Pests of Agricultural Plants
Grassland. The main insect pest is the grass grub, Costelytra zealandica (Wh.). This is a member of the Melolonthinae of the order Coleoptera or beetles. It is an endemic insect. The adult beetle is brown and somewhat less than 3/8 in. long. According to conditions in different places in New Zealand, it is on the wing from October to January, or even longer. The eggs are laid in the soil of grassland and the hatched larvae or grubs feed on the roots of grasses and may destroy them. In cases of bad infestations the root system is eaten through so thoroughly that the damaged turf can almost be rolled up like a carpet. This species is New Zealand's most important insect pest and, before a satisfactory control measure was devised, caused heavy losses in grassland farming estimated at £15 million a year, or even as high as £30 million. Control is gained by applying 2 lb p.p.i. DDT per acre, usually in superphosphate topdressing. There are two forms of this mixture: (1) the “dry”, where DDT of fine particle size is mixed with the finished superphosphate; and (2) the “wet”, where the DDT is incorporated during the manufacture of the superphosphate. An interesting new development is pelleted DDT, where fine particles of DDT are joined to a hard core. These dressings will give up to three years' protection from grass grub.
A pest second only in importance to the grass grub is the subterranean grass caterpillar, Oxycanus spp. There are several species of this, the most important of which are O. cervinatus (Wlk.), O. umbraculatus (Gn.), and O. despectus (Wlk.). They belong to the family Hepialidae of the order Lepidoptera and are endemic. Moths of the first named appear during September-November. Moths of O. umbraculatus are about one month later than those of O. cervinatus, while those of O. despectus are later still, large flights occurring in January-February. Moths lay their eggs on the surface of the ground. These hatch in about a month and the caterpillars emerge to live for some weeks on the surface of the ground under cover of a mass of webbing; later, they construct a more or less vertical tunnel in the soil and this is lined with silken webbing. From this tunnel the caterpillar emerges at night to feed on grasses. Oxycanus-infested pasture shows characteristic holes, the tunnel entrances, which are about the thickness of a pencil. “Casts” of soil (very like worm casts) are also present, but in the case of Oxycanus these are bound together by strands of webbing. As the caterpillars increase in size they do greater damage to the grass sward, until a point is reached where the increasing bare patches in the pasture may cause heavy economic losses. Control at the moment is the same as that for grass grub, Costelytra zealandica, but more work is needed on this as there have been a good number of instances in some parts of the country where inadequate control has been given.
Other pests of grassland are the exotic black beetle, Heteronychus sanctae-helenae (Blanch.); the exotic Argentine stem weevil, Hyperodes bonariensis, (Kuschel); and the black field cricket, Acheta commodus (Walk.), which was probably accidentally introduced from Australia many years ago. Then there are two species of army worm present: the native Persectania aversa (Walk.) and the introduced Pseudaletia separate (Walk.).
Field Crops. The most important pest of crucifers is the cabbage aphid, Brevicoryne brassicae (L.), which is particularly harmful in swedes and rape. Other pests of crucifers are white butterfly, Pieris rapae (L.), and diamond-back moth, Plutella maculipennis (Curt.), but these are generally well controlled by natural enemies. White clover seed crops are damaged by species of Coleophora. Lucerne suffers attack by the eelworm, Ditylenchus dipsaci (Kuhn), and other pests are the leaf roller, Epiphyas postvittana (Walk.); Heliothis armigera (Hb.); the hemipteron, Eurystylus australis (Popp.); the clover seed chalcid, Bruchophagus gibbus (Boh.), plays a relatively minor part. The tuber moth, Gnorimoschema operculella (Zell.), attacks potatoes and is the most serious pest of tobacco. Maize suffers from corn earworm, Heliothis armigera (Hb.), and black beetle, Heteronychus sanctaehelenae (Blanch.).
Fruit. The main pests are codling moth, Cydia pomonella (L.); the apple leaf roller, Epiphyas postvittana (Walk.); several species of mites, such as the European red mite, Panonychus ulmi (Koch); Bryobia arborea (M. and A.); and Tetranychus telariua (L.); the leafhopper, Edwardsiana australis (Froggatt); several species of aphids, for example, woolly apple aphid, Eriosoma lanigerum (Hausm.); green peach aphid, Myzus persicae (Sulz.); black peach aphid, Brachycaudus prunicola (Kltb.); and the black citrus aphid, Aphis citricidus (Kirk.). Scale insects, such as San Jose scale, Quadraspidiotus perniciosus (Comst.), occur on apple, pear, quince, plum, peach, apricot, nectarine, and cherry, the species Q. ostreaeformis (Curt.), Lepidosaphes ulmi (L.), and Parlatoria pittospori (Mask.) on apple; and red scale, Aonidiella aurantii (Mask.), Ceroplastes sinensis (De G.), and C. destructor (Newst.), occur on citrus; mealy bugs, Pseudococcus spp., and the borer, Oemona hirta (F.), are commonly troublesome.