To farmers, a pest is any organism (animal, plant or microbe) that damages their crops.
So all farmers, growers and gardeners need to use some form of pest control, or we wouldn’t be able to feed the world.
Pest control can be cultural(e.g. weeding or a scarecrow), chemical(e.g. pesticides) or
biological (e.g. predators), and modern practice is to combine all three in integrated pest management.
Cultural Control of Pests
This refers to any farming practices that reduce the problem of pests, other than chemical or biological methods. Cultural practices include:
• Weeding – physically removing weeds and diseased crop plants to prevent reinfection.
• Crop rotation – changing the crops each year to break the life cycle of host-specific pests.
Pest Control 2
- Intercropping – planting two crops in the same field e.g. sowing rye grass with wheat encourages ladybirds to control aphids on the wheat.
- Tilling – traditional ploughing and turning of the top soil layer to bury weed seeds and expose insects to predatory birds.
- Insect barriers – e.g. sticky bands on apple tree trunks stop codling moth caterpillars.
- Beetle banks – building strips of uncultivated rough ground around and through fields. These strips are breeding grounds for beetles and other invertebrates that may predate the pest and so keep their populations under control.
Chemical Control of Pests
- Chemicals that kill pests are called pesticides.
- Chemical pesticides include herbicides (anti-plant chemicals); insecticides (anti-insect chemicals); fungicides (anti-fungal chemicals); and bactericides (anti-bacterial chemicals).
- Pesticides have been used in some form for over 1000 years, and modern intensive farming depends completely on the use of pesticides to increase yields.
Pest Control 3
- 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, pesticides have saved millions of human lives.
Good pesticides must be:
- Selectively toxic, which means they kill their target but not the crop or other organisms including humans. Early pesticides were non-selective (or broad-spectrum), which means they caused a lot of harm to the environment. Broad-spectrum pesticides can kill useful pollinating insects and pest predators, so can actually cause the pest population to increase, Modern pesticides must be selective (or narrow-spectrum), which is better for the environment, but they are more expensive to produce.
• Biodegradable, which means they are broken down by decomposers in the soil. Early pesticides were not easily broken down (they were persistent), so they accumulated in food chains and harmed humans and other animals, but modern pesticides biodegradable so they do not leave residues on crops
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Different kinds of pesticides are used to control different kinds of pest:
- Insects are the most important group of animal pests, like aphids and leather jackets that eat the crop and so reduce yield.
- Insecticides can be contactor systemic. Contact insecticides remain on the surface of the crop and only kill insects that come into contact with it, so are not 100% effective.
- Systemic insecticides are absorbed into the crop and transported throughout the plant, so any insect feeding on the crop will be killed.
- Weeds are simply plants that the farmer (or gardener) doesn’t want.
- Plants like wild oats, cleavers, bindweeds and thistles compete with the crop plants for light, water and minerals, and so reduce crop yields.
- Weeds can also harbour pests and pathogens that can infect neighbouring crops.
Weeds usually arrive on farmland by wind-dispersed seeds or they can be sown accidentally with the crop.
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- How can a chemical kill some plants (weeds) but not others (the crop)?
- Fortunately, cereal cropsare narrow-leaved grasses (monocotyledons), while most weeds are broad-leaved (dicotyledon) plants, and these groups have different enzymes, so herbicides can be targeted at just one group.
- Fungi are the most important plant pathogens, causing diseases like mildew, rusts and
blackspot and rotting produce in storage.
- Crop seeds are often treated with fungicides before sowing.
Biological Control of Pests
As an alternative to chemical pest control, pests can be controlled using other living organisms to keep the pest numbers down – biological pest control.
The organisms can be predators, parasites or pathogens, and the aim is to reduce the pest population to a levelwhere they don’t do much harm – the economic
A new equilibrium should be reached where the pest and predator numbers are both kept low.
Biological pest control works particularly well when the pest has been introduced to the ecosystem and has no natural predators.
Biological Control of Pests 2
The control species has to be chosen carefully, to ensure that it :
- Attacks the pest only and not other native species
- Will not itself become a pest due to lack of predators or parasite.
- Can survive in the new environment.
- Does not carry disease
Control species should be trialled in a quarantine area, such as a greenhouse, before being released into the wild.
If proper precautions are not taken, biological control can lead to ecological disaster.
Integrated Pest Management
Modern intensive farming recognises the environmental dangers of the untrammelled use of pesticides, so is adopting Integrated Pest Management(IPM).
IPM attempts to bring together all forms of pest management to benefit from the best of each.
The aim is to reduce the effect of pesticides on the environment butwithout compromising the goal of maximising crop yield. There are 4 stages, each one more powerful than the one before:
Identify the pests and their population density at which they cause economic harm – the economic threshold. Only take action against the pest if its population is above the threshold.
Use suitable cultural methods to prevent pests reaching their threshold.
Intergrated Pest Management
If the pest population starts to exceed threshold the use biological control to bring it down.
If biological control doesn’t work then use chemical pesticides, but at low and carefully controlled dose, and at the best time of year to minimise damage to other organisms while maximising effect on pest.
At each stage the effect of that treatment is evaluated before deciding to proceed to the next stage.