e.g., Mytilaspis pomorum, Rhizococcus fossor-none, or scarcely any fluid, is excreted. But in no case does it appear that our Coccids[E] form honeydew to the same extent as the Aphides, which are stated to produce sometimes quantities that may be gathered from the leaves or the soil by the pound weight. It is not so much the amount exuding from each insect as the great number of insects on a plant which renders the Coccid honeydew obnoxious: each individual may excrete only a little, but when, as usually happens, there are many hundreds of individuals together, the result, for the reasons given below, becomes important to the tree.
[E] Gossyparia mannipara, an Arabian Coccid, is said to excrete so much that the Arabs "eat it with their bread like honey." Buckton, "Brit. Aphides," Vol. I., p. 42.
There is every reason to believe that the honeydew of Coccididæ is of similar character to that of the Aphididæ, and, according to analyses by Boussingault, of Paris, and Gunning, of Amsterdam (Buckton, "Brit. Aphides," Vol. I., pp. 42, 43), the Aphidian honeydew contains a very large quantity of sugar, and, curiously enough, cane-sugar. Some observers, noticing in its composition also glucose and dextrine, have considered it as of vegetable rather than animal origin; but the weight of evidence appears to make it certainly the product of the Aphides. As the present work is intended rather as a manual for gardeners and tree-growers than as a purely scientific publication, there is no need to enter more fully into the subject here: it may therefore be simply stated that the honeydew of Coccididæ probably contains a large proportion of sugar in various forms.
The mode in which this substance is excreted by the insects differs somewhat from that of the Aphididæ. On the abdomen of Aphis are seen two erect more or less prominent tubes, called "cornicles" or "nectaries," and it is the function of these to excrete the honeydew.[F] No European entomologist has, it is believed, seen or described the organ of honeydew-excretion in the Coccididæ. Some observations by the author of this work in 1886 demonstrate its existence as a cylindrical tube exserted from the anogenital orifice after the manner of a telescope, the furthest-extended tube being the most slender. This organ, extremely difficult of detection when not in use-except in the single genus Colostoma-is at intervals pushed out to its full extent, and at its further extremity there appears a minute globule of yellowish, nearly transparent, glutinous fluid, which rapidly expands like a soap-bubble, and then, suddenly breaking, falls in spray on the leaf beneath. In the second stage of the female of Colostoma zælandicum this organ may be detected more easily than in any other Coccid; but the act of protrusion of the organ and the formation of the drop of honeydew are apparently by no means frequent, and many long observations may be made without witnessing either.[G] (The organ and the honeydew-drop are shown in Plate xxii)
[F] The fluid also emerges from the anal orifice; but, seemingly, no mention is made by any observer of any special honeydew-organ protruding from the anus of Aphis.
[G] Mr. Comstock ("Report on Insects," U.S. Dept. of Agric., 1881, p. 22) states that on gently rubbing a Dactylopius two small drops of fluid, which he considers to be honeydew, can be seen to emerge from orifices on the dorsal side of the sixth abdominal segment; but he mentions no special organ in the body. The experiment has been tried on Dactylopius in this country without success.
For the purposes of this work further details as to the production of honeydew are not necessary. But as to its effect on plants it is requisite to be more particular, and the attention of tree-growers and gardeners is specially directed to the following points. It has been said above that when the bubble of honeydew has been expanded to its full size it breaks into spray. Now, as a general rule, Coccids are found almost exclusively on the under sides of leaves (when not on the bark). Some, as Lecanium hesperidum and a few others, may be seen on the upper side; but the general rule is as here stated. It follows that the spray of honeydew from the burst bubble falls, not on the leaf where the insect is, but on the upper sides of the leaves below it. These upper surfaces, being more exposed to light and air than the lower ones, are usually deserted not only by the Coccids but by other insects also, and so there is not much chance that the honeydew spray so falling will be disturbed. It consequently rapidly accumulates and forms a coating on the leaf where it rests. From this result two things, or, rather, the same injurious effect on the plant is produced in two ways. First, the honeydew itself, being, as stated, of a glutinous nature, tends to stop up and choke the stomata (or, in plain English, the breathing-orifices of the leaves) and so retard the growth of the tree. Secondly, the honeydew, being of a saccharine nature, is especially attractive to fungoid growths, and these fungi, rapidly increasing, tend still more to choke the leaves and hamper the proper functions of the tree.
The second of these is the most important, for, apparently, the honeydew is scarcely deposited before it becomes the receptacle for fungus-spores, and these grow with great rapidity. As a general rule, in New Zealand, these fungi appear to be mostly of the same family-the Physomycetes, and they are of a black or very dark brown colour. From the fact above stated, that the honeydew falls from the insects upon leaves beneath them, the lower leaves of a plant are more covered with it than the upper ones: these black fungi consequently discolour chiefly the lower leaves and branches; often the uppermost branches are nearly free from them. But the effect produced on the tree is not only unsightly, from the sooty blackness, but also injurious, from the choking-up of the stomata both by the honeydew and the fungus. As for ornamental plants, whether under glass or in the open air, the black coating is quite sufficient to spoil them.
These fungi are of various species, and specimens are given here in Plate xxii: on the leaves they form usually a hard, thin, black coat; while on the twigs and stem they are of a looser texture, forming masses of minute erect threads. They are not confined to New Zealand, and most writers on Coccididæ in Europe and elsewhere mention them, though only casually. They are, in fact, apparently, only the usual growths appearing on any decomposing substance, such as the honeydew is.
Gardeners and tree-growers ought to clearly understand that the appearance on their plants of this black sooty covering is almost always an indication of the presence of some Homopterous insects. In New Zealand, on account of the greater prevalence of Coccididæ, the insects will most probably be Coccids; but by no means necessarily so, for many Aphididæ, Psyllidæ, and Aleurodidæ produce the same effects. This is by no means as well understood as it should be, either by gardeners themselves or by those who write on trees and planting. The fungus growth is usually imagined to be in itself a disease of the plant, and efforts are made for its treatment without regard to its real origin, the insects on the leaves or bark. Under the names of "smut," "black blight," &c., it is often referred to as a destructive pest; and remedies are suggested which can, of course, have no permanent effect unless they are equally efficacious against the insect producing the honeydew. It is probably from this cause that sulphur, which is an excellent remedy against fungus, has been so great a favourite with those who pretend to have discovered what are called "scaly-blight destroyers;" and gardeners, seeing, perhaps, these nostrums clean some of the fungus from their trees, are under the false impression that the "scale" is also cleared away. The truth is, that the real remedy against "black blight" is to kill the insects on whose excreta it flourishes, if that can be done. As to the modes of doing this see Chapter V.
It is not, of course, pretended here that fungi of different kinds, and even those specially referred to, will not grow independently of insects and honeydew; and trees are, undoubtedly, subject to fungoid diseases which are not to be traced to any animal action. Still, the rule holds good; and the first effort of a gardener on the appearance of black blight on his plants should be to discover the insects on its leaves or bark, and deal directly with them. Once they are destroyed the fungus growth will in a short time...
