O Level Notes : Geography - Agriculture And Land Reform - Agriculture Pests and Diseases

Agricultural production in several countries and ecological regions has always been at stake due to the outbreak of pests and diseases, which reduces crops and animals quality and quantities.


  • Pests

More often, pests and diseases lead to crop failure and death of animals. In the long run, they lead to a decline in the agricultural yields thereby compromising food security and ushering several households into periods of starvation. 

The term pest refers to any animal or organism which is detrimental to both pests and deseases because their presence in the agricultural system often leads to some form of damage which reduces agricultural productivity. It is of paramount importance to note that there is a direct correlation between types of pests common in areas and their prevailing climatic conditions, for example, in Zimbabwe the common pest in Zambezi valley is tsetse fly which causes nagana in livestock and sleeping sickness in people. Tsetse fly is common in Zambezi area because it is characterised by relatively high temperatures. 

The Zambezi Valley tsetsefly affects agriculture

Cereal crops, leguminous and livestock are affected by different pests. The table below gives in brief the common pests affecting types of crops and animals


Pests affecting crops and animals





Description and effects on crops







It is a common pest in maize, larvae burrows maize stems and cobs

Place   trodden


g r a n u l e s   i n t o

stalk borer

maize funnel


Crop rotation


Spraying Rogor




Army worm

Army worm is a larvae which appears in large numbers.

It feeds on all plants like maize, wheat



Spray with dip terex or carbarly

and corn. They consume all   the leaf

parts leaving the mid rib



It burrows and feeds in dry grain. It deposits eggs within the grain where they fully develop into larvae and pupa

Treating grains

with dust pesticides


dust and Shumba dust





They transmit viruses when feeding. When a plant is highly affected it leads to stunted growth, reproductive failure  and leaf colouring


Spray using carbarly

and rogor


Quelea birds



The birds migrate in numbers. They mainly target wheat fields

Trapping birds with nets, scaring birds using metal haze or


using qualia box



Red spider mite

A spin fine web on leaves and plants mainly affects beans Suck cell sap among moulting of leaves


Spraying using dicofol such as Chirindamatura




Agricultural crops and animals are often affected by diseases which can be classified as follows: fungal, bacterial or viral. The table below shows the common agricultural diseases, their causes, effects and possible measures of combating them.

 Disease causes, effects and control




Cause and effect

on agriculture

Ways of controlling




Foot and mouth diseases



It is caused by a virus called picornavirus animals

Vaccinating animals before outbreak, Quarantine all suspected animals. Proper disposal or burning of dead animals, Control animal movement



Caused by a protozoa called trypanosoma


Use of barrel antecede



It is a bacterial discap which affects cow, sheep, pigs and goats spread through ingestion of contaminated pastures

Difficult to treat

Never open carcuses of dead animals. Burn completely or deeply bury carcuses Can use anti biotics, for example,

procane , penicillin, tetramycin and




Rodents and fungi

They often damage crops after harvest. They contaminate grains with urine and droppings

Killing rodents using manual traps, for example, rat traps. Use of poisoned baits, harvested grain should be stored in secure

store houses.

Pests control methods

To ensure high productivity in agricultural activities pests should be controlled. Farmers should understand that there are manual, biological and chemical ways which can be used to combat pests to obtain high yields of good quality per unit area.


Manual methods

When crops and animals have been affected by pests, farmers can combat these pests through manual methods, for example, wheat farmers in Birchenough bridge use traps to reduce quelea birds from damaging crops. Moreso, pests like mice and rats can be eliminated using rat traps.

The rat trap 

Farmers involved in livestock may control exogenous pests like ticks through the process of hand picking. Mostly affected crops and animals should be hacked off by the farmer to prevent further damage of the crops and animals. Hand picking is also useful in crop production. Some large pests like worms and beetles can be picked and thrashed, aphids are often rubbed on affected leaf resulting  in their death.

Pests can be combated through a manual method of shacking the plant. Shacking a plant is done to dislodge pests from the crop. Once shacked, the pest may fall on the ground where they are squashed or they fly away. This method is commonly used by fruit tree farmers.

Physical barriers are also designed to prevent the pests from reaching the crop. A common example of physical barrier used in Zimbabwe is the green house.

The Greenhouse

Other examples of physical barriers include weed free zones, row covers and fences.

Advantages of manual method of controlling pest

  • Manual method often deals with the targeted pests only rather than accidentally killing other organisms important for some processes in an ecosystem.
  • It is also very effective on small farm units, for example, a family garden.
  • Most manual methods are cheap since they utilise the farmers' knowledge and some resources found locally available and cheaply.
  • Manual methods are also environmentally friendly, quick and some pests trapped like mice and some insects, for example, locusts may be also used as food for the farmer.



Disadvantages of manual method of controlling pests

  • The method is only be applicable on small scale
  • Manual methods are often labour intensive and cumbersome



Biological methods of controlling pests

When pests are controlled biologically, natural enemies are incorporated in the farm system to militate against pests. The natural enemies introduced are biological control agents or bio agents. In biological control methods, pathogens can be used to control pests. In this method insect pathogens or diseases causing organisms such as virus, bacteria and fungi are used to control pests. For example, the bacteria called bacillus thruringienisis is commonly used to control moth, butterfly and beetles.

Biological pest control also involves the use of predators, for example, the lay bird and praying mantis feeds on aphids. The effect of predators is that it will consume a considerable number of pests.

Advantages of biological pest control method

  • Just like manual methods biological control method is cost effective thus it is cheap.
  • It is also often easy to apply since after introducing a new parasitic species in the environment, the system continues to operate independently.
  • The biological control agents are non- polluting hence the method is environmentally friendly.



Disadvantages of biological pest control method

  • Introduction of a new specie in the environment may disrupt and disturb the natural ecosystem.
  • It requires proper planning to develop a successful biological control system.
  • It is one of the slowest type of controlling pest so it requires a lot of time for biological agents to wipe away the pests. 

Chemical method of controlling pests

Chemical control of pests involves the use of chemicals in curbing agricultural pests. The chemicals used in controlling pests are called pesticides. There are different types of pesticides which are:

  1. Insectides- these are used to kill and control insects like flies and beetles
  2. Acatricides - these are used to control nematodes.
  3. Nematicides- these are used to control nematodes


It is of paramount importance to note that pesticides operate differently in the controlling of pests. Some are stomach poisons whilst some kill through contact. Most of the pesticides are used to control pests in cereals, vegetables and legumes but they should be used appropriately.

Advantages of chemical control

  • Chemical control methods often produce rapid response results in pest controlling.
  • They are also feasible to use on a large scale.
  • Chemical methods can be accessed in various forms such as dusts, fumigants and sprays.
  • It is one of the most effective ways of controlling pests.


Disadvantages of chemical control method

  • Pesticides are quite expensive to buy
  • They also have a tendency of killing none targeted organisms, for instance, bees which are of paramount importance in ecological systems because they facilitate cross pollination.
  • Their use requires expertise and precision in mixing.
  • Caution should be practised during application.
  • Chemical control methods need a safety harvesting interval hence it is not user friendly
  • They are unenvironmentally friendly; they pollute the air.