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.