David Miller

“ The fungi and bacteria are responsible for setting up the intricate reactions involved in the decomposition of organic matter, the bacteria being concerned in practically all of the chemical processes going on in the soil. Both fungi and bacteria are of two kinds: those that bring about decomposition, and those that live in a reciprocal relationship with plants upon the roots of the latter. Such relationship, which benefits both organisms and plants, is called symbiosis, the fungi being known as mycorrhiza, while the bacteria form nodules on the roots of such plants as the legumes. ”

“ Without a systematic study of the stomach contents of birds, it is not possible to decide when a species is beneficial or injurious, and in New Zealand no such study has been made; practically all the information we have is based on field observations, which are, unfortunately, influenced largely by the outlook of the observer, and are thus misleading. Though some species subsist for the most part on insects, most land-birds have a mixed diet of vegetable and animal food, but they specialise on an insect diet when rearing their young and when moulting. ”

“ Condition of Soil.—​The vigour of plants is dependent on the soil, which therefore must be kept in the right state; it must be well tilled, and must contain the requisite nourishment and moisture available for plant use, and as far as possible be free of an abnormal population of root feeding pests, such as eelworms and the larvæ of many insects. Proper cultivation is therefore the important factor in bringing the soil into the state most favourable to plant life, as all inimical factors, including pests, are reduced. ”

“ Scale insects and mealy-bugs, collectively known as coccids, are of very great economic importance on account, not only of their widespread depredations upon plants, few being free from infestation, but also of the commercial value of some species—​e.g., in the production of lac, cochineal, Chinese wax, etc.; it is with the injurious forms that the New Zealand horticulturist is concerned. The term “scale insects” is derived from the appearance of many of the species that are protected by a scale-like covering, which forms a conspicuous scaly incrustation when a plant is heavily infested. ”

“ In order to compete in the world’s markets, and to supply the growing demands of increasing population, a much higher and dependable standard of productivity is required than is found in nature. Consequently, whilst utilising, and increasing the efficiency of the so-called natural enemies as auxiliaries in his fight against destructive animals, man has found it necessary to develop an effective system of artificial control, involving chemicals, resistant plants, cultivation, crop rotation, etc., for the purpose of maintaining a more stringent balance to meet his requirements. ”

“ Insects, with the exception of certain species giving birth to living young, are reproduced from eggs laid by the females; with few exceptions, the latter take no further interest in the eggs beyond placing them in surroundings offering the most favourable conditions for their well-being, and a sufficient food supply for the forthcoming young; each egg is protected by a delicate shell, through which the young insect makes its way on hatching. ”

“ The Protozoa are single units of living matter (protoplasm) , and may be referred to as the one-celled animals; they are mostly microscopic, and lead an independent life, or are associated in colonies, but are capable, as a rule, of carrying on independently all the functions of life, though there are no organs such as those of digestion, respiration, and circulation, as we know them in the higher animals. It is amongst such simple forms that the distinction between the lowest animals and plants ceases to be clear. ”

“ The seed-feeding birds are a potential menace to the agriculturist, though in New Zealand the native species are fundamental to the well-being of the native forests; the insectivorous birds are obviously beneficial, though they devour both destructive and useful insects; while the omnivorous birds may be either useful or harmful, according to the circumstances. It should be remembered that, no matter what the food of the adult bird may be, most species give their young a diet of insects or other animal matter. ”

“ All insects do not follow the same method of development from egg to adult, and the adaptations of structure and habit are many and varied as well as simple and complex. Species having a complex development, during which they pass through stages, each differing in form from its predecessor, undergo what is known as a metamorphosis; contrasted with such insects are those developing in a simple manner without pronounced differences in the form of successive stages, the young resembling the adult in most features except size and maturity—​these insects are without a metamorphosis. ”

“ Overcrowding.—​The tendency to overcrowd, especially in household gardens, is to be avoided; a favourite habit is to plant something in every available space. Under such conditions pests and diseases will abound, and before attempting to spread over a large area, and so lessen the effect of their depredations, they concentrate in mass formation within the confines of the garden as long as the food supply lasts; further, plants tend to be less vigorous and more susceptible to infestation under crowded than under more open conditions. ”

“ Unlike the moth larva, that of the silverfish throughout its growth resembles the adult both in habit and form, the only marked differences being that of size and the absence of the abdominal leg-like appendages. During growth several moults take place, and at the final one the adult appears with all its characteristics. Some species take two years to reach maturity. In this type of insect there is, therefore, no pupal or resting stage, and the larval habits and food are the same as those of the adult insect, while there is but little difference in structure throughout all the stages. ”

“ Though gradation in size may be a sign of successive ages in certain insects, the presence of functioning wings denotes that growth has ceased; in the case of wingless insects, the characters of maturity may be less conspicuous. Although there may be at times a fairly wide range in size among fully-grown individuals of the one species, such variation is not due to age, but to certain factors influencing the insect during growth, such as the abundance or scarcity of food supply, and favourable or unfavourable climatic conditions. ”

“ So the process proceeds until a soil is formed of sufficient extent and quality for the support of a more extensive and increasingly complex vegetation; thus, in the cycle of life and decay, stores of organic compounds are elaborated by plants and returned to the soil, which they enrich, and where they are decomposed by organisms, and so maintain the supplies of food materials suitable for the maintenance of vegetation. ”

“ Living plants (in reference to green plants) utilise the sun’s energy in the manufacture of their complex food materials from comparatively simple chemical compounds. These latter compounds are carbon dioxide, derived from the air through the agency of leaves, and a weak solution of various chemical compounds in water, derived by means of the roots from the soil, and carried up through the plant to the leaves, where the elaboration into the complex compounds to be utilised by the plants as food takes place. ”

“ The best method of control is garden sanitation, all rubbish likely to harbour the woodlice being removed. Since they are nocturnal, the woodlice can be trapped by means of moss laid on the ground; the moss in which the woodlice have taken shelter is collected during the day and burned, or immersed in hot water to kill the animals, when it can be used again. Some good results have been secured by means of sliced potatoes dipped in arsenate of lead or Paris green; the potatoes are placed within reach of the woodlice, which are attracted to and feed upon the poisoned bait. ”

“ When fully developed, each caterpillar spins a silken cocoon on the under side of the leaf, and there transforms to the pupæ, from which a moth eventually emerges. The insect is small, narrow, and has a light-coloured, diamond-shaped marking along the back. The moth is nocturnal, and shelters amongst the denser foliage during the day; it emerges at night, and lays its eggs upon the leaves. The life-cycle from eggs to adult occupies some 36 days, more or less, according to the season, and there may be six or seven generations during the year. ”

“ The eggs hatch in the spring, and the young insects swarm over the host plant in search of a suitable place to settle. A continuous warm spell of weather in the spring will allow all the eggs to hatch almost at one time, but alternating cold spells will retard development, so that emergences take place over a longer period. After emerging from the egg until maturity, when egg-laying again takes place, a period of three months elapses; the insect is a slow breeder, and produces only one brood a year in colder climates, but is two-brooded in warm districts, such as Auckland. ”

“ According to the species of thrips, the female lays her eggs either in the plant tissues or upon the surface. The young insects are wingless, but attack the plant in the same manner as does the adult; as development proceeds, the insect transforms to a pupa, from which the adult ultimately emerges. A characteristic symptom of thrips infestation is a silvering of the foliage, while the leaves are further rendered unsightly by the minute specks of hardened excreta ejected by the insects. ”

“ Eelworms are minute, unsegmented worms, related to the parasitic thread-worms of animals, and are abundant in soil and water; it is usually the surface layers of the richer soils that are inhabited by them. Of the long list of species, only a few are destructive to vegetation, but these constitute one of the greatest problems of the horticulturist. It is thought that the injury caused to plants by eelworms is toxic rather than mechanical, and some plants apparently are capable of producing anti-toxins, which neutralise the toxins of the eelworms; such plants possess an immunity. ”

“ The adult female is oval and about one-third of an inch at its longest length; in shape it resembles a rather flattened turtle scale, but when laying eggs the body shrivels and numerous transverse wrinkles develop, although the margins of the scale remain smooth. There is at least one generation each year, and in warmer parts probably a second, which may reach maturity before winter or not till the following spring. The eggs are laid in an elongate, white, cottony sac secreted at one end of the female; this sac is sometimes as much as four to five times the length of the insect. ”

“ These comparatively simple compounds from which the plants elaborate their nourishment are the raw food materials, and that they must always be available for plant growth, is evident when one considers the vast areas of vegetation that cover, with the exception of desert regions, the surface of the earth. Under moist climatic conditions it has been calculated that some 500 tons of carbon dioxide and 1,000,000 tons of water, having the raw food materials in solution, are used annually by one square mile of dense forest. ”

“ The leaf-infesting stages of the insect cause pocket-like galls to form, which open on the upper surface of the leaf by a narrow aperture concealed under a tuft of delicate hairs (Fig. 9, 5) . In each gall the aphid matures and deposits several hundreds of eggs, from which wingless females hatch; these wander to other leaves, and each insect forms a new gall for itself. Several generations develop thus, but later many of the offspring migrate underground and join the root-infesting colonies. ”

“ Although in New Zealand there is usually only one generation produced each year, three or even four develop in other countries. The winter is passed by the larvæ in their cocoons, and pupation takes place just prior to the period when the moths emerge in the spring. As the moths continue to emerge and lay their eggs for a period extending from November to February, it is essential that regular applications of arsenate of lead be made during that time in order to protect the fruit from the larvæ hatching from the eggs laid by the moths. ”

“ Many are merely scavengers, but some insects, such as grass-grubs and the caterpillars of certain moths, and millepedes, feed upon living plants and so add organic matter to the soil in their excreta, which also contains quantities of soil swallowed with the food, this latter mechanical action aiding in the pulverising and opening up of the soil; certain eelworms, too, that attack living plants play a somewhat similar part, in that they are primary causative agents in the decay of healthy tissues. ”

“ After settling in the first larval stage, the insect becomes very sluggish, and does not move, except to migrate, as most do, from the leaves to the twigs, there to take up a permanent position. The legs and antennæ are retained throughout life, but in the adult are functionless, being folded against the body; in some species of intermediate forms the appendages become atrophied during development. In the olive scale, and related forms, the toughened cuticle not only serves as a protection to the insect, but also as a receptacle for the eggs (Fig. 5) ”

“ In dealing with the control of plant pests, the objective is to prevent attacks, or, when the attacks have established, to check them as much as possible. In the latter case the term “exterminate” is in too frequent use; it is not usually practicable to exterminate a pest, and the best that can be done is to check or control it. ”

“ Observers in other countries have noted that there is a summer migration of winged females to cruciferous plants, where colonies are established, and whence there is a return migration during the autumn to the original host. The wingless females are black, with part of the legs yellow, while the young individuals are pale in colour; the winged females have a green abdomen, from which arise the black “honey-tubes.” Since all the over-wintering eggs have hatched by the time the buds open, the insect can be then controlled by applications of nicotine-sulphate. ”

“ Earthworms may be correctly called the great soil builders; they burrow through it, allowing the free passage of air and water; they swallow large quantities, which they eject on the surface in the form of “worm-casts,” the soil materials being well mixed in the process; they pull underground leaves and other parts of plants from the surface and so increase the supply of organic matter for the action of the micro-organisms that bring about decomposition. ”

“ The organisms in the first group play a comparatively minor part in soil development, and influence its fertility to an almost negligible extent, the temporary scavengers, perhaps, being of more importance since they aid in the reduction of vegetable residues. The forms in the second group, however, are invaluable as soil-making agents and in the production of plant food materials, the least important among them being the insects, spiders and millepedes. ”

“ In New Zealand such beneficial animals are insects, birds, and the hedgehog.
Insects.
There is a wide range of insects that live at the expense of their fellows, and without these plant production would be impossible, either by Nature or by man. These so-called beneficial insects or parasites are the greatest factor in maintaining within reasonable bounds the insects that destroy vegetation; they are of much greater value in this respect than birds. ”