In agriculture and horticulture s such as those of the cotton bollworm moth
Helicoverpa armigera can devastate crops. Together pests and
diseases cause up to 40% yield losses every year. The animal groups of the greatest importance as agricultural pests are (in order of economic importance)
insects,
mites,
nematodes and
gastropod molluscs. Insects are responsible for two major forms of damage to crops. First, there is the direct injury they cause to the plants as they feed on the tissues; a reduction in leaf surface available for photosynthesis, distortion of growing shoots, a diminution of the plant's growth and vigour, and the wilting of shoots and branches caused by the insects' tunneling activities. Secondly there is the indirect damage, where the insects do little direct harm, but either transmit or allow entry of fungal, bacterial or viral infections. Although some insects are polyphagous, many are restricted to one specific crop, or group of crops. In many cases it is the larva that feeds on the plant, building up a nutritional store that will be used by the short-lived adult;
sawfly and
lepidopteran larvae feed mainly on the aerial portions of plants while
beetle larvae tend to live underground, feeding on roots, or tunnel into the stem or under the bark. The true bugs,
Hemiptera, have piercing and sucking mouthparts and live by sucking sap from plants. These include
aphids,
whiteflies and
scale insects. Apart from weakening the plant, they encourage the growth of
sooty mould on the
honeydew the insects produce, which cuts out the light and reduces photosynthesis, stunting the plant's growth. They often transmit serious viral diseases between plants. on
cherry caused by a mite,
Eriophyes cerasicrumena The mites that cause most trouble in the field are the
spider mites. These are less than in diameter, can be very numerous, and thrive in hot, dry conditions. They mostly live on the underside of leaves and puncture the plant cells to feed, with some species forming webbing. They occur on nearly all important food crops and ornamental plants, both outdoors and under glass, and include some of the most economically important pests. Another important group of mites is the
gall mites which affect a wide range of plants, several mite species being major pests causing substantial economic damage to crops. They can feed on the roots or the aerial parts of plants and transmit viruses. Some examples are the
big bud mite that transmits the reversion virus of
blackcurrants, the
coconut mite which can devastate coconut production, and the
cereal rust mite which transmits several grass and cereal viruses. Being exceedingly minute, many plant mites are spread by wind, although others use insects or other arthropods as a means to disperse. When its populations are high, the
potato cyst nematode can cause reductions of 80% in yield of susceptible potato varieties. The nematode eggs survive in the soil for many years, being stimulated to hatch by chemical cues produced by roots of susceptible plants.
Slugs and
snails are terrestrial gastropod molluscs which typically chew leaves, stems, flowers, fruit and vegetable debris. Slugs and snails differ little from each other and both do considerable damage to plants. With novel crops being grown and with insect pests having been brought more under control by biological and other means, the damage done by molluscs becomes of greater significance. One example being The giant apple snail,
Pomacea maculata which causes billions in loss in rice production each year.
Weeds , a native of South America, is an
invasive species in many other countries and is considered a
noxious weed as it is harmful to aquatic ecosystems, recreational activities, and favours the spread of
mosquitoes. Control is difficult. A weed is a plant considered undesirable in a particular situation; the term has no botanical significance. Often, weeds are simply those native plants that are adapted to grow in
disturbed ground, the disturbance caused by ploughing and cultivation favouring them over other species. Any plant is a weed if it appears in a location where it is unwanted;
Bermuda grass makes a good lawn plant under hot dry conditions but become a bad weed when it out-competes cultivated plants. A different group of weeds consists of those that are invasive, introduced, often unintentionally, to habitats to which they are not native but in which they thrive. Without their original competitors, herbivores, and diseases, they may increase and become a serious nuisance. One such plant is
purple loosestrife, a native of Europe and Asia where it occurs in ditches,
wet meadows and marshes; introduced into North America, it has no natural enemies to keep it in check and has taken over vast tracts of wetlands to the exclusion of native species.
In forestry tree killed by
emerald ash borer beetles In forestry, pests may affect various parts of the tree, from its roots and trunk to the canopy far overhead. The accessibility of the part of the tree affected may make detection difficult, so that a pest problem may already be far advanced before it is first observed from the ground. The
larch sawfly and
spruce budworm are two insect pests prevalent in Alaska and aerial surveys can show which sections of forest are being defoliated in any given year so that appropriate remedial action can be taken. Some pests may not be present on the tree all year round, either because of their life cycle or because they rotate between different host species at different times of the year. In forestry, pests may affect various parts of the tree, from its roots and trunk to the canopy far overhead. The larvae of
wood-boring beetles, for example, are notorious for spending years excavating tunnels under the bark of trees, leading to significant structural damage. These larvae only emerge into the open for brief periods as adults, primarily to mate and disperse. The import and export of timber has inadvertently assisted some insect pests to establish themselves far from their country of origin. An insect may be of little importance in its native range, being kept under control by parasitoid wasps, predators, and the natural resistance of the host trees, but be a serious pest in a region into which it has been introduced. This is the case with the
emerald ash borer, an insect native to north-eastern Asia, which, since its arrival in North America, has killed millions of ash trees.
In buildings Animals able to live in the dry conditions found in buildings include many
arthropods such as beetles, cockroaches, moths, mites, and silverfish. Another group, including
termites,
woodworm,
longhorn beetles, and
wood ants cause structural damage to buildings and furniture. The natural habitat of these is the decaying parts of trees. The
deathwatch beetle infests the structural timbers of old buildings, mostly attacking
hardwood, especially
oak. The initial attack usually follows the entry of water into a building and the subsequent decay of damp timber.
Furniture beetles mainly attack the
sapwood of both hard and
soft wood, only attacking the
heartwood when it is modified by fungal decay. The presence of the beetles only becomes apparent when the larvae gnaw their way out, leaving small circular holes in the timber.
Carpet beetles and
clothes moths cause non-structural damage to property such as clothing and carpets. It is the larvae that are destructive, feeding on wool, hair, fur, feathers and down. The moth larvae live where they feed, but the beetle larvae may hide behind skirting boards or in other similar locations between meals. They may be introduced to the home in any product containing animal fibres including upholstered furniture; the moths are feeble fliers but the carpet beetles may also enter houses through open windows. Furniture beetles, carpet beetles and clothes moths are also capable of creating great damage to museum exhibits, zoological and botanical collections, and other cultural heritage items. Constant vigilance is required to prevent an attack, and newly acquired items, and those that have been out on loan, may need
quarantining before being added to the general collection. There are over four thousand species of cockroach worldwide, but only four species are commonly regarded as pests, having adapted to live permanently in buildings. Considered to be a sign of unsanitary conditions, they feed on almost anything, reproduce rapidly and are difficult to eradicate. They can passively transport pathogenic microbes on their body surfaces, particularly in environments such as hospitals, and are linked with allergic reactions in humans. s are important commercial pests of grain storage. Various insects attack dry food products, with
flour beetles, the
drugstore beetle, the
sawtoothed grain beetle and the
Indianmeal moth being found worldwide. The insects may be present in the warehouse or maybe introduced during shipping, in retail outlets, or in the home; they may enter packets through tiny cracks or may chew holes in the packaging. The longer a product is stored, the more likely it is to become contaminated, with the insects often originating from dry pet foods. Some mites, too, infest foodstuffs and other stored products. Each substance has its own specific mite, and they multiply with great rapidity. One of the most damaging is the
flour mite, which is found in grain and may become exceedingly abundant in poorly stored material. In time, predatory mites usually move in and control the flour mites. ==Countermeasures==