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.