Carpet beetle larvae damaging a specimen of Sceliphron destillatorius in an entomological collection
A pest is any animal or plant detrimental to humans or human
concerns. The term is particularly used for creatures that damage crops,
livestock and forestry, or cause a nuisance to people, especially in
their homes. Humans have modified the environment for their own purposes
and are intolerant of other creatures occupying the same space when
their activities impact adversely on human objectives. Thus, an elephant
is unobjectionable in its natural habitat but a pest if it tramples
crops.
Some animals are disliked because they bite or sting; snakes, wasps, ants, bed bugs, fleas and ticks
belong in this category. Others enter the home thus invading our own
private space; these include houseflies, which land on and contaminate
food, beetles which tunnel into the woodwork, and other animals that
scuttle about on the floor at night, like cockroaches, rats and mice, which are often associated with insanitary conditions.
Agricultural and horticultural crops are attacked by a wide variety of pests, the most important being insects, mites, nematodes and gastropod molluscs.
The damage they do results both from the direct injury they cause to
the plants, and from the indirect consequences of the fungal, bacterial
or viral infections they transmit. Plants have their own defences
against these attacks but these may be overwhelmed, especially in
habitats where the plants are already stressed, or where the pests have
been accidentally introduced and may have no natural enemies. The pests
affecting trees are predominantly insects, and many of these have also
been introduced inadvertently and lack natural enemies, and some have
transmitted novel fungal diseases with devastating results. Humans have
traditionally attempted to control the agricultural and forestry pests
by the use of pesticides;
however, they have gradually come to appreciate that many of these have
unwanted consequences for the environment, and have tried to substitute
integrated pest management strategies with biological pest controls.
Concept
147.jpg)
Pests, such as these termites, often occur in high densities, making the damage they do even more detrimental.
A pest is any living thing, whether animal, plant or fungus, which
humans consider troublesome to themselves, their possessions or the
environment. It is a loose concept, as an organism can be a pest in one setting but beneficial, domesticated or acceptable in another. Microorganisms, whether bacteria, microscopic fungi, protists, or viruses that cause trouble, on the other hand, are generally thought of as causes of disease (pathogens) rather than as pests. An older usage of the word "pest" is of a deadly epidemic disease, specifically plague. In its broadest sense, a pest is a competitor to humanity.
Animals as pests
Feral pigeons can become very numerous in cities.
Animals are considered pests or vermin
when they injure people or damage crops, forestry or buildings.
Elephants are regarded as pests by the farmers whose crops they raid and
trample. Mosquitoes and ticks are vectors
that can transmit diseases, but are also pests because of the distress
caused by their bites. Grasshoppers are usually solitary herbivores of
little economic importance until the conditions are met for them to
enter a swarming phase, become locusts and cause enormous damage.
Many people appreciate birds in the countryside and their gardens, but
when these accumulate in large masses, they can be a nuisance. Flocks of
starlings
can consist of hundreds of thousands of individual birds, their roosts
can be noisy and their droppings voluminous; the droppings are acidic
and can cause corrosion of metals, stonework and brickwork as well as
being unsightly. Pigeons in urban settings may be a health hazard, and gulls
near the coast can become a nuisance, especially if they become bold
enough to snatch food from passers-by. All birds are a risk at airfields
where they can be sucked into aircraft engines. Woodpeckers sometimes excavate holes in buildings, fencing and utility poles, causing structural damage;
they also drum on various reverberatory structures on buildings such as
gutters, down-spouts, chimneys, vents and aluminium sheeting. Jellyfish
can form vast swarms which may be responsible for damage to fishing
gear, and sometimes clog the cooling systems of power and desalination
plants which draw their water from the sea.
Many of the animals that we regard as pests live in our homes.
Before humans built dwellings, these creatures lived in the wider
environment, but co-evolved with humans, adapting to the warm, sheltered
conditions that a house provides, the wooden timbers, the furnishings,
the food supplies and the rubbish dumps. Many no longer exist as
free-living organisms in the outside world, and can therefore be
considered to be domesticated. The St Kilda house mouse rapidly became extinct when the last islander left the island of St Kilda, Scotland in 1930, but the St Kilda field mouse survived.
Plants as pests
Caltrop, Tribulus terrestris, is sometimes considered a pest plant because of its sharp spiny burrs, shown here in a person's foot.
Plants may be considered pests, for example if they are invasive species.
There is no universal definition of what makes a plant a pest. Some
governments, such as that of Western Australia, permit their authorities
to prescribe as a pest plant "any plant that, in the local government
authority's opinion, is likely to adversely affect the environment of
the district, the value of property in the district, or the health,
comfort or convenience of the district’s inhabitants." An example of such a plant prescribed under this regulation is caltrop, Tribulus terrestris,
which can cause poisoning in sheep and goats, but is mainly a nuisance
around buildings, roadsides and recreation areas because of its
uncomfortably sharp spiny burrs.
Ecology
The term "plant pest", mainly applied to insect micropredators of plants, has a specific definition in terms of the International Plant Protection Convention
and phytosanitary measures worldwide. A pest is any species, strain or
biotype of plant, animal, or pathogenic agent injurious to plants or
plant products.
Plant defences against pests
The large and directly defensive thorn-like stipules of Vachellia collinsii are hollow, offering shelter for ants, which further protect the plant against herbivores.
Plants have developed strategies that they use in their own defence,
be they thorns (modified stems) or spines (modified leaves), stings, a
thick cuticle or waxy deposits, with a second line of defence being
toxic or distasteful secondary metabolites.
Mechanical injury to the plant tissues allows entry of pathogens and
stimulates the plant to mobilise its chemical defences. The plant soon
seals off the wound to reduce further damage.
Plants sometimes take active steps to reduce herbivory. Macaranga triloba for example has adapted its thin-walled stems to create ideal housing for an ant Crematogaster
spp., which, in turn, protects the plant from herbivores. In addition
to providing housing, the plant also provides the ant with its exclusive
food source in the form of food bodies located on the leaf stipules. Similarly, several Acacia
tree species have developed stout spines that are swollen at the base,
forming a hollow structure that provides housing for ants which protect
the plant. These Acacia trees also produce nectar in nectaries on their leaves as food for the ants.
Economic impact
In agriculture and horticulture
Caterpillars such as those of the cotton bollworm moth Helicoverpa armigera can devastate crops.
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' tunnelling 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.
The mites that cause most trouble in the field are the spider mites.
These are less than 1 mm (0.04 in) 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,[23] 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.
The potato cyst nematode can cause serious reductions in crop yield.
The nematodes (eelworms) that attack plants are minute, often too
small to be seen with the naked eye, but their presence is often
apparent in the galls or "knots" they form in plant tissues. Vast
numbers of nematodes are found in soil and attack roots, but others
affect stems, buds, leaves, flowers and fruits. High infestations cause
stunting, deformation and retardation of plant growth, and the nematodes
can transmit virus diseases from one plant to another. 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 differ from snails in having
shells that are too small for the animal to retract inside; the
shell-less state has evolved on multiple occasions during the
evolutionary development of molluscs, so there is little taxonomic
difference between the two, and both slugs and snails 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.
Terrestrial molluscs need moist environments; snails may be more
noticeable because their shells provide protection from dessication,
while most slugs live in soil and only come out to feed at night. They
devour seedlings, damage developing shoots and feed on salad crops and
cabbages, and some species tunnel into potatoes and other tubers.
Weeds
.jpg)
Alligator weed, 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
A green ash 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 year.
The larvae of wood-boring beetles may spend years excavating tunnels
under the bark of trees, and only emerge into the open for brief periods
as adults, 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
Termites can cause serious structural damage.
Animals able to live in the dry conditions found in buildings include many arthropods such as beetles, cockroaches, moths, and mites. 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 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.
Flour beetles 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
may be 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
Pest control in agriculture and horticulture
A row-crop sprayer applying pesticide to a young crop of maize
The control of pests in crops is as old as civilisation. The earliest
approach was mechanical, from ploughing to picking off insects by hand.
Early methods included the use of sulphur compounds, before 2500 BC in
Sumeria. In ancient China, insecticides derived from plants were in use
by 1200 BC to treat seeds and to fumigate plants. Chinese agronomy
recognised biological control by natural enemies of pests and the
varying of planting time to reduce pests before the first century AD.
The agricultural revolution in Europe saw the introduction of effective plant-based insecticides such as pyrethrum, derris, quassia, and tobacco extract. The phylloxera (a powdery mildew) damage to the wine industry in the 19th century resulted in the development of resistant varieties and grafting, and the accidental discovery of effective chemical pesticides, Bordeaux mixture (lime and copper sulphate) and Paris Green (an arsenic
compound), both very widely used. Biological control also became
established as an effective measure in the second half of the 19th
century, starting with the vedalia beetle against cottony cushion scale. All these methods have been refined and developed since their discovery.
Pest control in forestry
Forest pests inflict costly damage, but treating them is often
unaffordable, given the relatively low value of forest products compared
to agricultural crops. It is also generally impossible to eradicate
forest pests, given the difficulty of examining entire trees, and the
certainty that pesticides would damage many forest organisms other than
the intended pests. Forest integrated pest management
therefore aims to use a combination of prevention, cultural control
measures, and direct control (such as pesticide use). Cultural measures
include choosing appropriate species, keeping competing vegetation under
control, ensuring a suitable stocking density, and minimizing injury
and stress to trees.
Pest control in buildings
Pest control in buildings can be approached in several ways,
depending on the type of pest and the area affected. Methods include
improving sanitation and garbage control, modifying the habitat, and
using repellents, growth regulators, traps, baits and pesticides. For example, the pesticide Boron can be impregnated into the fibres of cellulose insulation to kill self-grooming insects such as ants and cockroaches.
Clothes moths can be controlled with airtight containers for storage,
periodic laundering of garments, trapping, freezing, heating and the use
of chemicals. Traditional mothballs deter adult moths with strong-smelling naphthalene; modern ones use volatile repellents such as 1,4-Dichlorobenzene. Moth larvae can be killed with insecticides such as permethrin or pyrethroids.
However, insecticides cannot safely be used in food storage areas;
alternative treatments include freezing foods for four days at 0 °F
(−18 °C) or baking for half an hour at 130 °F (54 °C) to kill any
insects present.
In mythology, religion, folklore and culture
Locust detail from a hunt mural in the grave-chamber of Horemhab, Ancient Egypt, 1422–1411 BC
Pests have attracted human attention from the birth of civilisation.
Plagues of locusts caused devastation in the ancient Middle East, and
were recorded in tombs in Ancient Egypt from as early as 2470 BC, and in
the Book of Exodus in the Bible, as taking place in Egypt around 1446 BC. Homer's Iliad mentions locusts taking to the wing to escape fire.
Given the impact of agricultural pests on human lives, people have
prayed for deliverance. For example, the 10th century Greek monk Tryphon of Constantinople
is said to have prayed "Snails, earwigs and all other creatures, hurt
not the vines, nor the land nor the fruit of the trees, nor the
vegetables ... but depart into the wild mountains."
The 11th-century Old English medical text Lacnunga contained charms and spells to ward off or treat pests such as wid smeogan wyrme, "penetrating worms",
in this case requiring a charm to be sung, accompanied by covering the
wound with spittle, pounded green centaury, and hot cow's urine.
The 20th century "prayer against pests" including the words "By Your
power may these injurious animals be driven off so that they will do no
harm to any one and will leave our fields and meadows unharmed" was
printed in the 1956 Rural Life Prayerbook.
