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
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.
0 Comments