Art. II.—On the “Honeydew” of Coccidæ, and the Fungus accompanying these Insects.
[Read before the Wellington Philosophical Society, 25th August, 1886.]
It has long been known that, in common with the Aphididœ, Psyllidœ, and other Rhynchota, the Coccidæ secrete a glutinous fluid, which has received the name of “honeydew.” In the other families mentioned, this fluid serves to attract and to feed various insects: thus, for example, Aphides are commonly visited by ants, which devour the honeydew, and even, it is said, go so far as to tickle the Aphides, in order to make them secrete additional fluid. Whether the secretion of Coccids may serve as food for other insects is not certain. I have never noticed anything tending to this idea: often, minute dipterous and hymenopterous insects may be seen amongst Coccids on leaves, but I incline to the belief that certainly the latter, and possibly the former, are seeking rather a place for depositing their eggs than a meal of honeydew. Hymenopterous insects are very often parasitic on Coccids: and out of a hundred pupæ of, say, Ctenochiton perforatus, as many as seventy-five may often be found with hymenopterous pupæ or larvæ inside them. Acarids of various kinds are also very numerous on leaves where Coccidæ are found; but, whilst it is quite possible that they may be attracted by the honeydew, this may not be the case, as Acarids are numerous and common on all plants, whether infested with Coccids or not. On the whole, I cannot affirm that the Coccid honeydew affords nutriment to any insects. I have never seen an ant amongst Coccids: but ants are not common in New Zealand, and this point is only negative, after all.
Still, the fact remains that Coccidæ exude, like other Rhynchota, a glutinous fluid. This is well known, but I think that nobody as yet has described either the mode or the organ of the secretion. An observation lately made enables me to fill this lacuna, and the points noted are not without interest in the study of the family. It happened that I was lately examining some specimens of the second, or pupa, stage of the female of Ctenochiton elœocarpi, mihi.* In one of these I noticed a sudden protrusion of an organ from between the
[Footnote] * “Trans. N.Z. Inst.,” vol. xvii., 1884, p. 26.
two dorsal abdominal lobes, and the excretion of a drop of honeydew.
Ctenochiton elœocarpi, like all Lecanids, exhibits at the abdominal extremity a deepish narrow cleft, on the dorsal side of which are two roundly-triangular protruding lobes. In a specimen of the second female stage it may be seen, under the microscope, that these lobes lie in a shallow groove formed by the sides of the cleft. At intervals, which may vary in length, the insect protrudes rapidly from beneath the lobes a cylindrical organ (figs. 1a, 1b,), composed of a basal, thickish tube, bearing at its extremity another, similar, but much thinner. The organ being pushed out to its full extent, a minute globule of transparent glutinous fluid appears at its extremity, rapidly expands, something like a soap-bubble, and then suddenly breaks and falls in spray on the leaf. The excreting organ is then rapidly withdrawn.
I do not entertain much doubt that the process just described is the same used by all the “honeydew”-secreting Coccids. Some of these, like Rhizococcus fossor, Planchonia epacridis, etc., produce much less than others; but even they, I think, excrete some. The observation which I have made as above throws a light upon an organ which I noted as occurring in the second female stage of Cœlostoma zœlandicum, in vol. xiv. of the “Transactions,” page 227, and in the adult female of the same insect in vol xii., p. 295. I then considered this organ as an oviduct, remarking, however, that I could not see its use in the pupal stage. I never saw it exserted, and was not aware that it ever was so; but it seems clear to me now that this is an organ similar to the one excreting honeydew in Ctenochiton elœocarpi, and it probably occurs in at least most of the other Coccids.
But observation of this organ is extremely difficult, if not in most cases impossible. The best and most careful work on the anatomy of Coccids is that of Professor Targioni-Tozzetti, “Studie sulle Coccineglie” (Milano, 1867); and there is no mention in it of any such organ. I have carefully, at various times, examined in every way many specimens of different genera, and, with the exception of Cœlostoma (an exceptionally gigantic species), I have not seen it.
There is no doubt of the quantity of glutinous matter secreted by Coccids; but, as observed above, the origin and mode of the excretion have not been noticed before. There does not appear to be any other organ, or any other portion, of the insect's body, producing this secretion; and it may well be that the absence of insects feeding on the honeydew may be due to the fact that the excreting organ, instead of being constantly protruded like the cornicles of the Aphididæ, is only now and then exserted and then withdrawn. In many genera,
such as Aspidiotus, Ctenochiton, Inglisia, Eriococcus; etc., the character of the shield or test, waxy or cottony or felted, would prevent the honeydew from exuding anywhere but at the abdominal extremity; there, however, there is almost always some kind of orifice or cleft permitting the extrusion of the excreting organ.
A point of considerable economic importance is connected with this honeydew of Coccids, namely, the growth upon it of various fungi; and I take this opportunity of drawing the attention of farmers and tree-growers to it, as I believe that a good deal of misconception exists in its regard. Everybody, doubtless, has observed how, in gardens or in greenhouses, in plantations or in forests, many plants have an unpleasantly blackened appearance; and it frequently happens that the true colours of the twigs and leaves are much, if not quite, obscured by the black coating on them (see figs. 2a and 2b). Now, it will generally be observed that this black coating is in most cases thicker and more unsightly on the lower than on the upper leaves and branches: sometimes, indeed, the uppermost leaves will be bright green, whilst the lowest look as if they were covered with soot. It will also be noticed that the black coating is abundant, not only in the damp recesses of the forest, but also in comparatively dry greenhouses, and just as much so in open-air gardens.
The primary cause of this is usually the presence of some homopterous insects, whether Aphididœ, Psyllidœ, Coccidœ or others. The “honeydew” excreted by them drops, or in the case of Coccids falls in spray, over the leaves beneath them: it very soon furnishes a glutinous and congenial soil for the spores of fungi. If one may judge from the quantity of black coating often found, the act of excretion above described must be repeated somewhat frequently, though so seldom observed.
As to the question, what is this black sooty coating: there is no doubt of its fungoid character, but I am not able to identify it precisely. There seem to be, in fact, several species in it. On the leaves, the fungus forms usually a flat, black, thinnish, closely woven covering; on the twigs and stem it has a looser texture, and generally forms a mass of small erect threads (see figs. 2a and 2b). These appearances are by no means confined to New Zealand, and most writers on Coccidæ devote some phrases in passing to the fungus. Signoret* calls it “Fumagine;” Comstock† calls it “Fumago salicina:” in an early paper of mine (1878) I attributed it to “Antennaria.” In point of fact, a good many of the Hyphomycetes and Physomycetes may be found amongst this fungoid growth. I have attached to this
[Footnote] * “Essai sur les Cochenilles.”
[Footnote] † Report of the Entomologist, U.S. Department of Agric., 1880.
paper (figs. 2c, 2d, 2e) rough representations of three different forms observed.‡
The economic point, however, to which I draw attention is that, as a general rule (I might even put it more strongly), the appearance of this black fungus is an indication of the presence of homopterous insects; probably, in New Zealand, of Coccidæ, because they are the most numerous of the order in this country. Now, a great deal has been heard of late years of the damages by scale-insects, and all sorts of people have written or experimented about them. I find that, amongst the diseases of trees, this black fungus-growth has attracted attention: but the misconception to which I alluded just now is that it has been considered as a separate, or primary, disease, which I do not think it is.
In Mr. Kirk's “Report upon the Diseases of Lemon and other trees in New Zealand,” the common notion is embodied, and, under the head of “Lemon Smut, or Black Blight,” the writer even emphasizes it, by saying that the excreta of Icerya purchasi (a scale-insect) had no share in its production, and that it was of “purely vegetable origin.” In answering, in 1885, questions by a Select Committee of the General Assembly upon tree-diseases, I combatted this view, and ascribed the black “smut” to fungoid growths, the result of the presence of homopterous insects. I believe that Mr. Kirk has now come round to this view of the question, which is, undoubtedly, the right one.
It stands to reason, then, that if a farmer, or orchard-grower, or gardener, observing this black growth on his trees, imagines it to be the primary cause of disease, and sets to work to eradicate it without attending to the insect pests, he will be simply throwing his labour away. And there is this important fact to be remembered, that it is not with scale-insects as with other insects. Coccidæ are impervious to many things which might, in other cases, be efficacious. There is a common belief which, like most superstitions, it is excessively difficult to destroy, that sulphur is an useful ingredient in what are called “scaly-blight destroyers.” I should not be in the least surprised to find that many who hold this belief do so because they have found the black fungus disappear, or lessen, after its use, for sulphur is undoubtedly a remedy for fungoid blights—e.g., oidium, etc.; and probably they never thought of looking more closely into the matter. All experience goes to show that sulphur, unless applied in such strength as to burn up insects, fungus, tree and all, is not a remedy against Coccids. Comstock, Riley, Hubbard, and others agree in this: and although, in newspaper
[Footnote] ‡ Boudier (Assoc. Fran. pour l'avancement des Sc., 1884) includes the European fungi developed in the honeydew of Aphis in the genus Cladosporium (Quart. Journ. Roy. Micros. Soc., Aug. 1886, p. 597).
accounts, and in the replies of farmers to questions from officials or committees, we find sulphur a common ingredient in the thousand-and-one mixtures recommended, I doubt very much if anybody could give an intelligent reason for its employment.
No harm will therefore have been done, I think, if this paper should induce gardeners and fruit-growers to go direct to the true origin of disease in their trees and neglect the secondary one. That the fungus will grow independently of scale-insects is, of course, indisputable: but those who wisely consider it as, in the vast majority of cases, merely an accessory to their presence, and who set themselves to destroy the Coccidæ or Aphididæ on the plants, will find the black fungus also very quickly disappear.
Explanation of Plate I.
Fig. 1a. Ctenochiton elœocarpi (larva) magnified, with waxy test removed; showing excreting organ, and bubble of honeydew.
Fig. 1b. Abdominal extremity of same, magnified, with waxy fringe; honeydew bubble bursting.
Fig. 2a. Fungoid growth on twig.
Fig. 2b. Fungoid growth on leaf.
Fig. 2c. Varieties of fungoid growths.
Fig. 2d. Varieties of fungoid growths.
Fig. 2e. Varieties of fungoid growths.