Environmental Impact Assessment (EIA) by b3JZgfan


									 BRE 211: Principles of
Agriculture and Forestry

       Lecture 4
Principles of Agricultural/
Crop Production
   They include:
       Land preparation
       Plant propagation
       Nursery establishment
       Transplanting seedlings
       Field Planting
       Fertilizer application
       Crop protection
       Harvesting
       Post harvest handling and storage
             Land Preparation
   Involves:
       Land Clearing
       Drainage
       Preparatory cultivation
   Land Clearing
           Existing vegetation is cleared.
           Intensity of clearing varying from clear
            felling to selective thinning depending on
            the crops to be grown.
           Intensively grown crops usually require
            total clearance of the vegetation.
 Techniques of Land Clearing
 Normally   two techniques including:
   Slash and Burn
   Clear Felling and Stumping

 Slash-and-burn    involves:
   Slashing  the herbaceous undergrowth
    followed by burning of the debris and
    standing shrubs and trees.
   In some instances the bigger trees are
    pruned or ring barked and fired to kill them
    in places where they endanger the growing
Advantages of Burning
 Releases   nutrients bound in the plant
  tissues in readily soluble ash and the
  planted crop may subsist on such
  nutrients immediately on germination.
 Acts as a disinfectant that destroys
  pests, disease causing organisms and
  weed seeds.
Disadvantages of Burning
   Destroys the potential organic matter in vegetation by
    releasing the volatile nutrients such as nitrogen and
   Some forest litter may resist burning thus result in
    poor land clearing.
   However,
       Burning is becoming less frequent on intensively farmed
        areas near homes and villages.
       Burning of clearings is still practiced in the establishment of
        tree crops.
 Clear felling and stumping:
 This is rare but under intensive or mechanized
  cropping it is done thoroughly in order to facilitate
  mechanical cultivation.
 After clearing, soil conservation and drainage works
   Causes of Poor drainage:
       Some soils have a water table permanently or
        seasonally high enough to adversely affect crop
        growth thus need drains to lower the water table.
       Physical condition of one or more horizons of the
        soil checks the downward movement of excess
        surface water thus impeding the drainage of
        excess surface water to a depth below the root
        range of crops.
       If the soil is wet and especially if it is high in clay
        content, a barrier impeding water percolation can
        also be formed by the smearing action of tractor
        wheel slip or of an implement which destroys
        structure and seals coarse pores and cracks.
Conditions for Drainage
   Where rainfall in abundant and well distributed
    throughout the year, it might be desirable to keep
    the water table below crop rooting depth at all
   Although it is necessary to improve drainage during
    part of the wet season in drier areas, it is
    undesirable to lower the water table more than is
    necessary since in such areas the objective is to
    conserve water.
   On peat soils, it is usually desirable to maintain the
    water table as high as crop requirements permit
    because draining peat too deeply enhances its
    tendency to dry, shrink and reach a high
    temperature during the day.
Conditions for Drainage
 Tree crops that need to develop a deep root
  system to anchor them firmly against strong
  winds will require a lower water table than
  some annual crops or surface-rooting
  perennials such as pineapple.
 Main effect of improving drainage is to
  improve the aeration of the soil enabling the
  crop to develop a deeper root system so that
  it can tap a larger volume of soil for nutrients
  and is better able to withstand periods of
Methods of improving drainage
   Sub-surface drains of tile, plastic pipe, rubble (bits
    of broken stone) and brushwood. Uncommon due
    to initial and maintenance cost
   Open surface drains: Made mechanically with
    scrapers, bulldozers, and drainage ploughs or
    dragline excavators. Cheaper in cost but take up
    land, inconveniences mechanized tillage
    operations, harbour obnoxious weeds and rodents
    requiring constant maintenance.
   Ridge and furrow system: Land is formed into
    broad ridges with a slight gradient on the furrows or
    ditches between them.
   Cambered bed system: This is a modification of
    the ridge and furrow system in which the land is
    raised into beds.

   Drains cannot help once the structure
    of the surface soil has been destroyed
   They only reduce the risk of such
    damage occurring by keeping the
    surface layer drier and thus prolonging
    periods during which it is not too wet
    for mechanized tillage.
        Preparatory Cultivation
   It is better to cultivate at the end of the rains in
    order to expose weed seeds, pests and pathogens
    to desiccation during the dry season.
   Extent and efficiency of preparatory cultivation
    depends on the type of soil and the cultivation
   Simple and light tools such as the hoe, panga,
    slasher, etc are associated with cultivation of soils
    in small-scale tropical agriculture.
        Human supplies the energy used in cultivation although
         farmers in the savanna also commonly use animal power.
   Tractors and their associated implements such as
    ploughs, harrows, and ridges are associated with
    highly developed agriculture of industrialized
    countries .
Effects of Cultivation
   Eliminates competition by weed species.
   Incorporates organic and inorganic manure.
   Improves the tilth or granular condition of the soil,
    which facilitates aeration, water percolation and easy
    rooting of seedlings.
   Helps destroy some pests and pathogens present in
    the soil by exposing them to the sun.
   Buries some weed seeds too deep for germination.
   However may turn up previously buried weed seeds
    which can dominate the re-growth after cultivation.
Plant Propagation
   Objectives:
       To increase the number of plants
       To preserve the useful characteristics of the
 Types:
 Sexual Propagation (Propagation by seed)
 Asexual Propagation (by use of specialized
  vegetative parts of the plants or by artificial
  techniques such as grafting, layering, cutting
  or budding)
Propagation by seed
 Commonest way of propagating self-
  pollinated and many cross-pollinated crops.
 Advantages:
       Seeds are usually not expensive
       Seeds can be stored for long periods
       Seeds can remain viable at least until the next
        planting season when stored in a cool dry place
       Seed does not usually carry over diseases and
        pests, which attack growing crops although some
        fungal spores infect seeds.
Propagation by seed
 Disadvantages:
   Development     of off-types and variation in
    plant populations when cross-breeds are
   Long juvenile period when plant is unable
    to produce seeds especially in tree crops
   Some seeds do not produce plants that
    resemble their parent plants
   Some seeds cannot produce yields during
    the first year e.g. Root and tuber crops.
Vegetative propagation
   Depends on the ability of plant parts to regenerate
    roots and shoots; and grow into new plants having
    the same characteristics as their parent plant.
   Advantages:
       Eliminates the problems of dormancy and reduces the
        juvenile period of plants
       Yields are obtainable easier and faster than seed
        propagated plants.
       Produces seedless crops e.g. banana easily through
       Maintains crossbred plants in heterozygous condition
   Disadvantage:
       Danger of transferring diseases to the new plants or
Methods of Vegetative
 Use of specialized vegetative parts or
  modified stems and roots such as bulb, corm
  runner, rhizome, suckers, tubers and root.
 Induction of adventitious roots and shoots by
  cutting or layering the stem e.g. in tea and
  sweet potato propagation.
 Grafting: Two plant parts are joined by
  regeneration of their tissues.
 Budding: Only the vegetative bud is joined to
  another plant where it regenerates a new
 Ratooning: Outgrowths from stools of a
  harvested crop are used.
Nursery Establishment
 A Nursery phase is an important part of the
  planting operation for most trees and some
  field crops.
 Seedlings can be better cared for and
  conveniently watered with less effort to
  increase growth and development.
 Characteristics of a good nursery site
       Should be near a source of water
       Should be as near as possible to the site of field
       Site should not be subject to planting other crops.
       Ground should be level or terraced.
Nursery Requirements
   Water:
       A small nursery can be watered using watering
        cans by family labour while a larger one may
        require more workers or the use of a pump and
        hose system.
   Polythene bags:
       Most widely used containers for nursery seedlings
        because they are cheap and durable and make
        handling of seedlings at planting very easy.
       size of bags used depends on the seedlings.
   Shade:
       Young seedlings do better under partial shade at
        the early stages. they will become self-shading
        when they get larger
Nursery Requirements
   Fertilizer:
       Usually nursery seedlings will grow faster if
        fertilized. The easiest way to fertilize seedlings is
        to dissolve a nitrogen fertilizer containing all the
        major nutrients in water and water lightly once
        every week.
       Fertilizer scorch can be avoided by immediately
        re-wetting with water alone.
       Sometimes micronutrient deficiencies will occur
        and these are best dealt with by watering with a
        complete/compound fertilizer dissolved in water
        and given periodically.
Transplanting seedlings
 Should be done during a sufficiently wet
 Before planting the seedlings, any weak,
  slow-growing, diseased, deformed or
  otherwise abnormal plants should be
  discarded as it affects the long-term yield of
  the field planting.
 Seedlings grown in Polythene bags are easy
  to transport to the field and lorry, tractor,
  trailer or hand may carry them depending on
  the situation.
Transplanting seedlings
 Use a box and double poles carried on the
  shoulders of two workers to carry seedlings
  over rough ground.
 Before planting the Polythene bags must be
  stripped off by slitting lengthwise with a razor
  blade or a very sharp knife.
 Although Polythene bags are widely used for
  nurseries some farmers may still prefer to
  plant seedlings directly in the soil.
 This often means that considerable damage
  is done to the seedling on extraction, thus
  delaying its development.
Field Planting
 Seed or planting material largely determines
  the quantity and quality of the harvested
 Good stocks of planting materials ensure
       Reduced costs of cleaning, standardization and
       Uniform germination thus eliminating replanting or
        supplying missing stands
       Vigorous seedling growth which reduces weed
        and disease damage
       Uniform growth rates, maturity and produce
Field Planting
   Low grade or poor-quality planting materials
    lead to:
       Uneven germination and establishment which may
        necessitate replanting or supplying missing stands
       Feeble seedling growth susceptible to disease and
        insect damage
       Costly disinfection or grading of planting materials
        against seed-borne diseases and pests
       Uneven growth rates due to lack of uniformity in
        genetic composition.
       Uneven maturity which affects the cost and
        efficiency of harvesting
       Lack of uniform produce due to a combination of
        all the above factors.
Field Planting
 This  affects not only the efficiency of
  primary processing but also the quality
  and market value of the products.
 Most farmers in the tropics still provide
  for their planting materials from their
  own harvests or purchase them from
  unregulated markets.
Methods of planting
   Broadcast:
       This is limited to crops that will be transplanted
        particularly small seed crops. It is unsuitable for
        large seeds and vegetative cuttings
   Drilling:
       This is planting seed in small furrows
   Dibbling:
       Holes are made and the seeds placed in these
        holes and covered. The crops could be planted in
        scattered like beans.
Sowing Practices
   The successful establishment of seedlings of annual
    crops depends upon
      Viable seed of adapted cultivar
      Uniform sowing & placement depth
      Firm seed-soil contact
      Availability of moisture and nutrients in the soil
   The factors involved in sowing management can be
    divided into two broad groups:
      Mechanical factors such as depth of planting,
       emergence habit, seed size, seedbed texture, and
       seed-soil contact.
      Biological factors such as companion crops (in
       mixed cropping and pastures) and competition for
Spacing and plant population
   Spacing crops optimally reduces interplant
    competition for sunlight, moisture, air and nutrients.
   The ultimate yield from a unit land area is contributed
    to by all the plants growing on it.
   Excessively wide or narrow spacing leads to
    reduction in yield.
   The exact spacing for any crop depends on:
       Soil productivity
       Location
       Time of planting.
 On fertile moist soils, closer spacing will give better
  results than on poor soils or soils susceptible to
  moisture stress.
 This explains the sustained high yields from close
  spacing and high plant populations in irrigated as
  compared to rainfed crops.
Fertilizer Application
 Fertilizers are chemical compounds
  containing the elements that are added
  to the soil to supplement its natural
 Fertilizers containing only one of the
  major elements (N, P, and K) are single,
  simple or straight fertilizers
 Those containing two or three elements
  are classified as mixed compound or
  complex fertilizers.
Fertilizer Application
 Nutrients in fertilizers must come within the
  feeding range of plants’ roots for maximum
  benefit from the application.
 The soluble constituents of fertilizers diffuse
  through the soil vertically and only slightly in a
  lateral direction.
 The method of application therefore must
  ensure distribution in a moist soil to reach the
  plant roots.
 Thus method of applying fertilizer is
Methods of Fertilizer Application
       Broadcast before planting: Fertilizer is spread as uniformly as
        possible over the field after ploughing land then mix it with
        soil by ploughs or cultivators.
          Gives good results with crops like millet which are planted in
           narrow rows.
          Suitable for those crops whose seeds are usually broadcast.
          However, it stimulates weeds.
       Row Placement: Fertilizer is placed in bands or in localized
        areas along rows at a calculated distance for maximum
        absorption by the plants.
       Advantages
          Fertilizer comes in contact with minimum amounts of soil
           particles reducing phosphorus fixation.
          Fertilizer is within reach of the roots and the plant can feed on it
          Fertilizer placed in bands does not supply nutrients to the weeds
           near the surface
Methods of Fertilizer Application
   Top-dressing: Second application by broadcasting
    on the soil surface close to the plants when the
    crop is 3-4 weeks old.
   Drill placement – Fertilizer is applied along with the
       Good for crops like wheat, maize and other cereals, which
        can withstand contact with the fertilizer.
   Band Placement: Fertilizer is placed in bands on
    one side or both sides of the row about 5cm below
    the seed and 4cm away from the seed or plant.
       The method is useful for cotton, tomato and potato crops
        that are sensitive to direct contact with fertilizer.
   Side-dressing: Second application when the crop is
    partly grown (4-8 weeks) given as a continuous
    band near the crop row to a depth of 4-5 cm.
Methods of Fertilizer Application
   Application by plough: Fertilizer is placed in
    a continuous band at the bottom of the
    plough furrow. Each band is covered as the
    succeeding furrows are turned over.
   Applying liquid fertilizer: Fertilizer is mixed
    with most fungicides and pesticides and
    applied simultaneously or dissolved in
    irrigation water and applied together in a
    process referred to as fatigation
       Used for high-value crops by direct spraying.
       Carbamide (urea) is the most commonly used in
        this way.
Types of Fertilizers
   Nitrogenous fertilizers
   Phosphatic fertilizers
   Potassium fertilizers
   Mixed or compound fertilizers.
   Farmyard manure
 Nitrogenous Fertilizers
 All major inorganic fertilizers in common use
  are synthetically produced.
 Many nitrogenous compounds are made from
  ammonia in an atmospheric nitrogen
  manufacturing process known as the harber
       Hydrogen is combined with nitrogen in the ratio
        3:1 by volume at high temperature (400-500 o C)
        and pressure (200-1000 atmosphere) in the
        presence of catalyst form ammonia.
   This ammonia is used directly as fertilizers or
    is converted into various nitrogen fertilizers.
Phosphatic fertilizers
 Areof three grades depending on their
   Water-soluble  phosphate
   Citrate-soluble phosphate

   Phosphates that are only soluble in strong
    mineral acids such as sulphuric acid and
    nitric acid.
 Thesolubility depends on the chemical
 composition or formula of the phosphate
 and the degree of fineness.
Potassium Fertilizers
 Manufactured from natural deposits of potassium
  salts found in various parts of the world.
 The crude potash minerals are dissolved in water
  and the various salts separated by fractional
 All are soluble in water and the potassium
  content is readily available to plants unlike the
  nitrogen fertilizers.
 Most potash fertilizers have no effect on soil pH.
 Most important potash fertilizer materials are:
       Muriate of potash
       Sulphate of potash
Mixed or compound fertilizers
   Contain at least two of the three major elements N, P,
   Advantages of compound fertilizers include:
       Can be applied by hand as well as a fertilizer drill since it is
        usually dry with fine and well-mixed granules.
       Is stable and does not cake up, form lumps or deteriorate in
        any way over time.
       Contains all the major plant nutrients in the right proportions.
       The ready-made mixture saves farmers the labour of mixing
       Save time and labour when applied in the calculated amount
        instead of using separate straight fertilizers.
   Disadvantages include:
   Cost slightly more than the total cost of three
    equivalents of NPK.
   May be unsuitable for many soils.
Farmyard Manure (FYM)
   FYM refers to all the refuse from farm animals.
   It is a by-product consisting of two components: solid and liquid
    in a ratio of 3:1.
       Solid part is made up of dung and straw that has been used
         for animal bedding while the liquid comes from the urine.
       Dung comes mostly from undigested material and the urine
         from the digested material that is absorbed by the animal
         and then excreted.
       More than 50% of the organic matter contained in dung is in
         the form lignin and protein similar to those contained in
       This material is quite resistant to further decay and therefore
         the nutrients present in this fraction of dung are liberated
         very slowly.
   Nutrients present in the urine are readily available either directly
    or after simple decomposition.
   Over half of nitrogen, almost all the phosphate and about two
    fifths of potash are found in the solid portion.
Farmyard Manure (FYM)
   Manure is essentially a nitrogen-potash fertilizer.
   Manure supplies nutrients required by plants.
   Because it originated from plants, FYM naturally
    contains all mineral constituents including trace
   FYM improves the physical properties of the soil by
    increasing humus content and consequently the
    water holding capacity of the soil.
   Carbonic acid helps to release minerals present in
   It releases these nutrients fastest when the soil
    provides warm moist conditions favourable for
    microbial decomposition.
Factors affecting Fertilizer Use
       Crop factors
          Certain crops need larger amounts of particular nutrients than
           others e.g. Legumes require large amounts of P whereas grains
           require proportionately more N.
          Crop variety: Recently developed varieties are more responsive
           to higher doses of fertilizer than traditional crop varieties.
       Soil Factor
          Soils differ in their potential for production.
          Large applications of fertilizer can be profitable on soils that have
           high potential but are low in fertility
       Climatic factor
          Soils in areas of low rainfall lose little by leaching thus nutrients
           level remains stable. If fertilizers are added, the limited amount of
           water available means that the plants are unable to respond.
          Soils of humid regions lose nutrients through leaching and
           weathering but their water supply is adequate for high crop
           production. Here, fertilizer application will show good results.
Factors affecting Fertilizer Use
   Economic factor
       Fertilizer use is increase by low prices and decreased by
        high prices.
       Crop prices have the opposite effect: High price for the
        crop will give a profitable return from large fertilizer
        application yield but follow a curve of diminishing returns
   Management factor
       Managers choose the input-output levels at which they will
        operate. Increased crop outputs usually require increased
        fertilizer inputs.
       Top yields depend on many factors including soil type,
        climate, cropping history, fertilizer history and soil
        amendments, tillage practices, weed control and timing of
        operations. Most of these are managerial factors.
Effects of Fertilizer Application on
 Use of mineral fertilizers boosts crop growth which
  harvested provides a considerable amount of residue
  adding the organic content of the soil.
 When well fertilized, cereals e.g. millet, maize and
  sorghum leave behind considerable organic residue
  in the form of roots, stumps and stalks
 When nitrogen is applied or a cereal-legume rotation
  is adopted, the organic residue undergoes rapid
  microbial decay to produce humus.
 Thus the use of fertilizers in conjunction with
  farmyard manure increases the efficiency of crop
Crop Protection
   Crops are protected against:
       Fire
       Pests
       Diseases
       Weeds
 Pests and diseases are among the most serious
  limiting factors to economically efficient crop
  production and utilization of natural resources in
  tropical agriculture.
 Crop losses through pests and diseases may
  sometimes be negligible but at other times total loss
  especially as a result of sporadic outbreaks of non-
  economic pests and diseases.
 Pests and disease-causing organisms or pathogens
  include rodents, bats, birds, insects, mites, molluscs,
  nematodes, weeds, parasitic plants, fungi bacteria,
  mycoplasma, viruses and sometimes humans.
 Some diseases are, however, caused by physical or
  soil factors.
Crop Pests
    Arise in two major ways.
     Natural occurrence: Sudden attack
       of crops by insect e.g. locust
       outbreaks in arid and semiarid
     Alteration of the ecosystem:
       People attempting to change the
       ecosystems for their own benefit
       creating conditions that favour the
       development of pests. Some of
       these human activities
Classification of Pests
   According to damage they
       Biting and chewing insects
       Piercing and Sucking insects
       Boring insects
Classification of Pests
    According to Severity of damage c
     Key or major pests: cause
      serious and persistent economic
      damage in an ecosystem in the
      absence of effective control
     Minor pests: Cause economic
      damage only under special
      circumstances in their local
Classification of Pests
   Numbers of organisms involved
   Frequency of occurrence
       Occasional pests
       Potential pests
       Migrant pests
        Fundamental Principles of
            Crop Protection
   Control of pests and plant diseases means the
    reduction in the amount of damage caused.
   Perfect control is rare, but there is economic control
    when the increase in yield more than covers the cost
    of chemicals, materials and labour used for the
    control operations.
   The fundamental principles of control include.
   Exclusion
       Preventing entrance and establishment of pests in farms,
        states or countries.
       Involves using certified seeds or plants, discarding any that
        are doubtful, possibly treating seeds or tubers before they
        are planted.
       For states and countries, exclusion also includes quarantine
        prohibition by law.
Fundamental Principles of
Crop Protection
   Eradication
       Eliminating pest once already established on a
        plant or in a farm through.
            Removal of the diseased specimens or cutting off
             cankered tree branches eradication could be aided by
             control viral diseases
            Cultivation and deep ploughing to bury plant debris
            Rotation of susceptible with non-susceptible crops in an
             attempt to starve out the pest
            Disinfection with chemicals
            Heat treatment
            Spraying or dusting foliage with pesticides
            Treating soil with appropriate chemicals to kill insects,
             nematodes and fungi
            Trapping rodents
Fundamental Principles of
Crop Protection
    Protection
      Placing a protective barrier between the
       susceptible part of the host and the pest.
      In most instances this is a protective spray or
       dust applied to the plant in advance of the arrival
       of the pest.
      Sometimes it means killing insects or other
       inoculating agents,
      Storage of food surpluses, preservation by
       freezing, canning, salting etc. can also protect
       the food items from pest damage.
Fundamental Principles of Crop
   Immunization
      Control by the development of resistant varieties
       or by inoculating the plant with something which
       will inactivate the pest or pathogen.
   Avoidance
      Growing crops during period when the pest
       population is low or absent in the field.
      Crop is planted to avoid the damaging pest
      The use of early and rapidly maturing varieties can
       also have profound effect on the degree of pest
       damage experienced by the crop
   Insects and other arthropods are the most serious pests of crop
    plants in tropical agriculture.
Pest Control
   Objective
       To reduce the population of the offending pests below
        the economic threshold when its damage becomes
        uneconomical i.e. does not cause losses in yield.
   A pest control method will therefore be
    considered successful if it can maintain the pest
    population well below the economic threshold.
   Complete eradication of a pest from an
    ecosystem is not readily practicable or even
   Basic principles of pest control include
       Preventing pests from gaining access to the host/ Pest-
        host interaction prevention.
       Killing the pest directly reduces the population of the
        pest on the host.
Methods of Pest control
   Include:
       Physical control
       Legislative control
       Cultural control
       Biological control
       Chemical control
       Integrated pest management (IPM)
Physical control
       Includes the use of various barriers to prevent pests from
        physical contact with their hosts and mechanical removal or
        destruction of the pest.
       Barriers may be mechanical, chemical or behavioral.
       Mechanical barriers include:
          Wire fences
          Mosquito nets
          Fine nylon net sleeves
          Nylon or paper bags
          Sticky bands
          Boots for waders
          Concrete foundation - prevent termites.
          Hand picking
          Flooding
          Lethal temperatures
          Hermetic storage – Tightly closed bins
          Radiation
Physical control
       Chemical barriers include:
       Prophylactic chemical treatment carried out
        on crops leaving a residual poison that is
        lethal to the pest.
       Behavioral barriers exploit the fact that
        pests locate their hosts by responding to
        external stimuli such as sight and oduors.
       They include
         Frightening devices.
         Traps
         Attractants
         Anti-feedants – inhibit feeding of pests
Legislative Control
       Is the use of laws and regulation to prevent
        the importation of pest organisms into a
        country and to restrict the spread of pests
        from areas where they are already
       Main objective is to prevent dangerous
        pests from colonizing new areas.
       Include:
         Quarantine: Restricts movement of produce
          from areas of infection to other areas
         Eradication regulations
         Certification regulation
Cultural Control
       Is the manipulation of regular agronomic practices to
        influence on the incidence and populations of crop pests.
       The basic principle of cultural control is the disruption of the
        development and life cycles of pests either by denying them
        their food or by exposing stages in then life cycle to adverse
        conditions so that they are killed.
       Advantages:
          Relatively cheap and effective.
          Poses minimal danger to the environment.
       Cultural control involves:
          Cultivation of the soil
          Variation in planting and harvesting dates
          Crop rotation
          Close season
          Trap cropping
          Resistant crop varieties
          Mixed cropping
          Good husbandry practices
Biological Control
 Is deliberate use of organisms (parasites,
  predators and pathogens) to reduce
  populations of pests.
 Such natural enemies may be arthropods
  (insects and mites), bacterial protozoan,
  fungi, viruses, nematodes or even vertebrates
  (birds, toads, fish).
 Method is usually used as a supplement to
  other methods of control.
 Successful biological control requires that:
       Pest population is reduced to levels well below the
        economic threshold.
       Population is maintained sufficiently low to allow
        the survival of the biological control agent.
Biological Control
 Biologicalcontrol requires thorough
  knowledge of the ecosystem, the
  ecology and behaviour of the target pest
  and the bio-control agent.
 Advantages:
   Itis safe and cost-effective
   Is devoid of environmental pollution
    problems associated with chemical control.
 Caremust be exercised, however, not
 to upset the ecosystem of the area by
 the manipulation of controlling species.
Chemical control
   Most common and easily applicable method for
    reducing or preventing economic pest damage is the
    use of toxic substances or pesticides to kill or repel
    pests on their host crops.
   Continues to play a significant role in solving the food
    and wealth problems of tropical countries.
   Advantages:
       Relatively easy method of pest control
       Produces quick and easy results
       Can be repeated as often as desirable
       Is cheap and individual farmers can take independent action
        on their own farms
       The broad-spectrum action of many pesticides makes it
        possible to control a complex of pests with one or a
        combination of pesticides.
Chemical control
   Disadvantages
       Is repetitive and must be applied whenever there is a pest
        outbreak. Thus it is wasteful
       Pesticide applied rarely kills all the pests and the residual
        population which survives soon develops to cause economic
       Pesticides can be toxic to beneficial insects especially
        parasites, predators and pollinators. They are potentially
        toxic to wildlife, fish and humans
       Cause environmental pollution and ecological disturbance.
        Toxic residues may remain in agricultural produce.
       Pests may develop resistance to a pesticide which reduces
        the effect of that pesticide on that pest
       Chemical control provides only a temporary solution to pest
       Pesticides are expensive to manufacture and usually have to
        be imported by tropical countries.

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