Arable Farming

(BiologyMad and Ian White)

Topic Notes

Additional Support Materials

i.e. animations, quizzes, pictures,  worksheets

Cereal Crops



Genetically Modified Crops 
(food and agriculture organisation of the united nations)

Major world Crop Areas

Farming Methods


Manipulating the Growing Environment Fertiliser and the Future 
(food and agriculture organisation of the united nations)

Harmful effects of excess Nitrogen and Phosphorous 

Biological Control of Pests Pest Animal Control (CRC)


Applied Botany – Arable Farming
Cereal Crops

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Agricultural production has increased enormously in the last few decades, and seems set to rise even further.  These increases have been brought about mainly by the development of new varieties of crops, animals and more intense methods of farming.

Maize  [back to top]
Originating in Central America, maize is now third only to wheat and rice in world production. It is widely grown in the USA (and Europe) as animal feed, and is also the basis of Corn Flakes and Sweetcorn for human consumption. This crop grows well where temperature is frost-free and light intensity is high.  There needs to be adequate water too – though not as much as rice needs.

It is grown as a staple food in much of Africa. However, for this purpose it has a major drawback, since it is deficient in the essential amino-acids tryptophan and lysine.  This causes children after weaning (i.e. about 4-7 years old) to become ill.  Their livers greatly enlarge in an attempt to synthesise the missing amino-acids and they suffer from Kwashiorkor.  The symptoms include stick-like arms and legs, thin, papery skin and a greatly swollen belly.  Note that these children are not ‘starving’ – they may have plenty of calories in their diet – but they are malnourished. 

Geese (and, less commonly, ducks) fed largely on a diet of maize develop similar symptoms.  Their grossly enlarged, fatty livers are then used to make paté de fois gras – mainly in the Bordeaux region of France.  This is a rare example of a farmer deliberately making an animal seriously ill in order to maximise profit – the reason why I will not eat it!  Another example is veal where calves are made anaemic, by depriving them of grass, in order to make their meat paler.

 One might assume that if maize crops are grown in conditions where light intensity and daytime temperatures are high; then the conditions should favour photosynthesis, however this is not necessarily the case as:

Adaptations include:

(Provided by: Illinois World Food and Sustainable Agriculture Program


Sorghum  [back to top]
Is the fifth commonly grown cereal in the world and is another tropical C4 cereal, like Maize (see above).  Sorghum is adapted to hot, arid, low-soil nutrient conditions, reflecting its origins in the Sudan region of Africa. In the drier regions of Africa and Central India it is often a staple food, being made into a tasteless porridge, but in the rest of the world it is used as animal feed or as a source of oil and fibre.  The USA is the major grower of Sorghum for this purpose.  In the UK we come across it as millet – used as budgie food! 

Sorghum is able to grow well in the very hot (over 35oC), dry regions of tropical Africa, southern USA and central India.  It is able to do this by synthesising special ‘heat-shock’ proteins very rapidly when the temperature rises.  It grows very high – up to 5 metres in a season – and the multiple seed-heads produce many thousands of small seeds from a single plant. 

Xerophytic (= a plant normally found in dry conditions) adaptations include:

World sorghum growing areas
(provided by: syngenta foundation for sustainable agriculture)

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The second most widely grown cereal (wheat is top!), rice is grown throughout the  tropical and Mediterranean regions of the world.  It requires a minimum temperature of 20oC in the growing season.  It is the staple food of half the World’s population, being highly nutritious and needs little post-harvest processing to make it edible.  Despite its Asian origins, it is a C3 plant; the USA is the world’s biggest exporter, since most poor countries can barely feed their own populations.  Of the two main types of rice (mountain and plains rice) only the latter has any adaptations to an unusual habitat - uniquely, it can grow in flooded conditions (though it is not a true hydrophyte or water plant).  This is the variety grown in SE Asia, where it is grown partly submerged in paddy fields for some of its life. These flooded soils ‘drown’ weeds, reducing competition.

The land is not needed for the seedling stage of growth either, so enabling up to 3 crops per year.  It is, however, expensive in labour – though this is the traditional way of life for women and children in this part of the world.

Adaptations of plains rice include:

Rice stem showing its hollow and aerenchyma tissue

Paddy fields are ‘bogs’ made by flooding the field with river water and also the local sewage (both animal and human).  This makes the water rich in organic matter (if rather smelly!).  Microbes break down the sewage whilst others use this energy to ‘fix’ nitrogen from the air.  Fish may also live there, feeding on the animal life.  When the crop is about to flower, the field is allowed to drain naturally and the bacteria break down in the soil releasing nitrogen for the benefit of the rice crop (catching the fish would be easy too!).  Thus a paddy field is both sewage and fish farm and a fertilizer factory.  Rice yield drops dramatically when weeds are present, so this ‘anti weed’ system is important.

(Provided by: Illinois World Food and Sustainable Agriculture Program)


Wheat  [back to top]

 This is the world’s most widely-grown crop and is extensively grown throughout the temperate regions of the world, both as human (flour) and animal feed.  Bread wheat – durum wheat - is a hard wheat, with a high protein (gluten) content, which enables the dough to stretch when rising.  It is also excellent for making pasta! Spring-sown, it is the preferred variety of Eastern Europe, Canada and the mid-west USA.  Winter wheat is a soft wheat, with a low gluten content and is good for making cakes and biscuits.  It is grown throughout the UK and Western Europe and in more temperate climates as it has a higher potential yield.  It is also ideal for animal feed, since it is easier to digest.

(Provided by: Illinois World Food and Sustainable Agriculture Program)

Adaptations and cropping yields of the major cereal crops

Crop Yield
Area grown
(ha x 106)
Growth requirements
Wheat 1000 - 4000 215 Warm, frost free climate, fertile soil, drought intolerant
Maize 1000 - 14500 139 Adapted to a wide range of temperate climates and soils

1500 (mountain)
4500 (plains)

155 Tropical, paddy varieties are aquatic, drought intolerant

300 – 2000
[6500 if irrigated]

43 Wide range of soils. Drought tolerant. Grown in regions too dry for maize.

Food values of some major crops


Energy (kJ/gm)

Protein (%)

Lipid (%)






























Farming Methods
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Intensive Farming   [back to top]

 After World War II intensive farming techniques flourished. Using high-yielding hybrid cultivars and large inputs of inorganic fertilisers; newly-developed chemical pesticides, and machine power, crop yields increased to 3 or 4 times those produced using the more extensive (low-input) methods of 100 years ago. Large areas planted with monocultures (single crops) are typical. Irrigation and fertiliser programmes are often extensive to allow the planting of several crops per season. Given adequate irrigation and continued fertiliser inputs, yields per hectare from intensive farming are high. Over time, these yields decline as soils are eroded or cannot recover from repeated cropping. More fertiliser leaches from the soil and enters groundwater as a pollutant.  Unemployment rises as workers are replaced by machines and the farmer becomes more and more dependent on fossil fuels and external inputs – he becomes a ‘land-slave’.  On the other hand, famine has become a thing of the past.

Extensive (or Organic) Farming  [back to top]

 Organic farming is a sustainable form of agriculture based on the avoidance of synthetic chemicals and applied inorganic fertilisers. It relies on mixed (crop and livestock) farming and crop management, sometimes combined with the use of environmentally friendly pest controls (e.g. biological controls and flaming), natural pesticides (nicotine and derris) and livestock and green manures. Note that ‘Organic’ is not the same as ‘pesticide-free’; to have that you must grow your own - and eat the occasional pest yourself!  Organic farming uses crop rotation and intercropping, in which two or more crops are grown at the same time on the same plot, often maturing at different times. If well cultivated, these plots can provide food, fuel, and natural pest control and fertilisers on a sustainable basis. Yields are typically lower than on intensive farms, but the produce can fetch high prices, and pest control and fertiliser costs are reduced.  It is labour-intensive, but requires little external input.



Comparison of agricultural methods  

Intensive farming

Extensive (organic) farming  
Advantages Advantages  
  • World grain production has almost tripled in the last 50 years.

  • Per capita grain production has increased, reducing global hunger.

  • The cost of food has declined: more food is now traded globally.

  • Yields increase more quickly and effectively than with the alternatives.

  • More closely matches crop type to appropriate season and soil.

  • Increases crop diversity and breaks up disease and pest cycles.

  • Soil quality and structure improve, reducing nutrient & water loss.

  • Farmers can still make use of new high yielding varieties of crops.

  • Produce is pesticide-free and produced in a sustainable way.  


  • Increases in yields may not be sustainable: the rate of gain in total production is slowing and per capita production is now declining.

  • Pesticide use is escalating yet pesticide effectiveness is decreasing.

  • Fertiliser use is increasing: soil & water quality continue to decline.

  • Poor countries are reliant on outside assistance and cannot afford the cost of the fertiliser required to achieve high crop yields  

  • Yields are lower and more land is required for the same output.

  • Organic produce may have more blemishes and a shorter shelf-life than sprayed produce.

  • Price may increase to offset losses.

  • High use of manures leads to bacterial contamination of produce.

  • The choice (especially of out-of-season crops) may be restricted.

  • use of ‘organic’ or old-fashioned pesticides may cause more damage to the environment than modern alternatives


    A fully-equipped modern greenhouse with all growth factors fully controlled by computer  
- shame about the flavour of the tomatoes!

Manipulating the growing Environment
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In the open field, Man can only manipulate the growing conditions to a certain extent.  The biotic component of the environment is under control – he can sow at the optimum density, weed and spray against pests and diseases.  But the abiotic component is much les amenable to change; there is only so much that we can do.  We cannot alter the climate (though, through Global Warming, we may be about to do so), so the sunshine, average temperature, days between first and last frosts are largely fixed.  True, we can plough (to aerate the soil); drain or irrigate (to seek optimum water levels); plant shelter-belts (to reduce wind speed); lime (to raise the pH – there is little we can do to lower it); and, of course, add fertilisers (to correct nutrient deficiencies).

But the three limiting factors for photosynthesis – light, temperature and carbon dioxide can only be controlled in a greenhouse or laboratory.  It is important to realise that the Laws of ‘Limiting Factors’ and of ‘Diminishing Returns’ apply here; unless increased profits more than cover the cost, there is no point in altering anything!


Carbon Dioxide  [back to top]

Photosynthetic organisms (Prokaryotes, Protoctista (= algae) and Plants) evolved when the concentration of carbon dioxide (CO2) in the atmosphere was very different from today.  Originally, the Earth’s atmosphere would have held up to 20% CO2 and, when the dinosaurs became extinct (65 million years ago), the level was still over 5% and oxygen levels would have been too low for matches to burn, for instance!

In more recent times, studies of the bubbles trapped in ice cores from Greenland and Antarctica confirm that CO2 levels were lower than today over much of the recent past – e.g. 280 ppm at the start of the Industrial Revolution (1750) – compared to the 375ppm of today (growing at 1.5 ppm each year).  These fluctuations were associated with climate change – but which was cause and which was effect is anyone’s guess!

In a greenhouse in full crop, the level of CO2 inside on a sunny day will be well below that outside and so photosynthesis is slowed considerably.  Growers can add extra CO2 by one of two main methods:

  1. burning fossil fuels and releasing the flue gases into the greenhouse (thus raising temperature as well – so useful in winter/spring

  2. adding pure CO2 from a tank of liquid (or ‘dry ice’ = solid CO2) outside.  This is very cold, so is more useful in summer.

Either method costs money and the levels of CO2 need to be monitored carefully.  It is pointless to raise the levels over 1000 – 1200 ppm, since photosynthesis does not increase, no matter what the conditions and there is no point at all if the temperature and light are below optimum (as is likely in winter).



Light  [back to top]
Light is obviously the key ingredient in photosynthesis – as the source of energy, without it photosynthesis (= ‘light building’) could not exist.  But there is more to light than day or night!

1.     Light intensity – our eyes and brain are very good at optimising our vision so that we do not realise just how gloomy it is inside.  Light intensity is measured in ‘lux’ and offices normally have around 5-600 lux at desk-top height.  In contrast, a sunny summer’s day can have well over 40,000 lux – and much more in the tropics!  In winter, lack of light seriously restricts growth and so growers try to site their glasshouses on south-facing slopes overlooking the sea (or a large lake). 

Every plant has a light level where photosynthesis = respiration and no net gas exchange takes place.  This is known as the Compensation Point and is passed every day at dawn and dusk.  Some plants (‘sun plants’) have a high light value (e.g. grasses); others (‘shade plants’) have a much lower value (e.g. ivy).  Trees can even have two types of leaf on the same plant!  The anatomy of the two types of leaf is very different – shade leaves are larger, with two or more palisade layers in the leaf and, consequently, more chlorophyll.  Research has shown that the yield of tomatoes etc. always rises with more light – gardening books recommending shading your greenhouse in summer are simply wrong!

2.   Colour – white light is a mixture of colours and plants cannot use them all.  In fact, they need roughly equal amounts of red and blue light, whilst they reflect green and so cannot use it at all.  [This is dealt with in Module 5!].  Artificial lighting can be used in winter to boost natural daylight levels – but it costs a lot!  Filament lamps give too much red, fluorescent tubes are OK, but give out too much heat, whilst metal halide and sodium lamps (as used in B&Q etc and in street lights) are fine – only about 35% of the electricity is turned to light, the rest wasted as heat.  Many commercial plants are raised in ‘growth rooms’ where the light is all artificial.  Sown in mid-winter, the plants get all the light they need for the first 2 months or so, before being moved to the glasshouses in February, prior to cropping in May.

3.   Day-length – plants grow best when the days are about 16 – 18 hours long, which is what we have in summer in the UK.  In winter, with 8 hours or less of daylight, plants scarcely grow at all – even when kept warm.  Surprisingly, plants generally do not like continuous light (they like to ‘sleep’ too!).  The length of day has a dramatic effect on the flowering of most plants.  Some like short nights and long days (runner beans); others like short days and long nights (Xmas cactus); by manipulating day-length (by the use of blinds as well as artificial lights) it is possible to have pot-grown chrysanthemums all the year round, when their natural flowering time is in October (only!)

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Photosynthesis is a series of chemical reactions and so is dependent on temperature.  As always, more kinetic energy means more collisions and so more enzyme-substrate complexes are formed and the action goes faster – up to the point (35 – 40o C) when the enzymes are denatured.  BUT – and it is a big but – that assumes that all other factors are optimal.  In the real world, this is not the case – CO2 is always limiting in summer and day-length and temperature are limiting in winter.  So how warm should you keep your greenhouse?  Well, 25-30oC is optimum (for C3 plants – for C4 plants it is 10oC higher), assuming you can raise the CO2 level, otherwise growth is no faster at 35+oC than at 20oC.  In summer, the main problem is keeping the temperature down.  This is where shading is supposed to help (less radiation = less heat absorbed); a better solution is more ventilation (use a fan) and pouring water on the floor, when its evaporation will cool the atmosphere.

Remember:     Growth (or net photosynthesis) = gross photosynthesis – respiration

But if the temperature rises, so does respiration, and if photosynthesis cannot go any faster (due to lack of CO2) then net photosynthesis (i.e. growth) will be lower at higher temperatures. 

Since there is no photosynthesis at night, the ideal is warm days (25+oC) and cool nights (10 - 15oC), together with long days (18 hours).  This is exactly what the northern parts of the UK and Canada experience each summer and explains why the highest yield of wheat and barley ever recorded was in these areas.

Fertilisers  [back to top]
Since the rate of plant growth in usually limited by the availability of mineral ions in the soil, then adding more of these ions as fertiliser is a simple way to improve yields, and this is a keystone of intensive farming.

Plants need mineral nutrients as well as carbon dioxide and water for photosynthesis. In addition to the products of photosynthesis plants also need:

Macronutrients are used in relatively large quantities i.e. Nitrate, Phosphate & Potassium (NPK)

Micronutrients are needed in very small amounts, e.g. iron, magnesium, sulphur. If plants lack these nutrients they show specific deficiency symptoms.

Macronutrient deficiency symptoms

Element   Function in plant   Deficiency symptoms  
Nitrogen   growth, proteins & nucleic acids stunted growth, yellow leaves  
Phosphorus   nucleic acids, ATP, membranes   poor root growth, blue-green colour to leaves  
Potassium enzyme activator   Poor flowering; susceptible to disease;
brown edges to leaves
Iron   manufacture of chlorophyll white veins in young leaves
Magnesium   contained in chlorophyll yellowing with green veins on old leaves
Sulphur   amino acids (and flavours in onions, garlic etc) yellowing and stunting of plant in spring

When plants are harvested the nutrients are removed with them. In a natural ecosystem the plants would eventually die and decay, with the nutrients being returned to the soil. Farmers need to use fertilisers containing these nutrients to maintain productivity. Farmers can use organic fertilisers or inorganic fertilisers.

The most commonly used fertilisers are the soluble inorganic fertilisers containing nitrate, phosphate and potassium ions (NPK). Inorganic fertilisers are very effective but also have undesirable effects on the environment. Since nitrate and ammonium ions are very soluble, they do not remain in the soil for long and are quickly leached out, ending up in local rivers and lakes and causing eutrophication. They are also expensive.

An alternative solution, which does less harm to the environment, is the use of organic fertilisers, such as animal manure (farmyard manure or FYM), composted vegetable matter, crop residues, and sewage sludge. These contain the main elements found in inorganic fertilisers (NPK), but in organic compounds such as urea, cellulose, lipids and organic acids. Of course plants cannot make use of these organic materials in the soil: their roots can only take up inorganic mineral ions such as nitrate, phosphate and potassium. But the organic compounds can be digested by soil organisms such as animals, fungi and bacteria, who then release inorganic ions that the plants can use (refer to the nitrogen cycle). Some advantages of organic fertilisers are:

Some disadvantages are that they are bulky and less concentrated in minerals than inorganic fertilisers, so more needs to be spread on a field to have a similar effect. They may contain unwanted substances such as weed seeds, fungal spores, heavy metals. They are also very smelly!

Increasing the amount of fertiliser increases yield, but only up to a point:

This graph shows the results of field trials using wheat – clearly, little is gained after in initial use and heavier seed density is actually counter-productive:

Problems with fertilisers

Leaching  [back to top]

  If the nutrients in fertilisers are not taken up by plants their is a danger that they will be washed out of the soil by rain water and that the run off will enter stream and rivers. This process is could leaching. The problem with this is that it may cause eutrophication.



Eutrophication  [back to top]

This is the process that takes place when freshwater is 'enriched' by nutrients, especially nitrates and phosphates.

The aquatic ecosystems naturally progress from being oligotrophic (clean water with few nutrients and algae) to eutrophic (murky water with many nutrients and plants) and sometimes to hypertrophic (a swamp with a mass of plants and detritus). This is in fact a common example of succession. In the context of pollution “eutrophication” has come to mean a sudden and dramatic increase in nutrients due to human activity, which disturbs and eventually destroys the food chain. The main causes are fertilisers leaching off farm fields into the surrounding water course, and sewage (liquid waste from houses and factories). These both contain dissolved minerals, such as nitrates and phosphates, which enrich the water.

Subsequently, this may lead to excessive plant growth (‘algal blooms’) and this, in turn, can lead to the deoxygenation of the water and the death of much of the animal life.  In cold Arctic waters this does not happen, though the water may still have high mineral content and so be eutrophic.

The main source of nitrogen is farming – either nitrates (from fertiliser over-use) or urea (from over-use of slurry/manure).  Correct timing of fertiliser application can reduce these problems.  Cold soils prevent crops absorbing nutrients, so there is no point applying them in winter or cold spring weather.  Heavy rain leads to the nutrients being washing into groundwater before the plants have had a chance to absorb them; so do not apply if heavy rainfall is forecast.

The main source of phosphates is sewage.  Phosphates are added to detergents to improve washing performance – particularly in hard water areas.  Sewage is also warm too, so the combined effect on plant growth downstream is quite marked.  Aeration of the sewage outfall (by weirs or spraying) will increase oxygen levels and improve water quality for animal life, but do nothing to the mineral load added to the water.

Chemical Control of Pests  [back to top]

To farmers, a pest is any organism (animal, plant or microbe) that damages their crops. Some form of pest control has always been needed, whether it is chemical (e.g. pesticides), biological (e.g. predators) or cultural (e.g. weeding or a scarecrow). Chemicals pesticide include:

Pesticides have to be effective against the pest, but have no effect on the crop. They may kill the pests, or just reduce their population by slowing growth or preventing reproduction. Intensive farming depends completely on the use of pesticides, and some wheat crops are treated with 18 different chemicals to combat a variety of weeds, fungi and insects. In addition, by controlling pests that carry human disease, they have saved millions of human lives. However, with their widespread use and success there are problems, the mains ones being persistence and bioaccumulation

Competition between crop plants and pests

Weed species compete with the crops for light, space, water and nutrients.

Flax is grown for its oil – linseed – as well as for its fibre, used to make linen.  It is not common in the UK, but has pale blue flowers, so is readily identified in early summer.  A common arable weed is known as Wild Oat, which competes with it.  Wild Oat seeds ripen earlier than flax and so carry forward to the next crop. The effect of Wild Oat on flax yield is shown below:

Wild Oat density (no./m2) Fertilised plots, yield of flax
Unfertilised plots, yield of flax
Average reduction in yield (%)
0 19.5 17.9 -
10 13.4 14.3 26
40 6.7 8.0 60
70 4.3 6.3 72
100 3.5 4.2 80
130 3.4 3.4 82
160 2.9 2.3 86

Like all crops these days, farmers normally use ‘direct drilling’, spraying the land with glyphosate (= ‘Roundup’), a weed-killer that kills all germinated weeds and allows the crop to germinate in weed-free conditions. ‘Tram-lines’ are used to confine wheel-tracks to the smallest possible area.  In addition, ploughing is no longer used, since it:

‘GM’ crops have been modified to allow this weed-killer to be used on the growing crop too (it has no effect on seeds), thus further improving crop yield and Monsanto’s profits!

Animal pests (mainly insects) damage crops by feeding on them. They lower crop yield by:

  1. Eating their leaves and so reducing photosynthetic area (e.g. caterpillars)

  2. Sucking their sap (e.g. aphids or greenfly).  This removes the products of photosynthesis; however, the main problem with these insects is that they act as vectors for virus diseases.

Use of chemical pesticides  [back to top]
Insecticides act either by contact or are distributed inside the plant (systemic) and are then eaten by the insect.  The latter are a more recent development and have the advantages that only harmful insects are killed and rain does not wash the chemical away, so less spraying is needed and young plant growth is continually protected.

There are four main modes of action of insecticides:

Problems with insecticides  [back to top]
Some pesticides are persistent so do not break down in the environment or within the tissues of living organisms. This gives rise to two potential problems:

a)  Bioaccumulation - the accumulation of a substance in living tissue. Organisms at any trophic level may be capable of bioaccumulation.

b) Biomagnification is the increasing concentration of a substance up a food chain - i.e. from one trophic level to the next . Animals at the higher trophic levels will be most affected (e.g. us!).

Some of the earliest insecticides were organochlorines, such as DDT, which kills by both contact and stomach ingestion.  DDT was the first known contact insecticide, synthesized in 1874. It has saved more human lives than any other chemical; it widespread use against mosquitoes briefly eradicated malaria in many parts of the world (resistance rapidly built up, however, and Malaria (= ‘bad air’) is once again on the increase).  Unfortunately, DDT persists in fatty tissues (bioaccumulation). Larger, long lived predators at the end of a food chain may accumulate a lethal quantity of DDT as a result of eating large numbers of smaller species (biomagnification).

These dangerous effects coupled with the appearance of resistant insects soon led to the banning of DDT and related compounds (e.g. Lindane) except in specialised environments, such as wood-worm treatment of loft timbers.  Even that use has now ceased in the EU.

  Water à zooplankton à small fish à large fish à birds  
                   0.04ppm          0.5ppm         2ppm         25ppm

Sparrow hawks are long lived and eat large numbers of smaller species in their lifetime.


Fungicides   [back to top]

Fungi are the main plant pathogens.  Able to grow in a wide variety of conditions and able to produce cellulose to digest cell walls they cost farmers and growers millions each year.  

people preparing to leave Ireland forever 
during the potato famine

  Phototgraph showing potato blight - phytophthora infestansIn 1848, potato blight (Phytophthora infestans) came to Ireland, where the staple crop of the local people was potatoes.  Over the next few (wet) summers, it spread and destroyed the crop, for which there was then no cure.  The Irish resented growing corn, which was unaffected and shipped abroad, when they were starving and riots followed.  In 5 years, half the population of Ireland starved or emigrated (mainly to the USA – as the Kennedy and Clinton families did).  Much of the bitter hatred of the English that still exists today in ten conflicts of Northern Ireland have their origins in the effects of a fungus!  By 1852, the use of Bordeaux Mixture had transformed the outlook and potatoes could again be grown with success.

Blight turns the leaves of the plant brown and the plant diverts all its sucrose energy to the infected leaves, rather than to the growing tubers.  After a few weeks, the whole of the plant is infected and it dies.

   The tubers become infected too and turn to a brown, stinking, inedible mess.

Traditional fungicides e.g. Bordeaux Mixture. These are based on heavy metals (e.g. copper, manganese), together with a ‘sticking’ agent so that it stays on the surface of the leaves.  These chemicals are purely preventative, and must be sprayed on the crop before infection.  They have the advantage that resistance cannot build up, but the drawback that they are washed off when it rains and that new growth is unprotected.  They can, of course, be easily washed off the food before consumption.   Some other old-fashioned remedies were based on tar-oil compounds (e.g. creosote) and their impact on the environment can only be guessed at – certainly, some of their ingredients are carcinogenic!

Systemic insecticides – e.g. Benlate.  These are systemic in their action i.e. they are absorbed and carried around the plant.  In addition to being preventative, they are also curative (at least in part) and so preventative spraying can be reduced.  They also protect new growth as it appears and so, again, less spray is needed.  The old idea of ‘preventative spraying’ has long been abandoned – it merely leads to the build-up of resistance.  They have the drawback that all the washing in the world cannot remove them from our food.  In the past few years, nearly all the fungicides available to the gardener have been withdrawn from the market.  

Biological control of pests  [back to top]

1. What are the characteristics of pest species?

They cause economic damage (e.g. eating growing crops, stored crops or damaging buildings) or have health implications (e.g. vectors of disease). Many are capable of very rapid population growth. Often the normal factors which would regulate their numbers (i.e. natural enemies) are not present. This may be because they have been imported into a part of the world where their natural enemies do not exist, or because these enemies have been suppressed in some way (e.g. use of insecticides).

 2. What is the aim of biological control?

This is not always easy to determine. For example if you live in a house infested with cockroaches your pest control aim might be to eliminate all of them. However, in general it is accepted that the aim is to depress the pest population below the Economic Injury Level (EIL):- That is where the costs of the control measures start to exceed those of the extra revenue.

The graph shows an idealised situation. The introduction of a control measure depresses the pest population size below the EIL, but does not eliminate the pest completely.

3. What is a suitable biological control agent?

Introduced control organisms should preferably:

                so that they keep the pest at low numbers

If the pest species is alien  (i.e. not indigenous) then the following programme is usually followed:

  1. Search in the pest species native country for a suitable organism

  2. Identify if the geographic, climatic and political conditions are right for release

  3. Quarantine the organism, rear the control organisms for release to ensure any unwanted parasites are not introduced with it.

  4. Select a suitable release area.

  5. Extensively trial the proposed control agent, within a contained environment

  6. Seek EU Commission approval (UK government not allowed to)

  7. Convince Greenpeace etc that what you are proposing is safe

  8. Market the proposed product


4. What groups of organisms can be used for biological control?

1. Insect parasites These have the advantage that they are generally specific in the host on  which they lay their eggs. The larvae eat the host from the inside once the eggs hatch.

2. Predators. These are carnivorous and so may, in turn become a pest if, having reduced the original pest to a low level, then attack other species e.g. Cane Toad in Queensland, Aus.

3. Pathogens The best known example is the bacterium Bacillus thuringiensis, the toxin from which kills a wide range of caterpillars.  If a virus is used, they are generally specific in action.

5. What examples of biological control are there?

1.         The control of White Fly by the parasitic wasp Encarsia formosa in greenhouses.
                     This is now widely used as an alternative to pesticide control.

2.         The control of water hyacinth, which was introduced to the USA from S. America, by a weevil.

3.         The use of grass carp in the UK to control weed growth in ponds and waterways. It is claimed that water temperatures in the UK are too cold for them to breed. However, the use of an exotic species such as this is still extremely controversial.

Can things go wrong? Yes, especially if a non-native species is used. On Granada mongoose were released to control the rat population, however, they have had a devastating effect on native species- especially some ground-nesting birds.


Advantages and disadvantages of biological control.

Advantages   Disadvantages  
1. It should not intensify or create new pest problems – the organisms used are selective.   1. Control is slower.  
2. No manufacturing of new chemicals: the organisms are already available and so ‘organic’.   2. It will not exterminate the pest.  
3. Control organisms will increase in number and spread.  3. It is often unpredictable.  
4. The pest is usually unable to develop resistance.   4. It may well require training in its use.  
5. Control is largely self-perpetuating  

5. It is difficult and expensive to develop and supply – and where’s the developers’ profit?

Integrated Pest Management   [back to top]

It is now recognised that the most cost-effective form of pest control is achieved by using several different approaches together: a combination of the following is known as Integrated Pest Management.


Crop rotation

Fields of most crops attract pests in large numbers. If the same crop is grown on the same piece of ground year after year pests persist in crop debris, in the soil or in hedgerows from one year to the next, with the result that infestations increase in severity with yields and quality suffering. Crop rotation has been widely practised from the earliest times. Today, one common 4-crop rotation is: cereal A; legumes, cereal B; oilseed rape or other ‘break’ crop, following one another in a regular sequence. This limits the population density of pests and diseases, especially those with annual life cycles associated with specific crops – such as potato blight.


Pest-resistant varieties

Some varieties of crop plants are naturally resistant to certain pests and diseases. For many years selective breeding has been used to transfer the genes that confer this natural protection into high-yielding strains. More recently, genetic engineering (using recombinant DNA) has served a similar purpose. The transfer of a gene across the species barrier from the bacterium B. thuringienis into cotton is an interesting example of conferred insect resistance – and most cotton is now ‘GM’. In the future, more pest-resistant varieties should result from the applications of biotechnology.


Biological Control see notes


Pesticide application

The primary aim of integrated pest management is to hold pests and diseases below the Economic Injury Threshold.  Providing this can be achieved by other means, pesticide application can be held in reserve. However, if a farmer receives advance warning of an impending increase in a particular pest or disease, effective control can usually be achieved by applying a pesticide at the optimum time.  The old-fashioned practice of ‘preventative spraying’ was both costly and an invitation to the pest to become resistant.



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Last updated 20/06/2004