CHECKS TO INCREASE OF COCCIDIDÆ, PARASITES, ETC.
The Coccididæ, like all Homoptera, produce great numbers of young; but
their increase does not appear to be as rapid as that of some other
families. The numbers of some Aphididæ or Aleurodidæ produced from
a single female in the course of a single year have been calculated
at hundreds of thousands, if not millions; and as many as eleven
generations have been produced in little over half a year. Coccids,
however, as a rule, do not propagate at this alarming rate. Many,
if not the great majority of them, produce in this country but one
generation in the year, e.g., _Mytilaspis pomorum_, _Cœlostoma
zælandicum_, &c. Others, such as _Icerya purchasi_, breed more often;
and probably climate has a good deal to do with the frequency, for
Mr. Comstock says that in the United States _Mytilaspis pomorum_
breeds once a year in the North and twice in the South. In point of
fact, it does not seem possible to lay down any rule on the subject.
Unfortunately, _Icerya_ is not only a frequent breeder, but also the
most destructive insect of the family in New Zealand.
The number of young produced by each female seems also to vary. The
author has counted from 30 to 80 eggs in the puparium of _Mytilaspis
pomorum_; as many as 350 in the ovisac of _Icerya purchasi_, and about
the same number of _Cœlostoma zælandicum_: and a female of _Lecanium
hesperidum_ examined in spring contained 93 embryos. These figures do
not denote any remarkable fertility; but, as in the majority of cases
males are but seldom met with, sometimes even entirely unknown (e.g.,
_Lecanium hesperidum_), it follows that nearly every insect is capable
of propagation, and the increase in numbers is therefore more rapid
than might be anticipated otherwise. How the females in the species
apparently destitute of males are enabled to produce young is perhaps
one of the most mysterious things in Nature. The male of _Mytilaspis
pomorum_ has never been found in New Zealand or Europe, and doubtfully
in America. _Lecanium hesperidum_ has been known and studied for nearly
two hundred years without any male, pupa or adult, being discovered.
Yet both of these species go on increasing regularly and in great
numbers, and show no signs of extinction.
In spite of this absence of males in some cases, and of the
comparatively small numbers of eggs, Coccids would naturally increase
at an exceedingly rapid rate if left undisturbed, on account of the
great proportion of females. They are, moreover, protected, to a great
extent--First, by the fact that birds do not, as a rule, care to eat
them. The "blight-bird" or "white-eye," _Zosterops lateralis_, has
been noticed in this country pecking about in holly-hedges infested by
_Lecanium hesperidum_; but it is not absolutely certain whether it was
eating the Coccids or the other more easily picked-off insects on the
plant, such as _Psocus_, which is very commonly observed among Coccids.
And other birds seem not to devour them at all. Secondly, the usual
position of these insects, on the under side of the leaves, or in the
crevices of bark, is a great shelter and protection for them against
birds or ordinary accidents. Again, they are in many cases effectually
covered by the waxy or fibrous shields, or by the masses of cotton with
which they surround themselves. In countries like the South of France,
California, or the greater part of New Zealand, the winters do not
appear to be sufficiently severe to injure Coccids, and many of them
breed as much in winter as they do in summer. It would therefore seem
that everything combines to assist these insects in their career, and
in their propagation. Nature, however, has provided a check which is
to some extent effective, at least against several species, although,
unhappily, against some of the most noxious--such as _Mytilaspis
pomorum_, the Dactylopii (mealy bugs), _Icerya purchasi_, &c.--it
is not energetic in this country; and this remedy is the attacks of
other minute animals, whether by direct devouring of the Coccids or by
parasitism.
Direct attacks from animal enemies are not frequent. Whether from some
inherent distastefulness, or from the difficulty of getting at them,
Coccids are scarcely subject to being directly devoured. There are a
few exceptions. Under the puparia of _Mytilaspis pomorum_ a minute
white Acarus (mite) may often be found, and it is noticeable that
where it abounds the eggs of the Coccid are often shrivelled or empty.
It is not improbable that this Acarus may feed on the eggs. It appears
to belong to the genus _Tyroglyphus_, a mite which is not usually
carnivorous; but Mr. A. Michael, an authority on mites, seems to be
uncertain whether _Tyroglyphus_ may not make a meal of the Coccid.[H]
No others amongst the Diaspidinæ appear to be directly attacked, nor
any of the Lecanidinæ; but amongst a number of _Dactylopius glaucus_
on a leaf there may often be seen a minute caterpillar, apparently
covered with many tufts of hair. This is the larva of the common
ladybird (_Coccinella_), a beetle which, in both the larval and perfect
states, feeds on Aphides, Coccids, and other insects. The larva may
sometimes be seen holding a _Dactylopius_ in its powerful jaws and
devouring it. Another larva, smooth and without hairs, performs the
same functions--it is the young of a small dipterous fly, apparently
one of the _Syrphidæ_, insects also predatory; but this seems to be
rare. In America, similar larvæ are said to feed upon the "black scale"
(_Lecanium oleæ_). In this country, as far as is yet known, Lecanidæ
are not directly attacked by the ladybirds.
[H] Quart. Journ. Royal Micros. Soc., Feb., 1885.
But this direct warfare by other animals is of small consequence. A
more important check on the increase of many Coccids is afforded by
the indirect action of minute hymenopterous insects, which make use of
them as receptacles for their eggs. This plan is adopted by several
Hymenoptera, of the families Chalcididæ, Ichneumonidæ, Proctotrupidæ,
&c. They do not devour their prey; they allow it to live that they may
live on it. By means of their long ovipositor they pierce its body, and
deposit in it an egg. As the victim grows the egg matures, changes into
a larva, and still remains in the body of the Coccid; changes again
into a pupa, and by this time the Coccid is at liberty to die, for the
parasite has no further use for it except as a shelter; then, when the
proper time arrives, the perfect fly emerges and departs. All Coccids
are by no means equally subject to this system. In the United States,
according to Mr. L. O. Howard,[I] parasites are found in nearly all
genera--Diaspidinæ, Lecanidinæ, or Coccidinæ. In this country, as far
as observation shows at present, the Lecanidinæ are the most liable
to attack, some of the Diaspidinæ next, and the Coccidinæ least of
all. _Mytilaspis pomorum_, so hurtful to apple-trees, does not seem
to be attacked. _Icerya purchasi_, the worst species of all, has not
yet furnished a single parasitic fly. A few specimens of _Dactylopius
glaucus_ contain parasites; a good many of _Fiorinia asteliæ_ and _F.
stricta_; while _Ctenochiton perforatus_ and _C. viridis_ appear to
be the favourite victims, as sometimes scarcely any specimens on a
plant can be examined which do not contain either a larva or a pupa
of a parasite. It is to be observed that in no case is more than one
parasite to be found in a Coccid.
[I] Report of the Entomologist, U.S. Dep. of Agric., 1880, pp. 350-371.
Unfortunately, although this provision of Nature must have a very
considerable effect in preventing the increase of Coccididæ, it is
subject to two drawbacks. In the first place, as just observed, some of
the most injurious pests appear to be unaffected by it. Probably, up to
the present time the worst plant-enemies[J] in New Zealand have been
_Mytilaspis pomorum_, _Aspidiotus coccineus_, _Aspidiotus camelliæ_,
_Lecanium hesperidum_, _Lecanium oleæ_, _Lecanium hemisphæricum_,
_Pulvinaria camellicola_, and _Icerya purchasi_. Here reference is made
not so much to insects which render plants unsightly as to those which
seriously injure its growth: many others, such as _Fiorinia asteliæ_ or
_Ctenochiton viridis_ are ugly enough, but have not been destructive.
Of the injurious species above named none, apparently, are troubled in
this country by parasitic insects up to the present time, at least to
any appreciable extent.
[J] Speaking of Coccids only; _Kermaphis pini_ is equally, if not more,
destructive.
A sketch of Ctenochiton enclosing a parasitic pupa, and of the perfect
fly, will be found in Plate xxiii. In a work like this the generic and
specific characters of these parasites need not be given: they do not
seem to differ much from hymenopterous and dipterous insects of other
countries.
Another mode by which the too rapid increase of Coccids is checked is
by the attacks of vegetable parasites--fungoid growths which permeate
the whole body of the insect, and soon kill it. As far as experience in
New Zealand extends as yet the genera _Ctenochiton_, _Lecanium_, and
probably _Eriochiton_ are the only ones so attacked. On certain plants
in the forests, notably Hedycarya and Coprosma, circular spots may
be commonly found on the under side of the leaves: some dark-brown,
somewhat convex, some bright yellow and often quite globular. In
spring, examination of a young larva of _Ctenochiton viridis_--a
species very common on the above plants--will frequently show, either
within the insect, or on its waxy test, or between the test and the
insect, minute specks, which under a high power of the microscope,
prove to resemble the filaments composing the brown or the yellow spots
just mentioned. On turning over one of the brown fungi, or on pulling
it to pieces, the dead body of a young _Ctenochiton_ or _Lecanium_
larva will always be found in the middle. Apparently this brown fungus
does not attack any but young larvæ; but the bright yellow fungus will
be found filling the bodies also of the females in the second stage,
and the globular portion of the fungus will stand out above them.
These fungi are not of the same genus as _Empusa_, the fungus which so
frequently kills the house-fly; but they seem to act in much the same
way within the insect.
Probably a good many of the _Lecanodiaspidæ_ are preyed on and
destroyed by these fungoid parasites, of which figures are given in
Plate xxiii.