Chemical and biological control
of agricultural pests
5.5
Learning Objectives
All students should know:
• What pests and pesticides are.
Most students should know:
• The features of an efficient pesticide.
• How biological agents are used to control pests.
Some students should know:
• How integrated pest control systems are managed and what
it involves.
Success Criteria
All Students:
• I can match the key terms associated with its definition, by
completing the cross word activity correctly.
Most students:
• I know the features of pesticides and how they work to control
pests, this was demonstrated by correctly matching the
explanations of these specific terms with the cut and stick
activity.
• I can describe and explain how biological agents are used to
control pests by correctly answering plenary questions.
Some students:
• I can write a detailed account on, why isn’t the use of the cane
toad in Australia a good example of an ‘integrated pest control
system’?
Starter Activity
Across
7. can not be broken down by
micro-organisms.
Down
1. large area of land that contains
only one type of species.
2. toxic chemical used to kill pests.
3. toxic chemical used to kill weed.
4. can be broken down into nontoxic substances by microorganisms.
5. the build up of toxic chemicals.
6. an unwanted organism that
interferes with human activity
and reduces crop yield.
Starter activity: Answer
Across
7. can not be broken down by
micro-organisms.
Down
1. large area of land that contains
only one type of species.
2. toxic chemical used to kill
pests.
3. toxic chemical used to kill
weed.
4. can be broken down into nontoxic substances by microorganisms.
5. the build up of toxic chemicals.
6. an unwanted organism that
interferes with human activity
and reduces crop yield.
Management of Pests
• Pests are organisms that compete with humans for
food sources and space.
• Controlling pests is important in the agricultural
industry. If not treated properly pests will invade our
crops, damaging human food sources.
• There are two main ways to manage pests:
1. Using pesticides (chemical control)
2. Using predators or parasites (biological control)
Chemical Control
• Pesticides are toxic chemicals that kill pests. Ideally pesticides
should be:
Selective
only kills the species it is supposed to.
Biodegradable
can be broken down into non toxic substances.
It should be non toxic to other organisms
Cost effective
development cost of pesticides are high and is
only useful for a limited period of time as pest
will become resistant to it over time. How does
this happen?
Not accumulate
build up of pesticide in specific parts of an
organism is not ideal as it will be passed along
food chains.
Non mobile
stays where it is applied.
Bioaccumulation
• Bioaccumulation can occur where the pesticide is not broken
down.
• Pesticides tend to be fat soluble and accumulates in the
environment and in food chains. It builds up in the fatty
tissues and can not be excreted as waste product.
• This problem becomes worse as the pesticide progresses
along the food chain. This is because organisms from higher
trophic levels will eat large numbers of organisms from the
lower trophic level.
Example of Bioaccumulation (1)
Low
concentration
of pesticide on
crop
→
→
Some pests not
killed, still
containing low
concentration
pesticide
Small birds feed on
many insects.
Building up higher
concentrations of
pesticide in their
fatty tissues.
→
Birds of prey eat
many small birds
accumulating very
high concentrations
of pesticides in their
fatty tissue.
But not toxic enough These toxic levels
may kill the bird.
to kill small bird.
Example of Bioaccumulation (2)
→
DDT Insecticide
in water
(3 x 10-6 ppm)
→
DDT in
zooplankton
(0.04 ppm)
→
DDT in large fish
( 2ppm)
DDT in fish-eating
birds
(25ppm )
Another example of bioaccumulation is DDT, it was used as an insecticide to kill
mosquitoes. But insects slowly became resistant to it. High toxicity of DDT was
found in fish which was consumed by birds. As you can see in this food chain
the fish has about 10 times more DDT than zooplankton
Biological control
• Predators and parasites of the pests can also be used
to control pests. An example of this could be
ladybirds controlling insect pests on citrus fruits.
• Using a biological control does not eradicate the
parasite but controls it, so it is manageable and
natural.
• Ideally the parasite and pests should be in balance
with each other. Where the pest has little effect.
Differences between chemical and
biological control
Biological control
• Can have some effect on
other non-targeted
organisms.
• Pests can develop genetic
resistance to pesticides
overtime.
• Control organism
reproduces itself once it has
been introduced therefore
cheaper more cost
effective.
Chemical control
• Reapplied at intervals 
expensive
• Very specific, chosen
predator would only eat the
pest.
• Pests does not become
resistance.
• No pollution
Differences between chemical and
biological control
Biological control
• Very specific, chosen
predator would only eat the
pest.
• Pests does not become
resistance.
• Control organism
reproduces itself once it has
been introduced therefore
cheaper more cost
effective.
• No pollution
Chemical control
• Can have some effect on
other non-targeted
organisms.
• Pests can develop genetic
resistance to pesticides
overtime.
• Reapplied at intervals 
expensive
Integrated pest-control systems
• These systems include both types of controlling methods to
produce a better result for the management of pests.
• This type of control aims to reduce the effects of pest with minimal
impact on the environment.
• Integrated control involves:
1. The introduction of an organism that is better suited to the local
environment and is pest-resistance.
2. Provide suitable habitats for the crops.
3. Monitor crops for signs of pests regularly for earlier prevention.
4. Remove pests when it exceeds the acceptable population level.
5. Use biological agents if necessary
6. If the situation starts to get out of control use pesticide as last
resort.
Cane Toad Video
You will watch a video on the use of
the cane toad as a method of
biological control. After the video
you will be expected to write the
following account: Why shouldn’t the
cane toad have been used as an
‘biological control system’ in
Australia?
Pest and Productivity
• Pests often competes with crops for resources. These
resources are often related to the rate of photosynthesis,
mineral consumption or space.
Can you think of any limiting factors of photosynthesis?
• Pests can also affect the human food source by consuming it
themselves, directly competing with humans.
• Both points above shows how pests interfere with crops to
reduce productivity.
• The overall aim of pest control is to balance the cost and
benefits it brings. Farmers often have to think about how to
produce cheap food to satisfy their costumers and make
profit.
Plenary: Summary Questions
1. Pesticides are used to increase productivity.
Explain how their use might sometimes
reduce productivity. (3 marks)
2. State two advantages and two disadvantages
of biological pest control. (4 marks)
3. Weeds are growing amongst wheat in a field.
Explain how these weeds might reduce the
productivity of a crop. (3 marks)
Peer Assessment: Answers
1.
2.
3.
Pesticides will kill most of the pests-reducing its population. This in turn
reduces the population of its predators. (1 mark) However, with no
predators controlling pests after pesticide was introduced, the
population of pests starts to increase. (1 mark) This could be higher than
the pest population before, increasing the effect of pests on the crop –
reducing productivity.(1 mark)
Advantages: any 2 from: highly specific, targeted only by pest/once
introduced it reproduces itself and does not need to be reapplied/pests
do not become resistant. (2 marks)
Disadvantages: any 2 from: effect id slow as there is a time lag between
application and results/ may itself become a pest or may disrupt the
ecological balance. (2 marks)
Weeds compete with the crop for sources (e.g light, water, mineral ion,
space, carbon dioxide..) limiting the supply for crops. (1 mark) This limits
the rate of photosynthesis and therefore reduces productivity. (2 marks)
Learning Objectives
All students should know:
• What pests and pesticides are.
Most students should know:
• The features of an efficient pesticide.
• How biological agents are used to control pests.
Some students should be able to:
• Know how integrated pest control systems are managed and
what it involves.
Success Criteria
All Students should know:
• I can match the key terms associated with its definition, by
completing the cross word activity correctly.
Most students should know:
• I know the features of pesticides and how they work to control
pests, this was demonstrated by correctly matching the
explanations of these specific terms with the cut and stick
activity.
• I can describe and explain how biological agents are used to
control pests by correctly answering plenary questions.
Some students should be able to:
• Write a detailed account on, why isn’t the use of the cane toad in
Australia a good example of an ‘integrated pest control system’?