Question: What is the number one pest that affects wheat crops?

Pest (organism)




A pest is an animal or plant detrimental to humans or human concerns including crops, livestock, and forestry. The term is also used of organisms that cause a nuisance, such as in the home. An older usage is of a deadly epidemic disease, specifically plague. In its broadest sense, a pest is a competitor of humanity.[1][2]

Concept
A pest is any living organism, whether animal, plant or fungus, which is invasive or troublesome to plants or animals, human or human concerns, livestock, or human structures. It is a loose concept, as an organism can be a pest in one setting but beneficial, domesticated or acceptable in another.

Animals are called pests when they cause damage to agriculture by feeding on crops or parasitising livestock, such as codling moth on apples, or boll weevil on cotton. An animal could also be a pest when it causes damage to a wild ecosystem or carries germs within human habitats. Examples of these include those organisms which vector human disease, such as rats and fleas which carry the plague disease, mosquitoes which vector malaria, and ticks which carry Lyme disease.
A species can be a pest in one setting but beneficial or domesticated in another (for example, European rabbits introduced to Australia caused ecological damage beyond the scale they inflicted in their natural habitat). Many weeds are also seen as useful under certain conditions, for instance Patterson's curse is often valued as food for honeybees and as a wildflower, even though it can poison livestock.
The term "plant pest" 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.[3] Plants may be considered pests themselves if an invasive species.
The animal groups of greatest importance as pests (in order of economic importance) are insects, mites, nematodes and gastropods.[4]  
Plant pests can be classed as monophagous,  oligophagous, and polyphagous according to how many hosts they have. Alternatively, they can be divided by feeding type, whether biting and chewing; piercing and sucking; or Lapping and chewing. Another approach is to class them by population presence as * key pests,  occasional pests, and potential pests. In terms of population biology, there are population growth rate (r) pests;  carrying capacity (k) pests; and  r-k pests.

Animals

Vertebrates
Birds
Pigeons and seagulls eat human food and carry disease
Many birds, such as crows, eat crops
Canada goose; widely regarded as pests in the United States (see Canada goose#Relationship with humans), and in New Zealand (see Canada geese in New Zealand).
Woodpeckers peck at rooftops and also nest in them. They cause structural damage to houses.
The common myna was declared by IUCN Species Survival Commission as one of the world's most invasive species and one of only three birds in the top 100 species that pose an impact to biodiversity, agriculture and human interests.[5] In particular, the species poses a serious threat to the ecosystems of Australia where it was named "The Most Important Pest/Problem".[6]
Amphibians
Cane toads have had serious negative effects on many ecosystems to which they have been introduced, especially in Australia. The toad's skin is toxic, killing many wild and domestic animals that attempt to eat it.
Mammals
Mice, rats, and other small rodents damage crops and stored produce
Rabbits decimate native plant populations in Australia, where they are an introduced species.
Foxes, opossums, raccoons, and bears eat waste
Beavers destroy trees
Gophers, groundhogs, and moles destroy lawns
Feral cats and feral dogs eat human food and carry disease
White-tailed deer are now seen as pests in many suburban and exurban areas of the US, though not in more rural areas.
Coyotes, wolves and lions prey upon livestock
Vampire bats drink blood of livestock[7]
Eastern grey squirrels are seen as pests in Britain and Ireland because of the decline of red squirrel populations
Wild boars damage crops, spread disease, and prey upon livestock
Invertebrates

Insects and arachnids
Agricultural and domestic arthropods


Examples of agricultural and horticultural insect pests include: 
aphids and other Hemipteran pests,
thrips,
Lepidopteran, Dipteran, and Coleopteran larvae,
spider mites,
locusts and crickets.
Ants, cockroaches, flies and wasps
Termites, woodworm and wood ants cause structural damage
Bookworms, silverfish, carpet beetles and clothes moths cause non-structural damage
Tree and forest pests

Gypsy moths attack hardwood trees (see Gypsy moths in the United States)
Ectoparasites
Chiggers cause skin irritation
Sarcoptes scabiei causes scabies
Ticks and mites cause irritation and can spread disease
Lice, fleas and bed bugs can all cause skin irritation
Mosquitoes, tsetse flies and kissing bugs cause irritation and carry disease
Nematodes
Root-knot nematode
Soybean cyst nematode
Potato cyst nematode
Gastropod molluscs
These include slugs and land snail pests:
Some slugs are pests in agriculture and gardens.[4] Deroceras reticulatum is a worldwide slug pest.[4] Local importance slug pests include: Deroceras spp.,[4] Milax spp.,[4] Tandonia sp.,[4] Limax spp.,[4] Arion spp.[4]  and some species of Veronicellidae:[4] Veronicella sloanei.[8]
Helix aspersa damages citrus fruits in California,[4]
Cernuella virgata, Theba pisana and Cochlicella spp. decrease quality of grains when harvested with the product in South Australia.[4]
Achatina fulica damages vegetables and ornamental plants in the Pacific region.
Succinea costaricana damages ornamental plants in Costa Rica.[9]
Ovachlamys fulgens damages ornamental plants and orchids in Costa Rica.[10]
Other pest species include Amphibulima patula dominicensis,[8] Zachrysia provisoria[8] and Bradybaena similaris.[8]
Plants and plant diseases

Weeds

Plant diseases

See also
Home stored product entomology
Invasive species
List of common household pests
List of parasites of humans
Nuisance wildlife management
Pest control
Pesticide
Pesticide application
Synanthrope
Urban wildlife
Vermin


Further reading
Burch, John B.. 1960. Some snails and slugs of quarantine significance to the United States. U.S. Dept. Agriculture, Agricultural Research Service 82(1): 73 pp
Hockings, F.D, 2014,"", CSIRO Publishing, Melbourne, ISBN9781486300211

 Media related to Pest insects at Wikimedia Commons
 Media related to Weeds (plants) at Wikimedia Commons
Pests travel guide from Wikivoyage
 - arthropods, nematodes and plant diseases affecting humans, livestock/pets, agricultural and ornamental plants.