Definitions and Importance crop pests


Definitions and Importance  crop pests

Before any attempt is made to control an insect or other organism, it should first be established that it is a pest and that it would be profitable to attempt control.

In this article, you will be acquainted with the fundamentals of crop pests and the general methods for their management and control.

This post, you should be able to define and describe crop pests, discuss the importance of Crop pest, Categorize crop pests, Name the different types of crop pests and discuss the damages caused by insect pests.


Definitions of crop pest

Pests are usually defined in terms of the degree and importance of crop damage or loss. Most often, the definition of a pest depends on individuals and the prevailing conditions.

Examples of the many definitions of a pest include the following:

i) A pest is any animal or plant which harms or causes damage to man, his animals, crops or possessions, or even just causes him annoyance.

ii) A pest is any organism detrimental to man, whether it is an insect, disease organism, weed, rodent, or other.

iii) A pest is any form of plan or animal or pathological agent injurious or potentially injurious to plant or plant products, livestock or man.

iv) An organism is a pest when the level of damage it causes is sufficient to warrant control measures.


Importance of Crop pest

Definitions and Importance  crop pests

Crop productivity in most of Africa is generally low. This is due to losses from pests and diseases. 

Therefore, the reduction of losses due to pests and diseases is an important element in increasing the efficiency of crop production. 

These losses occur from planting of the seeds through field phases of production to storage and processing.


Conditions which promote pests

a) Favorable climatic conditions: The most common way in which organisms attain pest status is simply by an increase in number. Seasonal increases in pest numbers are usually controlled by climatic conditions and biological pressures.

These climatic conditions include temperature, humidity, rainfall and sunlight. Aphis gossipy (the cotton aphid) outbreaks commonly occur on young plants in spells of dry weather, but clear up rapidly with the onset of the rains.

In Great Britain, outbreak of certain aphids can be expected in years following a mild winter. The reverse is the case after a very severe winter.

b) Biological change: When the environmental conditions are favourable, an ecological change can covert a harmless organism into a pest.

The major ecological reasons for an organism developing pest status include:

i) Change in cultural practices: E.g. monocultures represent a concentration of plants of the same species over a wide area and this is beneficial to the insect or organism which will thereby have little difficulty in finding its host plant.

ii) Change in the character of the food supply: Plants grown for agriculture have normally been selected for their nutritive value and therefore more attractive to pests than their wild relatives. E.g. Sorghum and maize are more attractive to stem borers than are wild grasses.

iii) Introduction to new environments: Insects and other organisms become established as pests when taken to countries where they did not previously exist.

In the new country, the natural enemies (parasites and predators) and competitors for food are often absent, hence allowing the population of the new pests to increase dramatically

e.g. icerya purachasi mask. (Cotton cushion scale) is a native of Australia but was introduced into California in 1868.

By 1887, it has become a serious pest of citrus in its new environment. Most storage pests exist in small population in the field but increase economically in numbers and become serious pests in the favorable climatic and abundant food of a grain store.

E.g Sitophilu oryzae (L) on maize cob, Stotroga cerealella (oliv.)  on sorghum and the bruchids on cowpea.

iv) Climate in host/natural enemy relationships: The application of pesticides on a large scale in agricultural operations generally affects natural enemies more than the pests.

E.g Ascotis selenaria (the giant cooper) is normally a minor pest of robusta coffee in Uganda. It became a serious pest of Arabica coffee in Kenya after very frequent use of parathion in coffee plantation.

v) Loss of competing species: Under monoculture conditions, there are fewer insect species than under natural conditions and many species now become pests which were not pests under natural conditions.

vi) Economic change: A pest may arise purely for economic reasons because of a change in the value of a crop. Damage that is not serious when prices are low can be very important when prices are high. If the crop is in short supply, consumers overlook a little damage.

Generally, organisms which cause significant economic loss in quantity and/or quality of crops and plant products are widely recognized as pests and disease organisms.


Categories of crop pests

Pests are categorized according to several factors, including their abundance, damage caused, etc. we have already seen that the number of organisms causing damage or loss is considered to be of great importance in determining which organisms are pests. Very often, the degree of seriousness of damage is related to numbers.

However, there are exceptions, e.g. disease – transmitting organisms, which the effect of organism on crop is not directly proportional to numbers or in special quality products where a slight contamination may lead to serious financial loss.

In spite of this, the concept of economic threshold based on the population levels of organism, or level of incidence of a disease, is still the most acceptable in categorizing pests.

Economic threshold is the population density at which control measures should be applied to prevent an increasing pest population from reaching economic injury level or the population level of the organisms or level of disease incidence above which economically significant damage or loss is caused, and below which damage or loss is negligible or the population level above which it will pay the farmer to control his pests and below which it is uneconomical.

The concept of economic threshold is based on the fact that organisms over a long period of time and in a relatively undisturbed environment reach a state of equilibrium with their environment. 

This is a dynamic state of equilibrium, which means that although population densities vary from season to season, year to year or place to place, for a particular place, there is an average population level which is reasonably stable over a long period of time.

The economic threshold and economic injury level (the lowest population density that will cause economic damage or injury that will justify the cost of artificial control measures) are usually above this average population level. 

These levels are not constant for any pest, disease or environment, but they can be worked out from an intimate knowledge of the organisms, the crops which they are attacking and other components of the environment.

Economic damage is the amount of injury which will justify the cost of artificial control measures. 

On the basis of the concept of economic threshold and depending on the severity of damage caused, the number of organisms involved, frequency of occurrence and the prevailing circumstances, pests are categorized as follows:

1) Key pests (major pests, regular pests): These are perennial pests which cause serious and persistent economic damage in the absence of effective control measures. The population of the damaging stage stamp above economic injury level.

Examples: The variegated grasshopper Zonocerus variegates is a key pest of cassava, vegetables, citrus and many cultivated crops in West Africa. Maruca testulalis, the cowpea borer is a major pest of cowpea. Dydercus volkerii, the cotton stainer on cotton.

Some major pests cause economic damage at low populations and are therefore called low – density pests, e.g. cocoa mirids.

Other pests like locusts and grasshoppers usually occur in very dense populations and are therefore described as high – density pests. Key pests are the main target of pest control operations.

2) Minor pests:  Some organisms cause economic damage only under certain circumstances in their local environment. Under normal conditions, their populations are low and the damage they cause is insignificant.

Examples: The cocoa – pod husk minor Marmara sp. is a minor pest of cocoa in Nigeria and Ghana. Minor pests are usually not the focus of pest control operations.

3) Occasional pests: Populations of occasional pests are normally below the economic threshold level, occasionally rise above it.

Examples: Many lepidopterous defoliators and stem borers occur at irregular intervals and cause economic damage to crops.

4) Potential pests: Potential pests are those species whose population level are usually far below the economic threshold but can become highly injurious under changed cultural practices or as an introduced pest.

Example: The giant looper (Ascotis selenari reaprocaria (wlk) became a major pest of coffee estates in Kenya following indiscriminate and uncontrolled use of pest pesticides in the agro – ecosystem.

5) Migrant pests:  These moves from one area to cause damage to crops in another area. They are a special group of key pests which are classified as migrant pests. Their control normally involves international cooperation between the members countries affected.

Examples: The African migratory locust is jointly tackled by the West Africans which form the OICMA organization with headquarters in Bamako, Mali. Army worms (Spodoptera spp.) are jointly monitored by the West Africa Armyworm Forecasting Programme involving many East African countries. The village weaver birds, Queen sp. are also migrant pests.


General methods for the prevention and control of crop pests

1. Prevention: The best way of controlling pests is to prevent their attack. To prevent new insect pests from spreading, all animals which harbour the pests must be properly treated.

2. Chemical methods: These involve the use of chemicals to kill the pest. This method is the most effective of all the methods. The chemicals are generally called pesticides and include insecticides (for the control of insect pests), rodenticides (control of rodents), herbicides (control of weds). They may be applied inform of powder or dust, smoke of spray or may even be used as stomach or contact poison. Examples of pesticides are Gammalin 20, Alfrex – T, Aldrin, Ventox 25, Phostoxin, Parathion, dual, Pimextra etc.

3. Physical: This involves use of scare crows, hand picking, use of trap, fencing and burning of debris.

4. Cultural method: This is the use of good cultural practices to enable crops escape the attack of pests. They are ploughing, proper tillage and disposal of refuse, regular weeding, planting of resistant varieties of crops, early planting and harvesting and practice of crop rotation.

5) Biological method: This involves the use of natural predators and parasites of the pests. Example is the use of snakes and frogs on insects, use of African marigaid for the control of nematodes, use of cats to control rodents in stores and use of tiny wasp to control bugs on cassava.


Conclusion on Definitions and  Importance of crop pests

Pest is an all-encompassing word that includes insects, fungi, bacteria, viruses, phytoplasmas, nematodes, mollusc, vertabrates, weeds and parasitic flowering plant (striga). 

All organisms causing damages to crops can be regarded as potential pests, but it is usual to use the term strictly for organisms causing significant damage in quantity and or quality of crops and plant produce.

In this post, we learnt about the different types of crop pests. We learnt about their definitions, categories, importance and conditions which promote their activities. The damages caused by pests and control methods were also discussed.

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