PRODUCTS OF THE COCCIDIDÆ.
[Waxy or cottony matter: the "honeydew" and the black fungus--"smut" or
"black blight"--growing upon it.]
The Coccididæ, in some parts of the world, excrete various substances
which are of commercial value, as, for example, shellac, "manna,"
candle-wax, &c. Cochineal is not in the same category, as it appears
to be a colouring-matter pervading every cell of the tissues of the
insect from which it is extracted--_Coccus cacti_. But there is no need
to dwell here upon the ordinary excretions of the New Zealand insects,
as they appear to be not sufficient either in quantity or quality
for any practical service. The fibrous puparia of the _Diaspidinæ_
appear to be quite useless. The tests of the _Lecanodiaspidæ_, such as
_Ctenochiton perforatus_, _Inglisia ornata_, &c., although more or less
waxy (but of very brittle material, often more like glass) are much too
insignificant to repay any trouble taken to collect them. Of all the
family, _Cælostoma zælandicum_, in its second stage, seems to produce
the greatest amount of material, its large, hard, waxy tests being
very thick and solid, and often clustered in hundreds on a root or a
twig of Muhlenbeckia; but, supposing this substance (of which the true
chemical nature[D] is not yet known) to be fit for some purpose, there
does not seem to be any means of cultivating the insect to profit.
_Dactylopius alpinus_ produces in alcohol a rich red tint, and this not
by way of excretion, but from the colouring matters of its tissues,
as in the case of _Coccus cacti_; but here, again, the rarity of the
insect and its out-of-the-way habitat would be a bar, even if nowadays
it were worth while to cultivate a New Zealand cochineal. At present,
therefore, there seems no reason to believe that the Coccididæ of this
country are likely to furnish any products of a useful or commercial
character.
[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 _Cœlostoma_--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 _Cœlostoma 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 disappear.