Pest of Crops – SS2 Agricultural Science Lesson Note (Second Term)

A crop pest is any organism that is capable of causing damage to a crop.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define a crop pest and list the types of crop pests;
2. classify insect pests and give examples of each class;
3. describe the damage caused by, and the control of, important pests of major crops;
4. state the economic importance of insect pests;
5. describe the four methods of pest control and their side effects.

What are the types of crop pests?

There are six classes of crop pests: insects, birds, rodents, monkeys, man and nematodes.

• Insects: the largest group, e.g. grasshoppers, termites, aphids and stem borers.
• Birds: feed on grains and seedlings, e.g. weaver birds and quelea birds on rice and millet.
• Rodents: gnaw seeds, stems and tubers, e.g. rats and grasscutters.
• Monkeys: eat and carry away fruits, pods and maize cobs.
• Man: damages crops through theft, careless handling, trampling and bush burning.
• Nematodes: microscopic roundworms that feed on plant roots and cause galls and poor growth.

How are insect pests classified?

Insect pests are classified into three groups by how they feed: biting and chewing, piercing and sucking, and burrowing.

Biting and chewing insects

• Feature: strong mandibles and maxillae (mouth parts) bite and chew plant parts.
• Examples: termites, grasshoppers, leafworms, mantids, locusts and beetles.

Piercing and sucking insects

• Feature: a strong mouth part called the proboscis pierces plants and sucks out liquid materials.
• Examples: aphids, cotton stainers, mealybugs, scale insects, capsids, mirids and whiteflies.

Burrowing insects

• Feature: the insects and their larvae bore into the tissue of plant parts, fruits or seeds.
• Examples: bean beetles, stem borers, maize weevils and rice weevils.

What are the important pests of major crops?

Each major crop has its own important pests, which cause different damage and need different control.

What is the economic importance of insect pests in crop production?

Insect pests cause economic loss to farmers because they reduce the yield, quality and market value of crops.

• Field damage: they destroy crops by biting, chewing, boring, sucking and defoliation.
• Storage loss: they reduce the viability of stored produce.
• Disease entry: spot injuries predispose crops to pathogen attack.
• Disease vectors: some pests carry disease from plant to plant.
• Higher cost: money must be spent on controlling them.
• Poor appearance: they make vegetables and fruits unattractive and unmarketable.
• Lower quality: they reduce the quality of produce in the store or in the field.
• Lower yield: they generally reduce the yield of crops.
• Crop death: they can cause the total death of crop plants.
• Reduced profit: the farmer earns less from the harvest.

What are the methods of pest control?

There are four methods of pest control: physical, cultural, biological and chemical.

Physical control

Physical control is the direct removal or keeping out of pests by hand or by simple devices.

• Hand-picking: collecting insects and larvae by hand.
• Trapping: setting traps to catch rodents.
• Shooting: killing rodents with a gun.
• Fencing: putting wire nets around the farm.
• Scarecrows: frightening birds away from the crops.

Cultural control

Cultural control is the use of normal farm practices to prevent or reduce pest attack.

• Crop rotation: growing different crops in turn so that pests lose their host.
• Resistant varieties: planting crop varieties that pests do not easily damage.
• Tillage: appropriate tillage operations expose or bury pests.
• Burning crop residues: destroys pests that remain on the farm.
• Timely planting: crops grow well before pests build up.
• Weeding and sanitation: removes weeds that shelter pests.
• Timely harvesting: crops are removed before heavy pest attack.
• Close season: no living plant is allowed on the land for a certain period.

Biological control

Biological control is the introduction of natural enemies of pests to keep the pest population low.

• How it works: the natural enemies eat or feed on the pests and reduce their numbers.
• Example: ladybird beetles feed on aphids.

Chemical control

Chemical control is the use of chemicals called pesticides to kill or control pests of crop plants.

What are the forms and modes of action of insecticides?

Insecticides come in four forms: powder, liquid, granules and gas.

• Powder: a dry dust applied to plants, setts or stored produce.
• Liquid: a spray mixed with water and applied to the crop.
• Granules: small solid pellets applied to the soil or the plant.
• Gas: a vapour used in closed spaces such as stores.

Insecticides kill pests in four ways, called modes of action:

• Contact: kills the pest when it touches the chemical.
• Systemic: the plant absorbs the chemical, and pests die when they feed on its sap.
• Stomach (ingestion): kills the pest when it swallows the chemical with plant parts.
• Fumigation: a gas that the pest breathes in and that kills it.

What are the side effects of pest control methods?

Each pest control method has side effects that farmers must manage.

Chemical control

• Beneficial organisms: kills some beneficial insects and soil organisms.
• Poisoning: exposes animals and man to toxic chemicals.
• Residue: leaves chemical residue in the environment.
• Water pollution: chemicals wash into water bodies and harm aquatic life.

Biological control

• Crop attack: the introduced organisms may attack cultivated crops or stored grains.
• Wrong target: predators may not feed on the target pests and may feed on beneficial organisms.
• Imbalance: the new organisms may upset the balance of the ecosystem.

Cultural control

• Bush fire: burning may get out of hand and spread to other farms.
• Soil damage: burning destroys soil structure and causes loss of organic matter.
• Microbe loss: burning kills beneficial microbes.

Evaluation

1. Define a crop pest and list six classes of crop pests.
2. Name the three classes of insect pests and give two examples of each.
3. State five ways in which insect pests are of economic importance.
4. Describe the four methods of pest control.
5. State two side effects of chemical control.

Assignment

Choose any three pests from the table of important pests of major crops. For each one, state the crop attacked, the nature of the damage and two ways of preventing or controlling it.