Methods of Weed and Pest Control – JSS1 Agricultural Science Lesson Note (Second Term)

A weed is a plant that grows where it is not wanted. A pest is a living organism, plant or animal, that damages cultivated crops, farm animals or humans. This note describes the characteristics, uses and classification of weeds, and the types of insect pests by feeding habit and location.

Behavioural objectives

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

• define a weed and a pest;
• state the characteristics of weeds;
• list the uses of weeds;
• classify weeds by life cycle and give examples of each class;
• classify insect pests by feeding habit and by location, with examples.

What is a weed?

A weed is a plant that grows in a place where it is not wanted, for example a vegetable plant growing on a cassava plot.

Characteristics of weeds

1. High reproductive capacity: Weeds are always the first to emerge and grow when the necessary conditions are provided. They have many seeds. For example, a single grass head may produce thousands of seeds.
2. High resistance capacity: Weeds can withstand adverse conditions because of their tough protective seed coats, for example the seeds of some legumes.
3. Ability to regenerate: Some weeds can regenerate and are therefore very difficult to eradicate. Commelina species, elephant grass and spear grass are examples.
4. Easy dispersal: Weed seeds have various devices for dispersal. These include spines, hooks, parachutes of hair, and wind, with which they are easily dispersed, for example tridax, goat weed and desmodium.
5. Highly competitive and aggressive growth habit: Weeds can easily smother crops. For example, spear grass and elephant grass can easily overrun a farm if not regularly checked.

Uses of weeds

1. Food: Some weeds are used as food for people, for example African spinach (Amaranthus caudatus) and Corchorus olitorius.
2. Animal feed: Some weeds are used for feeding farm animals, for example guinea grass, sedge and elephant grass.
3. Medicine: Many weeds can be used as medicine. Drugs can be extracted from such weeds, for example lemon grass.
4. Erosion control: Some weeds are used to control light erosion, for example Bahama grass.
5. Source of ornamental plants: Some weeds are sources of ornamental crops, for example pride of Barbados, canna lily, rose, croton and hibiscus.

How are weeds classified by life cycle?

Weeds are grouped into three classes: annual, biennial and perennial weeds.

Annual weeds

These are weeds that complete their life cycle in one year. They grow, reach maturity and die within a year. Annual weeds include tridax, goat weed and spigelia.

Biennial weeds

These are weeds that complete their life cycle in two years. The first year is used by the weeds to grow and store food. Reproduction and maturity take place in the second year. Examples are wild carrot and yellow rocket.

Perennial weeds

These are weeds that take more than two years to complete their life cycle. Examples include giant star grass, guinea grass, carpet grass, centrosema and tropical kudzu.

What are some common weeds?

What is a pest?

A pest is a living organism, plant or animal, that can cause damage to our cultivated crop plants, farm animals and humans. Examples of common pests include insects such as grasshoppers and beetles, mammals such as rats and monkeys, and birds such as weaver birds.

How are insect pests classified by feeding habit?

According to their mode of feeding, insect pests are classified into three groups:

1. Biting and chewing insect pests
2. Piercing and sucking insect pests
3. Boring insect pests

Biting and chewing insect pests

Insect pests in this group have mouth parts made for biting and chewing plants. The mouth parts consist of a pair of very hard biting jaws called mandibles, a second pair of jaws called maxillae, a flat upper lip called the labrum, and a lower lip called the labium.

The mandibles are used to bite off and chew parts of the crop. The maxillae are used to hold the food and push it into the mouth. Examples are crickets, grasshoppers, beetles, termites, armyworms and mantises.

Piercing and sucking insect pests

The insects in this group have mouth parts shaped like injection needles, called the proboscis. Their mouth parts are adapted to pierce into plant parts and suck the sap and juice of plants. Examples include aphids, cotton stainers, mealybugs, white flies and capsids.

Boring insect pests

These are insects that bore holes into plant tissues and seeds. They are also called burrowing insects. Examples are bean beetles and weevils such as the rice weevil and the maize weevil.

How are insect pests classified by location?

Based on location, insect pests are classified into two groups: field insect pests and storage insect pests.

Field insect pests

These are insects that attack crop plants on the field, that is, before harvesting. Important field pests include:

1. Stem borers
2. Fruit and seed feeders: examples are fruit beetles, fruit flies and fruit piercing moths.
3. Root feeders: examples are yam beetles.

Storage insect pests

This group of insects attacks harvested products in the store. Examples include bean beetles and weevils such as the rice weevil and the maize weevil.

Evaluation

1. Define a weed and give one example.
2. State three characteristics of weeds.
3. Name the three classes of weeds based on life cycle and give one example of each.
4. List the three groups of insect pests according to their feeding habit.
5. Name the two groups of insect pests based on location and give one example of each.

Assignment

1. Write the common names and botanical names of five weeds from the table.
2. Describe the mouth parts of biting and chewing insect pests and give three examples of such pests.