Art. XXXIII.—On the direct Injuries to Vegetation in New Zealand by various Insects, especially with reference to Larvœ of Moths and Beetles feeding upon the Field Crops; and the Expediency of introducing Insectivorous Birds as a Remedy.
[Read before the Philosophical Institute of Canterbury, 12th December, 1872.]
The little time and attention that I have been able to afford to its investigation precludes my treating exhaustively a subject so comprehensive as that of Injuries caused by Insects, and Benefits derived from Insectivorous Animals; but, should the members of the Society be sufficiently interested, I hope, on some future occasion, to enlarge upon the subject in a series of papers.
It has been observed by the authors of a valuable work on entomology, that if it were not for certain counter-checks restraining them within due limits, insects would drive mankind, and all the larger animals, from the face of the earth.—That “the common good of this terraqueous globe requires that all things endowed with vegetable, or animal life, should bear certain proportions to each other, and if any individual species exceeds that proportion it becomes noxious, and interferes with the general welfare.” And they ask, “How is it that the Great Being of beings preserves the system, which he has created, from permanent injury in consequence of the too great redundancy of any individual species, but by employing one creature to prey upon another, and so overruling and directing the instincts of all that they may operate most where they are most wanted.”
So long as this balance remains undisturbed, so long will harmony prevail; and whenever we suffer excessive injuries from insects, or other animals, the cause may generally, if not invariably, be traced either directly or indirectly to the agency of man alone. Man, in his blindness, is ever breaking, or throwing out of gear, some wheel of the great cosmical machine, and disorder necessarily follows.
In illustration of this, I would point to the great increase of caterpillars, and other larvæ, in the neighbourhood of Christchurch during the last four or five years—an increase attributable in all probability to the following simple causes:—
In the early days of the Canterbury settlement, quails, larks, and other birds that fed upon insects and their larvæ, abounded on the plains; but the quails have been exterminated, the larks have become comparatively scarce, and the other birds have almost disappeared. So long as the plains remained open and uncultivated, extensive grass-fires sweeping over the land consumed an enormous amount of insect life, and took the place of that counter-check which was being removed by the decrease of the birds; but within the last
few years inclosures and cultivation have been rapidly extending around Christchurch, and forming a nursery for the preservation and increase of the insect race. A luxuriant and abundant vegetation has sprung up for its food and shelter, and it is comparatively freed from the ravages of fire and the attack of its feathered foes. What can we expect under such circumstances but to be visited with an insect pest? Unless some remedy were applied, or some special intervention of Providence occurred, the evil would inevitably increase with each succeeding year, and the farmer would ultimately find that his money and labour were providing but a harvest for the caterpillar and grub.
Some idea may be formed of the enormous increase of herbivorous insects if we take, for example, Plusia gamma, one of the moths of the Noctuœ family (a family extensively represented in this neighbourhood) and the common Aphis or plant louse. Réaumer has proved that from a single pair of Plusia gamma moths, 80,000 might be produced in one season, and the rapidity of production of the Aphidœ is so enormous, that nine generations have been produced in three months; and, each generation averaging 100 individuals, it has been calculated that 10,000 million millions may be generated in that period from a single Aphis.
So far as I have been able to ascertain, from inquiry and from my own personal observations, the insects which appear to have been the most injurious to the farmers of this neighbourhood are of the following kinds, namely:—
Of moths five species, namely—Pielus umbraculatus, Gu., Pielus variolaris, Gu., and Cloantha composita, Gu., (all named and described by M. Guenèe as “new species” from specimens taken by me in this province), and Heliothis armigera and Sesia tipuliformis (a species found also in England).
Of Beetles two species, namely — Odontria striata and Odontria (n.s. undescribed).
Of Aphides, several species.
Several specimens of the perfect insect of each of the above species of moths and beetles I now place before you for inspection, and in order that you may identify the species to which I allude.
Pielus umbraculatus and P. variolaris make their appearance on the wing, in great numbers, in the evening twilight, and in the daytime are found at rest on posts and rails and palings and such like places, and numbers may be seen entangled or wound up in the webs of spiders. These moths are very abundant in the months of October and November. The family to which they belong has received the common name of “swifts” from the rapidity of their flight. The larvæ of these species are short fleshy grubs, having six pectoral, eight ventral, and two anal feet; they are subterranean, and feed
principally upon the roots of grasses, coiling themselves up when disturbed. The transformation to the pupa state takes place under ground, and the pupæ are of a chestnut colour and glossy.
Cloantha composita not only flies at night, but also may be frequently seen on the wing in the daytime, flying briskly from flower to flower, and feeding upon the nectar, which it extracts with its long proboscis. The larvæ are more slender than those of Pielus, of a variety of colours, and striped longitudinally with numerous thread-like lines. They have sixteen feet, and feed principally on grasses and standing corn—especially rye-grass and oats—eating off the heads and stems of the grass, and the ears and leaves of the corn, sometimes resting on the stems during the day, but generally hiding in the grass, and coming out at night to feed. They commit immense damage, and when they have consumed the grass of one field they may be seen in prodigious numbers marching over the ground to another. The pupa is found under ground; and is of a dark glossy chestnut colour.
Heliothis armigera makes its appearance on the wing by day, as well as at night, and particularly delights in the brightest sunshine, when it may be seen, like Cloantha composita, flying about the flowers in search of nectar. The caterpillars are of various colours, have sixteen legs, and feed on low plants and vegetables, particularly peas, the pods of which they perforate and devour the contents. The colour of the pupa is glossy chestnut or brown.
Sesia tipuliformis has undoubtedly been brought into this country with the currant tree, upon which it feeds, and it shows how careful we ought to be when introducing anything useful that we bring not with it a grievous pest. The ravages of this insect have so increased that I question if we shall be able much longer to grow the red currant unless some check is imposed. The larvæ (whitish fleshy grubs) perforate the stems and branches of the trees, and eat away the pith. The perfect insect would be mistaken by the uninstructed for a species of fly or hymenopterous insect, so little does it resemble the ordinary appearance of a moth.
The Odontria beetles may be seen in the dusk of evening, flying in swarms over the grass, and humming like a hive of bees. The larvæ are subterranean, and are particularly destructive to clover and grasses, devouring the roots and leaving the upper part of the plant loose upon the ground as if cut off with a knife. They are soft fleshy grubs of a whitish colour, with brown horny heads. They have six legs, one pair on each of the three first segments, but none on the hinder. When disturbed they lie motionless, in a recurved position, having the hinder part bent inwards towards the head in the form of a hook. They seem, though not half their size, to be almost as destructive as the larvæ of the cockchafer (Melolontha vulgaris) which does such immense damage to pastures in England. Most, if not all, of the several species of this
family continue in the larva state for several years before transformation to the pupa, and it is probable that such is the case with the two species we are now considering. The perfect insect feeds upon the leaves of various trees, but I do not think that any serious injury in that respect has yet occurred in this province. Dr. Carpenter, in his work on zoology, revised by Mr. Dallas, referring to the cockchafers, says, “Their excessive multiplication is usually prevented by birds; but if these be kept away they increase very rapidly and become a complete pest to the cultivator. The perfect insects sometimes' make their appearance in such swarms as to devastate an entire forest.”
A species of Aphis appears to have become very injurious to our corn crops, and we all know what pests we have in two other species, the one Aphis lanigera, commonly called “American blight,” which infests the apple trees, and the other the “cabbage blight.”
In addition to moths, beetles, and Aphides I may mention locusts and grasshoppers, the latter of which are very abundant on the plains, and devour a considerable quantity of grass and herbage. Fortunately for us the locusts are not yet so numerous as to do any considerable mischief, but I have noticed their increase of late years. These insects are so well known in their perfect state that I may pass over them with the single remark that their larvæ and pupæ resemble the perfect insects, except that the wings of the pupæ are rudimentary only and the larvæ have none.
There is also a most destructive species of saw-fly, identical either with Selandria cerasi, or the North-American “slug-worm,” or closely allied thereto. The larva of this fly is covered with a greenish-black viscid matter which exudes from its body, and to a cursory observer resembles a small black slug. It feeds upon the upper surface of the leaves of its food plant. Cherry, plum, pear, hawthorn, and sometimes other trees, become completely stripped of their leaves by these larvæ, and when it occurs early in the summer, as it frequently does, the trees are compelled to put forth fresh foliage, thereby weakening them, and lessening the production of fruit in the succeeding year.
Lastly, there is an insect which appears identical with, or allied to, Coccus arborum linearis. It infests the pear and ash, and some other trees, and has the appearance of a small scale shaped like a mussel-shell. These insects thickly cover the bark of the trees, to which they closely adhere and exhaust the sap.
Such as I have above described are, I believe, the most injurious of the insects we have to contend with, but there are numerous others of minor importance that I must defer for future observation.
We will now proceed to the consideration of the expediency of introducing insectivorous birds and animals as a remedy.
The increase of insects is so enormous and rapid, and their location so
intimately connected with the things they destroy, that we cannot effectually apply any direct remedy, without at the same time destroying or injuring what we attempt to preserve. It is an error to suppose that caterpillars, or the larvæ of insects, are to any considerable extent affected by atmospheric forces. The severest frost does not destroy their vitality, for if they fail to find a sufficient shelter torpidity only is produced. I have myself taken caterpillars from the snow so entirely frozen as to have become brittle as glass, and yet, when exposed to the warmth, they have quickly revived and resumed their activity, without having suffered any apparent injury. When inundations or heavy falls of rain take place, and the ground becomes completely covered with water for days or weeks, considerable mortality is probably caused amongst the caterpillars; but such occurrences are only occasional and local.
Our only remedy is an indirect one, and that I conceive to be the employment of insectivorous animals to do the work for us; and for this purpose insectivorous birds stand prominent. I consider it to be our duty not only to protect the few indigenous birds that yet remain, but to continue to introduce others, until we have restored the balance which has been disturbed.
Of indigenous insectivorous and insect-destroying animals already existing in this locality, the following list, I think, comprises most of those which are of prominent importance, namely:—
Bats—entirely insectivorous, and of which, I believe, we have more than one species, but the individuals do not appear to be very numerous.
Gulls and terns, or sea-swallows—visiting the fields in flocks, and picking up slugs, worms, and grubs.
Larks—of which we have one species only—and a few small birds seen in the bush, and in our gardens.
We have also the Zosterops, commonly called “blight-birds,” from their feeding on the American blight (Aphis lanigera), but they, though there is no direct evidence of their introduction, are considered not indigenous.
Lizards, spiders, dragon-flies, which are all entirely insectivorous.
Beetles, of which we have numerous species of the Carabidœ family, whose habit is (to use the words of Dr. Carpenter) “to prowl about on the surface of the ground, under stones, etc., beneath the bark of trees or moss growing upon their roots, in search of their insect prey, which consists chiefly of herbivorous species of their own order. Some of them nocturnal in their habits, feeding on cockchafers and other species of herbivorous beetles that fly abroad during the night.” And two species at least of the family of the Coccinellidœ, commonly called “lady-birds,” or “lady-cows,” whose larvæ feed entirely upon Aphides.
Flies—of which we have two species most destructive to moths and flies, namely, Asilus varius, and Doctria, (? species) which dart from their resting place with exceeding rapidity, and seize their prey on the wing.
Ichneumons—a tribe of parasitic insects the most valuable of all, for scarcely an insect exists that is not exposed to the attack of one species or another of them. Every species of ichneumon has its particular species of insect upon which its larvæ exist. The victim is generally the larva (in some cases the egg or pupa) of some other insect. The egg of the parasite having been deposited by means of a long ovipositor, and hatched in the body of its victim, the parasite grub there feeds upon it, for days and months, devouring all but the vital organs; and so accurately is the supply of food proportioned to the demand that the victim lives just long enough for the parasitic grub to become full-fed and ready to assume the pupa state.
I have now only to indicate what birds are most valuable for us to introduce and acclimatize.
Thanks to our Acclimatization Society, many useful birds have already been introduced, and thoroughly established. Pheasants, sparrows, and chaffinches are plentiful; and many other birds (included in the list below), though at present scarce, seem to have obtained a firm footing.
To enumerate all the useful birds it is desirable to introduce would occupy more space than can be afforded in this paper, and I, therefore, confine myself to suggesting the few I have named in the list below; and, in selecting from such list, it should be a matter for consideration what species will increase the most rapidly, and spread over the country; and it should be borne in mind that many of the birds which live entirely on insect food are less valuable, for the purposes for which we require them, than others not wholly insectivorous, and that gregarious birds are preferable to those comparatively solitary.
The following is the list of birds recommended as insectivorous in their habits:—Rooks, jackdaws, partridges, landrails, starlings, skylarks, quails, plovers, redpolls, swallows, martens, swifts, blackbirds, thrushes, pipits, wagtails, nightingales, tits and their allied species, and wrens.