In this chapter will be grouped for convenience mites, woodlice, millepedes, slugs, snails, and eelworms.

Mites.

Mites, together with spiders and ticks, belong to a group of animals distinct from the insects, from which, they differ in many respects; for example, they possess four, and not three, pairs of legs in the adult state, no head separated from the body as a movable, distinct region, while in many cases, especially in mites and ticks, the abdomen and thorax are continuous; in no case are wings developed.

Mites are of small size, some being microscopic, while others are just discernable by the unaided eye. All species have the mouth-parts developed for the purpose of feeding upon liquid food—?e.g., blood (in the case of those species that attack animals), decaying vegetable matter, or the saps of plants. It is the last—?that is, those parasitic upon plants—?with which we are here concerned.

The life-history of mites presents some variability, and, though there are fundamentally four stages of development, additional stages have been developed by some species which tend to complicate the cycle. The principal stages in development are as follows (Fig.13, 1–5):—?In practically all cases eggs are deposited, but few species being viviparous. The larva, on hatching, possesses but six legs, and resembles an insect in this respect; the larva then becomes quiescent, and after moulting the eight-legged nymph appears. While in the nymphal state the mite may undergo one or more moults, giving rise to additional nymphal forms, that may complicate the life-history. From the final moult of the nymph the adult mite emerges.

Perhaps the best-known mite in New Zealand is the European red mite of apple trees (Paratetranychus pilosus), though it attacks a wide range of plants apart from deciduous fruit trees, which it favours; it has been found on grape vine, raspberry, rose, hawthorn, citrus, etc. This mite (Fig.13, 6) occurs in Europe, Russia, British Isles, North America, Australia, and New Zealand, and it causes considerable injury to foliage, which assumes a brown appearance, owing to the tissues drying up where they have been punctured by the mouth-parts of the mite.

In the case of heavily-infested trees, the red eggs of this mite form conspicuous patches on the bark during winter; these winter eggs are laid from January onward till leaf-fall, and from them the young mites hatch in the spring, when the foliage is again attacked. The red mite develops rapidly, and reaches the adult stage in about two weeks; several generations are thus produced from spring to autumn, when the eggs are laid upon the foliage.

The eggs (Fig.13, 7) are very small, globular, and ribbed on the surface; from the centre of each projects a hair-like stalk, somewhat bent at the tip. The colour is bright red, changing to a deep orange. The red mite lives freely upon the foliage, and does not produce a web, as do related species; the adult female is bright red to dark brownish-red, rather globular in shape, with comparatively stout legs and numerous spine-like hairs on the back. Although the eggs of the European red mite are exposed on bark and readily accessible to sprays during the winter, no effective winter wash for their control is yet known; the most satisfactory method for checking the pest is to spray the active stages of the mite with summer oil.

Another species of mite, having much the same habits and host plants as the European red mite, is the brown mite (Bryobia prÆtiosa). The eggs of this species are of a deep red, with a yellowish tinge in many cases, but differ from those of the European red mite in the absence of the polar-stalk and ribbed surface. The brown mites (Fig.13, 8) are of a dull red or greenish colour, lack the spine-like hairs on the back, are decidedly flattened, and have the front pair of legs abnormally long.

The common red spider (Tetranychus telarius) is a species of mite frequently met with on a wide range of plants too numerous to mention here; in New Zealand it frequently injures violet, hop, currant, willow, and many weeds. This mite is to be found in all stages practically all the year round; during the spring it is mostly found on weeds and such cultivated plants as strawberry and violet. It is a web-spinning species, and the minute yellowish-red eggs are to be found scattered among a fine web attached to the lower surface of leaves as a rule. The adult mite (Fig.13, 9) is very active; it is somewhat larger than the two foregoing species, and of a yellowish-green colour, with a pair of conspicuous dark spots on the back. Though this mite can be held in check by the application of lime-sulphur sprays, advantage should be taken of thorough cultivation during the dormant season, since the mite hibernates on weeds and among dead leaves and in the soil.

A mite very often met with by bulb growers is the bulb-mite (Rhizoglyphus hyacinthi), now found in most parts of the world. Although this mite may possibly be able to attack practically all tubers or bulbs, it is commonly found infesting narcissus, hyacinth, tulip, crocus, and Easter lily; it is especially abundant in bulbs with loose scales, and has been found to be capable of attacking healthy tissue. The life-history of this species is complicated at times by the development of additional stages; one of these—?the hypopus—?is of particular interest, as it shows more activity than the others, and attaches itself to the bodies of insects, and is so transported. The mite develops from egg to adult within a period of nine days under favourable conditions, or as long as six weeks at other times. All stages of the bulb-mite occur at the same time in infested bulbs, which become soft and rotten. The adult mites (Fig.13, 10) are smooth, yellowish-white, tinged with pink, and have legs and mouth-parts reddish. Symptoms of their presence are to be found in stunted growth and yellowing leaves, failure of flower development, reddish spots on bulb scales, or a softening of the bulbs. All seriously-infested bulbs should be destroyed, and the ground where they were grown treated with calcium cyanide. For the treatment of bulbs, they should be immersed for ten minutes in a two per cent. solution of formalin heated to 122 deg. Fahr., or simply in water at a temperature of 131 deg. Fahr.

Another group of mites of importance to the horticulturist is that of the blister mites; they are so minute—?measuring about a hundred-and-fiftieth of an inch long—?as to be invisible to the unaided eye. Though so minute, however, their damage to foliage is characteristic and conspicuous, so that their presence is easily detected. The most important blister mite in New Zealand is the pear-leaf blister mite (Eriophyes pyri); it differs from the other mites described above in having a long and cylindrical body, with only two pairs of legs crowded at the head end, the elongate abdomen having the appearance of being composed of innumerable segments (Fig.13, 11). This mite lives in colonies in blisters formed on the leaf, and sometimes on the leaf petioles. In the spring the yellowish-green blisters will give the upper surface of an infested leaf a spotted appearance, and as the season advances these blisters become reddish and finally brown; in the case of severe infestation, the blisters become so crowded as to merge into masses.

During the winter the mites lie in the shelter of the bud scales; as soon as the foliage begins to develop in the spring the over-wintering mites attack the leaves, each mite forming a blister, in which it produces a colony of young. The offspring then migrate from the parent blister and form blisters for themselves, and this goes on until autumn, when the last generation of mites migrates for the winter to the shelter of the bud scales.

Owing to the mites being protected within the leaf blisters, summer sprays are not effective as a means of control, which can be effected, however, by spraying with lime-sulphur in the autumn, when the mites are taking up their winter quarters, and again at bud movement in the spring.

Woodlice.

Woodlice are so well known, that but little description is necessary here. However, the following features are of interest. They belong to the group of animals known as the Crustacea, which also includes the crabs; these animals breathe by means of gills, and are usually aquatic, but some forms, such as the woodlice, have become adapted to a life on land. In outline (Fig.13, 12) the woodlice are more or less oval, with the upper surfaces somewhat arched, and the lower flat; the body is divided into several segments, which may enable the animals to curl up in the form of a pill. There is a distinct head, bearing a pair of antennÆ and the mouth-parts, followed by seven large thoracic segments, to each of which a pair of legs is attached; finally, the remaining six segments are more or less crowded together, and constitute the abdomen.

Since woodlice are terrestrial gill-breathing animals, moisture is essential for them, and it is in moist places that they abound. They depend upon a mixed diet, being carnivorous, as well as herbivorous; though normally scavengers, their attacks upon seedlings and tender parts of plants bring them into the ranks of important garden pests.

Woodlice hibernate under any convenient shelter; in the spring, eggs are produced and carried by the female on the under side of the body until the young woodlice hatch. During growth the cuticle or shell is periodically cast, and a freshly-moulted woodlouse is white in colour.

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. Horse manure should not be used in seed beds likely to be infested by woodlice.

Millepedes.

Millepedes are short, worm-like animals, with a fringe of numerous short legs on each side (Fig.13, 13), and have a characteristic habit of curling up when disturbed. Though scavengers for the most part, feeding upon decaying vegetation and on small organisms, they may do considerable damage to sprouting seeds, seedlings, and to tender plants; they are particularly abundant in damp and warm soil, where there is an abundance of rotting vegetable matter.

Having a keen sense of smell, millepedes are readily attracted to poisoned bait in the form of sliced potato spread with Paris green: another method is to place a piece of freshly-cut potato under an inverted flower pot, to which the millepedes will be attracted, when they can be collected and destroyed. A satisfactory control measure is to treat infested soil with black-leaf 40, using one part in one thousand parts of water.

Slugs and Snails.

Plants are very often greatly damaged by the depredations of slugs and snails; frequently young plants are devoured as soon as they appear above ground. These animals attack the plants after nightfall, and during the day seek cover. Though slugs will shelter in the soil, they, together with snails, will shelter in any convenient place, such as under old boards, sacking, bricks and stones upon the ground, or under large leaves or amongst rank herbage—?indeed, in almost any place that affords cover and moisture. Slugs and snails are especially active during wet weather, and at such times, owing to the overcast conditions, they will continue their depredations in the daytime.

Though slugs are active throughout the year, and even during winter when the temperature is favourable, snails pass the winter, as well as hot, dry spells in summer, in a dormant state, often being found together in sheltered positions where the conditions are dry.

Both slugs and snails copiously secrete a slimy substance, that affords them protection against chemicals used for purposes of control. In the case of the slug (Fig.13, 14), the shell is small and inconspicuous, but the large spiral shell of the snail (Fig.13, 15) affords the animal adequate protection, into which it withdraws itself in times of danger. Both slugs and snails reproduce by means of eggs; these are white, spherical and opaque, and are deposited in the soil or under decaying vegetation.

One of the best means of control is to dust the plants with powdered tobacco. Another method is to treat infested plants with soot or lime, but this must be done at night, and the material used must come into actual contact with the pests. An effective poison bait, but one that requires to be carefully handled, owing to its poisonous nature, is a mash made of 6lb. of bran mixed with 1lb. of arsenate of lead and an equal weight of treacle; this is made into a stiff paste, water being added if necessary. Lumps of this mash are placed about the plants to be protected. As a barrier to prevent the inroads of slugs and snails, plants may be surrounded by a belt of calcium cyanide; this would have to be replaced each night, and the utmost care taken in handling, since the substance and the gas evolved from it are highly poisonous; out of doors, however, the gas, being diluted with air, would not be very injurious as long as one did not stand over the treated ground longer than was necessary for laying the cyanide.

Apart from the above methods, the key to the control of slugs and snails is “clean farming”—?that is, the removal of all places, such as rubbish and rank vegetation, where the animals will find shelter; the compost heap is a favourite breeding place, and this should be turned over at intervals and dressed with lime.

Eelworms.

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. There are three important species in New Zealand.

The so-called bulb-eelworm (Anguillulina dipsaci) attacks more than two hundred kinds of plants, but is of especial interest to the horticulturist on account of its attacks upon hyacinths, daffodil, narcissus, and gladiolus, causing deformity and rotting of the tissues (Fig.13, 16). It has been found that this eelworm develops from egg to adult within a period of between three and four weeks; the eggs are capable of lying dormant in the soil for as long as seven years. Infested bulbs and corms should be treated by immersion for three hours in water heated to 110 deg. Fahr.

Potatoes are often damaged by the beet eelworm (Heterodera schachtii), which causes what is known as “potato sickness,” when the growth is retarded, and wilting takes place; the root-system shows an abnormal development of secondary or “hunger-roots.” The eggs are retained in the body of the female, which forms a protective sack or cyst (Fig.13, 17 and 18), and in this state the eggs pass the winter in the ground, where they are known to remain dormant for a period of ten years; under favourable conditions in the spring, the larvÆ emerge from the eggs and attack the rootlets of suitable host plants, entering them at the extreme tip. Satisfactory methods of control have not yet been developed under field conditions, but a four-year crop rotation following potatoes is suggested; seed potatoes from infested ground should not be used.

The roots of tomatoes are often found to be a mass of galls, due to attack by the root-knot eelworm (Heterodera radicicola), which also infests tobacco roots as well as other plants (Fig.13, 19 and 20). All stages of this species are to be found in the root galls; the female lays her eggs in a gelatinous egg sack, which remains attached to the parent. The larvÆ, on hatching, either remain within the parent gall or leave it and enter the soil, where they seek out and attack the roots of another plant. In tomato gardens steam sterilisation of the soil is the most effective means of control.