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Unit 3: Sustainability and Interdependence Sub Topic 3: Crop Protection
Unit 3: Sustainability and Interdependence
Sub Topic 3: Crop Protection
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 2 of 12
On completion of this sub topic I will be able to:
State that weeds compete with crop plants.
State that pests and diseases damage crop plants.
State that weeds, pests and diseases reduce plant productivity.
Explain the properties of annual weeds include; rapid growth, short life cycle, high seed output and long-term seed viability.
Explain the properties of perennial weeds and their competitive adaptations such as storage organs and vegetative reproduction.
Give examples of crop plant pests that include invertebrate animals such as insects, nematode worms and molluscs.
State that plant diseases can be caused by fungi, bacteria or viruses, these are often carried by invertebrates.
Explain cultural methods of controlling pests, weed and diseases are ploughing, removal of weeds and crop rotation.
Describe that selective plant protection chemicals are similar in chemical structure to plant hormones.
Explain the advantages of selective plant chemicals.
Describe that systemic plant protection chemicals are absorbed by the weed and taken into their circulatory system.
Explain the advantages of systemic plant protection chemicals.
State that protective applications of fungicide based on disease forecasts are often more effective than treating a diseased crop.
Give examples of the problems with plant protection chemicals that include toxicity to animal species, persistence in the environment, can accumulate or be magnified in food chains, produce resistant populations.
Explain the term biological control.
Explain the term Integrated Pest Management.
Describe the risks associated with biological control.
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 3 of 12
Prior Learning
National 5 – Unit 3, Sub Topic 5: Human Impact on the Environment
The increasing human population requires an increasing food yield.
Methods of increasing food supply have included the use of fertilisers,
pesticides, biological control and genetically modified (GM) crops.
Pesticides are chemicals used to protect crops against damage by pests e.g.
insects.
Pesticides can be slow to break down and so can accumulate in the bodies of
organisms, building up along the food chain.
Toxicity levels of pesticides in the bodies of carnivores can build up to the
extent that the animal’s ability to successfully reproduce is affected. Levels can
become lethal.
Biological control may be an alternative to the use of pesticides.
Biological control relies on natural solutions to pest problems, such as
making use of natural predators or parasites.
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 4 of 12
Crop Protection
Natural Ecosystems
In a natural ecosystem a balance exists between producers and consumers. There
is also a diverse variety of plant species that co-exist with insects and
microorganisms. The diversity and genetic variety amongst species make them
more resilient to weeds, pests and fungal infections.
Monocultures
In a monoculture only one species of crop plant is grown, usually over a large area.
The crops are often genetically identical. This makes them very susceptible to
weeds, pests and disease. Weeds, pests and fungal infections have to be strictly
controlled to ensure the greatest yield of the crop.
Weeds
Plants compete for light, space, water and soil nutrients. Weeds (and plants
growing where they are not wanted) will compete with crop plants. Competition in
monocultures can be reduced by spacing out seeds when sowing, by planting the
crops in rows. However, there is a risk that weeds can fill the gaps, which may
reduce crop yield.
Economic Impact of Weeds
In agriculture, weeds have a significant economic impact. They may:
cause a significant reduction in crop productivity due to competition
release chemical inhibitors into the soil, further reducing crop growth
contaminate grain crops with their seeds and reduce the crop’s value
act as hosts for crop pests and diseases
The picture shows the effect of disease on a
banana plantation in Mozambique. The fungus
Fusarium was responsible for wiping out 230,
000 plants across 2 plantations.
Apple scab (fungal disease)
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 5 of 12
Activity: Use pages 266 and 267 of Torrance to complete the table showing the properties of annual and perennial weeds.
Annual Weeds Perennial Weeds
Colonisation Vacant, “empty” fields Long established in the habitat
Description of life cycle
Growth rate
Method of Reproduction
Seed Output
Seed Viability
Food Source
Invertebrate Pests
There are three main groups of invertebrate pests:
Decreased Yield
Damage sustained by pests can reduce photosynthesis and less glucose is
produced. The loss of glucose reduces energy supply to tissues and as a result,
there is a reduction in vigour and yield. Some pests can also be a vector
(carrier) for diseases which can damage the crop. For example, aphids can transmit
a virus that causes potato leaf roll.
The effects of potato leaf roll on
the leaf and the tuber
Insects such as aphids (feed on sugary sap); caterpillars (eat
leaves)
Molluscs such as snails and slugs (eat leaves)
Nematode worms (root parasites).
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 6 of 12
Plant Diseases
Plant diseases are caused by pathogens such as fungi, bacteria and viruses. These
can be spread through the air, the soil or by invertebrate vectors (carriers).
The economic effects can be devastating: poorer yield; reduced marketability
(blemished); reduced storage life (degrade too quickly).
Controlling Weeds, Pests and Diseases Weeds, pests and diseases can be controlled in two ways: cultural and chemical. Cultural Crop Protection
These are techniques for controlling weeds, pests and diseases which are non-
chemical. They have developed over a long period of time from traditional farming
methods, some by trial and error.
Ploughing - by turning over the top 20cm of the soil every time a field is
ploughed, many weeds are buried deeply enough for them to die and
decompose.
Weeding - removal of weeds early in the life of the crop reduces
competition. Weeds removed from edges of fields as they provide a
breeding ground for pathogens.
Crop rotation - a series of different crops are grown one after the other on
the same piece of ground, over 4 growing seasons. Pathogens in the soil
from the first crop cannot then grow on the second crop. Growing
leguminous (nitrogen fixing) plants can add nutrients to the soil.
Clearance of crop residue – debris such as stubble or straw left on the
ground after harvesting can harbour fungal spores.
Cover crop - planting of crop such as clover that prevents growth of weeds
on fallow ground.
FIELD 1 FIELD 2
FIELD 3 FIELD 4
In this model, the cabbages follow the pea plants and
are followed by the potatoes in the rotation. A pest
that attacks the brassicas for example, may be
controlled effectively because they are unlikely to
survive the 4 years until the host returns.
This method works best against soil-inhabiting pests
that attack a narrow range of hosts.
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 7 of 12
Chemical Crop Protection
Herbicides, pesticides and fungicides can be used to control pests when cultural
means of control have failed.
Herbicides can be selective, systemic or contact.
Selective herbicides mimic the action of plant growth hormones. This
speeds up the metabolism of broad leafed plants to the extent that they use
up their food reserves and die. Narrow-leaved plants e.g. cereal crops, are
not affected.
Systemic herbicides are absorbed by the plant and quickly transported to
all areas of the plant. This has a lethal effect on the leaves and the roots of
the plant.
Contact herbicides kill all green plant tissue they come in contact with.
They are biodegradable so their effect is short-lived. However, the roots
survive and plants can regrow.
Pesticides
The three main pest groups can be eradicated (killed) by different pesticides;
insecticide, molluscicide or nematocide.
Each is used extensively in agriculture and helps prevent around 30% of crop loss in
Scotland. Pesticides can be either contact or systemic.
Contact pesticides work in two ways:
• killing the invertebrates when they come into contact with the spray
• leaving a protective residue on the plant which kills future invertebrates
Systemic pesticides are absorbed by the plant and only kill invertebrates when
they ingest plant material.
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 8 of 12
Fungicides
Fungicides can also be contact or systemic.
Contact fungicides are sprayed onto crops and absorbed by fungal spores when
they start to germinate. This causes the fungi to die. They are easily washed away
by the rain and need to reapplied to crops regularly.
Systemic fungicides are absorbed by the plant and are therefore not washed
away by the rain. They give better protection and only kill fungi that are affecting
the crop plant.
Fungicides can be used to protect crops when environmental conditions and disease
forecasts suggest that infection is likely.
Example
The airborne spores of the fungus causing potato blight are more likely to infect
plants when certain temperature and humidity levels occur over two days. Farmers
can apply fungicide in advance when these conditions are forecast. Prevention is
more effective than treatment.
Activity: Complete the diagram on the next page to summarise the key points of
“Controlling Pests” using the information on pages 6 to 8.
Potatoes infected with potato
blight virus which caused the
potato famine in Ireland in 1845
CONTROLLING PESTS
Cultural Methods
Chemical Methods
pesticides
herbicides
fungicides
ploughing cover crop
weeding crop rotation
Contact:
Selective:
Systemic:
Used to kill ________________
Used to kill ________________
Contact:
Systemic:
Contact:
Systemic:
Used to kill ________________
polyculture/ companion
planting
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 10 of 12
The Problems with Plant Protection Chemicals
Ideally a plant protection chemical should be:
• specific to the pest concerned
• short-lived (i.e. should not persist in the environment but be broken down
into harmless by-products)
• safe for animals and human consumption
However, some problems associated with their use include:
• toxicity to animal species
• persistence in the environment
• accumulation in food chains
• resistance in pest populations
Pesticide Persistence and Accumulation
Many plant protection chemicals have been found to persist in the environment.
Whilst they are found at low concentrations in the environment, they accumulate
along food chains and become more concentrated at each level as the chemical
persists in cells.
The once widespread use of DDT, a powerful insecticide, resulted in the death of top
predators such as sparrowhawks. DDT is now banned in many countries, but is still
widely used in developing countries.
Effect of DDT accumulation in an ecosystem:
25 ppm
2 ppm
0.5 ppm
0.04 ppm
Sparrow Hawks
(fish eating birds)
large fish
small fish
Zooplankton
(producers)
A 10 million times
increase in DDT
concentration
KEY:
ppm = parts per million
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 11 of 12
Resistance to Pesticides
A few individuals in a crop may have resistance to a pesticide. They survive the
pesticide and pass their resistance on to the next generation (natural selection), this
eventually produces a population of resistant pests after continued use of the
pesticide.
Biological Control
Biological control is the introduction and use of a natural enemy as a control agent
for a pest population.
Biological controls can be:
predators - (ladybirds used to control aphids)
parasites - (Ecnarsia a parasitic wasp that lays eggs inside whitefly and
destroys it)
pathogens - (Bacillus thuringiensis which can infect caterpillars with Bt
toxin).
Timing of the introduction of the biological control agent is very important, firstly
the predator must find its prey. The control agent is introduced when crop
infestation has begun and environmental conditions are favourable for its
introduction. Biological control works well in enclosed systems such as
glasshouses where the temperature can be controlled and control agents can be
kept confined.
Biological Control: the risks
If a control agent escapes into an environment free from its predators, parasites or
disease then its numbers can increase rapidly and threaten indigenous species.
Integrated Pest Management (IPM)
IPM uses a combination of chemical, biological and cultural methods and plant
resistance to improve yield. IPM aims to: reduce chemical use, only uses
infrequently needed and biodegradable chemicals and reduces pest numbers to a
level where economic damage is minimised and at which biological control can then
be adopted.
Higher Biology Unit 3 Sustainability Sub Topic 3.3 Crop Protection
and Interdependence Pupil Course Notes
Duncanrig Secondary School CG 2017 Page 12 of 12
3.3 Crop Protection
How well do you rate your knowledge and understanding?
1 2 3
State that weeds compete with crop plants.
State that pests and diseases damage crop plants.
State that weeds, pests and diseases reduce plant productivity.
Explain the properties of annual weeds include; rapid growth, short life cycle, high seed output and long-term seed viability.
Explain the properties of perennial weeds and their competitive adaptations such as storage organs and vegetative reproduction.
Give examples of crop plant pests that include invertebrate animals such as insects, nematode worms and molluscs.
State that plant diseases can be caused by fungi, bacteria or viruses, these are often carried by invertebrates.
Explain cultural methods of controlling pests, weed and diseases are ploughing, removal of
weeds and crop rotation.
Describe that selective plant protection chemicals are similar in chemical structure to plant hormones.
Explain the advantages of selective plant chemicals.
Describe that systemic plant protection chemicals are absorbed by the weed and taken into
their circulatory system.
Explain the advantages of systemic plant protection chemicals.
State that protective applications of fungicide based on disease forecasts are often more effective than treating a diseased crop.
Give examples of the problems with plant protection chemicals: toxicity to animals, persistence in the environment, can accumulate in food chains, produce resistant populations.
Explain the term biological control.
Explain the term Integrated Pest Management.
Describe the risks associated with biological control.
Complete: Column 1 - before your Unit assessment Column 2 - before your Prelim
Column 3 - before your May exam
