Pest (organism)

A pest is any organism that humans consider harmful to themselves, their possessions, or their environment.

This term is often applied to creatures that damage crops, livestock, or forestry. It also describes animals that cause a nuisance in human homes. Because humans modify the environment for their own objectives, we often become intolerant of other species occupying the same space. For example, an elephant is not a pest in its natural habitat. However, it becomes a pest when it enters a field and tramples crops. In a broad sense, a pest can be viewed as a competitor to humanity.

In agriculture and horticulture, pests cause significant economic damage.

Pests and diseases together can cause up to 40% of annual yield losses. The most economically important groups of agricultural pests are insects, mites, nematodes, and gastropod molluscs. Insects cause damage in two primary ways. First, they cause direct injury by feeding on plant tissues. This can reduce the leaf surface available for photosynthesis or cause the wilting of shoots. Second, they cause indirect damage by transmitting infections. Insects can carry fungal, bacterial, or viral pathogens into a plant.

Insects have different ways of attacking plants depending on their life stage and type. Many insects are polyphagous, meaning they can eat many different types of plants. Others are restricted to one specific crop. Often, the larva is the stage that does the most feeding. For instance, sawfly and lepidopteran larvae usually feed on the aerial parts of a plant. In contrast, beetle larvae might live underground to feed on roots. Some insects, known as Hemiptera or true bugs, use piercing and sucking mouthparts. These insects, such as aphids and whiteflies, suck sap from the plant and can cause the growth of sooty mould. This mould blocks light and further reduces photosynthesis.

Mites are another major group of pests that can devastate crops.

Spider mites are especially troublesome because they can be extremely numerous. They thrive in hot, dry conditions and live on the underside of leaves. They puncture plant cells to feed, and some species even produce webbing. Gall mites are also significant pests. They can feed on roots or aerial parts and transmit various viruses. Examples include the big bud mite, which transmits the reversion virus of blackcurrants, and the coconut mite, which can devastate coconut production.

Some animals become pests because of how they interact with human structures or urban settings.

Pigeons in cities can become a health hazard, while gulls near coasts can be a nuisance to people. Birds also pose a serious risk at airfields if they are sucked into aircraft engines. In buildings, woodpeckers may excavate holes in fencing or utility poles. Even jellyfish can be pests when they form vast swarms. These swarms can damage fishing gear or clog the cooling systems of desalination plants and power plants.

Plants can also be classified as pests, especially if they are invasive species or weeds.

There is no single universal definition for a pest plant. In Western Australia, authorities can label a plant as a pest if it is likely to affect the environment, property value, or the comfort of inhabitants. An example is the caltrop, which has sharp, spiny burrs that are a nuisance near roadsides and recreation areas. This plant can also cause poisoning in sheep and goats.

Climate change is currently altering the ranges of many pests.

Pest ranges are heavily determined by climate, especially rainfall. While drought can weaken a plant's resistance, it can also slow down the spread of infections. More recently, climate change has been pushing many pest ranges toward the North and South Poles. Between 1960 and 2013, many species shifted poleward. However, viruses and nematodes have shown the opposite trend by moving toward the equator. This might be because they do not disperse through the air and instead move along with human activity.

Plants have evolved complex strategies to defend themselves against these attacks. They may use physical barriers like thorns, which are modified stems, or spines, which are modified leaves. They also use a thick cuticle or waxy deposits to protect their surfaces. Some plants even use chemical defenses, such as toxic or distasteful secondary metabolites. Some species, like the Acacia, use a more active strategy. They provide housing and food, such as nectar or food bodies, to ants. In exchange, the ants act as a defense force by protecting the plant from herbivores.