A weed is any plant that grows where it is not wanted. Weeds compete with crops for nutrients, water, sunlight, and space, which can reduce crop growth and yield.

Example: If a cowpea plant grows in a cassava field, it is considered a weed in that field.

• Annual Weeds: Complete their life cycle (germinate, grow, produce seeds, die) within one year or one cropping season.


• Perennial Weeds: Live for more than one year. They may regrow from underground parts such as tubers, rhizomes, or roots.

1. Produce many seeds and spread easily.


2. Grow very fast and compete strongly with crops.


3. Can survive in poor soil and adverse conditions.


4. Seeds can remain dormant for long periods before germinating.


5. Can spread by wind, water, animals, or farm activities.

1. Guinea grass (Panicum maximum)


2. Bahama grass (Cynodon dactylon)


3. Elephant grass (Pennisetum purpureum)


4. Spear grass (Imperata cylindrica)


5. Goat weed (Ageratum conyzoides)


6. Water leaf (Talinum triangulare, when growing in crop fields)

1. Compete with crops for nutrients, water, light, and space.


2. Make cultivation, planting, and harvesting more difficult.


3. Act as hosts for pests and diseases.


4. Reduce crop yield and quality.


5. Increase the cost of production.

Note: Some weeds can be useful as animal feed, mulch, or medicine, but they are mostly harmful in crop production.

Methods of Weed Control

1. CULTURAL CONTROL

Cultural control involves using farm management practices to prevent or reduce weed growth naturally.

Techniques include:

1. Mulching: Covering the soil with dry grasses, leaves, or crop residues to block sunlight and prevent weeds from growing.


2. Crop Rotation: Growing different crops in succession to break the life cycle of weeds that prefer a particular crop.


3. Flooding: Drowning weeds in crops like rice, which weeds cannot survive in waterlogged conditions.


4. Cover Cropping: Planting fast-growing crops like legumes to occupy soil space and reduce weed establishment.


5. Close Spacing of Crops: Planting crops close together to shade the soil and reduce space for weeds.


6. Using Clean Seeds: Ensuring seeds are free from weed seeds to prevent introducing new weeds into the farm.

2. MECHANICAL / PHYSICAL CONTROL

Mechanical control uses physical removal or destruction of weeds.

Techniques include:

1. Hand Weeding: Manually pulling out weeds; effective for small farms or gardens.


2. Hoeing / Uprooting: Using tools like hoes or cutlasses to remove weeds and disturb the soil to kill seedlings.


3. Ploughing or Tillage: Turning the soil with a plough or tractor to uproot weeds and expose them to sunlight, killing them.

3. CHEMICAL CONTROL

Chemical control uses herbicides to kill or inhibit weed growth.

Details:

1. Pre-emergence Herbicides: Applied before weeds germinate to prevent growth.


2. Post-emergence Herbicides: Applied after weeds have grown to kill them directly.


3. Advantages: Fast and effective for large infestations.


4. Disadvantages: May harm crops if misapplied; can pollute soil and water; may affect beneficial organisms.

4. BIOLOGICAL CONTROL

Biological control involves using natural enemies of weeds to reduce their population.

Examples:

1. Certain insects that feed on specific weeds.


2. Grazing animals like goats that eat weeds.


3. Microorganisms that attack weeds.

Advantages: Environmentally friendly and sustainable.

Disadvantages: Slower effect; careful management is needed to avoid harm to crops.

5. PREVENTIVE CONTROL

Preventive control focuses on stopping weeds from appearing or spreading.

Techniques include:

1. Using clean, weed-free seeds for planting.


2. Removing weeds from surrounding areas before they spread.


3. Maintaining proper farm hygiene.


4. Quarantine measures to prevent the introduction of new weed species.

6. INTEGRATED WEED MANAGEMENT (IWM)

Integrated Weed Management combines two or more control methods to maximize effectiveness and sustainability.

Example:

1. Using crop rotation, mulching, and manual weeding together.


2. Reduces the need for chemical herbicides.


3. Environmentally safe and long-lasting.

1. Chemical control: May pollute soil and water or harm beneficial organisms.


2. Mechanical control: Overuse of machinery can damage soil structure.


3. Burning: May destroy soil organisms and reduce soil fertility.


4. Biological control: Predators may attack crops if not carefully managed.

A pest is any living organism (animal, insect, or microorganism) that damages crops, farm animals, or stored produce.

Pests reduce crop yield and quality, increase production costs, and may spread diseases.

1. INSECT PESTS

Insects are the most common pests in agriculture. They damage crops in different ways depending on their feeding habits.

(a) Chewing Insects

Chewing insects feed on leaves, stems, fruits, or flowers of plants. They have strong mandibles (jaws) to cut and chew plant tissues.

Examples:

• Caterpillars – feed on leaves of maize, tomatoes, and cabbage.


• Grasshoppers – eat leaves and stems of crops like millet, maize, and vegetables.


• Locusts – form swarms and destroy large areas of crops.

Effects on crops:

• Remove leaves, reducing photosynthesis.


• Damage stems or flowers, which can reduce yield.


• Severe infestations can destroy entire fields.

(b) Piercing and Sucking Insects

These insects have specialized mouthparts to pierce plant tissues and suck sap. They usually weaken the plant over time and can transmit diseases.

Examples:

• Aphids – suck sap from stems and leaves; transmit viral diseases.


• Whiteflies – feed on the underside of leaves; produce honeydew that encourages mold growth.


• Mealybugs – feed on sap and damage fruits and shoots.

Effects on crops:

• Yellowing and curling of leaves.


• Stunted growth due to loss of nutrients.


• Transmission of plant diseases from infected to healthy plants.

(c) Boring Insects

These insects bore into stems, roots, fruits, or seeds. Their larvae usually live inside plant tissues, making control difficult.

Examples:

• Maize stem borer – larvae bore into maize stems, killing growing points.


• Weevils – attack stored grains, seeds, and sometimes roots.

Effects on crops:

• Wilting and stunted growth.


• Direct loss of yield as fruits, seeds, or stems are destroyed.


• Plants may fall over due to weakened stems.

2. NON-INSECT PESTS

These are pests that are not insects but still damage crops or stored produce. They include animals and other organisms.

(a) Vertebrate Pests

Vertebrates are animals with backbones that feed on crops.

Examples:

• Rats – eat seeds, grains, and seedlings in the field or in storage.


• Birds – consume seeds and fruits, especially rice and maize.


• Monkeys – raid farms and damage fruits and vegetables.

Effects on crops:

• Direct loss of crops before or after harvest.


• Contamination of stored grains with droppings or urine.


• Can spread diseases to humans or livestock indirectly.

(b) Invertebrate Pests

These are animals without backbones that damage plants.

Examples:

• Nematodes – microscopic worms that attack roots and stunt growth.


• Slugs and snails – feed on leaves, seedlings, and fruits.


• Mites – suck plant sap and damage leaves, reducing photosynthesis.

Effects on crops:

• Reduced root development leading to poor nutrient and water uptake.


• Deformation of leaves and fruits.


• Stunted growth and reduced yields.

3. MICROBIAL PESTS

These are microorganisms that damage plants by causing diseases.

Examples:

• Fungi – cause leaf spots, rust, mildew, and rots.


• Bacteria – cause wilts, blights, and soft rots.


• Viruses – stunt plant growth, distort leaves, or reduce fruit quality.

Effects on crops:

• Wilting and yellowing of plants.


• Rotting of fruits, roots, and seeds.


• Overall reduced yield and poor quality produce.

1. Reproduce quickly and in large numbers.


2. Have the ability to move to new areas and infest other crops.


3. Prefer specific crops or plant species, but some are general feeders.


4. Cause both direct damage (eating crops) and indirect damage (spreading diseases).

1. Direct damage to plant parts: Pests eat or destroy leaves, stems, roots, flowers, and fruits of crops.


2. Reduces photosynthesis: When pests eat leaves, the plant cannot make enough food, so growth slows down.


3. Stunts plant growth: Pests damage stems or roots, making plants weak and short.


4. Reduces the amount of harvest: Damaged plants produce fewer crops.


5. Lowers crop quality: Fruits, seeds, or grains may get holes, spots, or rot, making them less useful or unsellable.


6. Spreads plant diseases: Some pests carry viruses, bacteria, or fungi that infect healthy plants.


7. Causes post-harvest losses: Pests like weevils or rodents eat or spoil stored grains after harvest.


8. Increases production costs: Farmers spend more money on pesticides and pest control.


9. Can destroy entire crops: Severe pest attacks can kill all the plants in a field.

1. Use of pest-resistant crop varieties to prevent infestation.


2. Crop rotation to break pest life cycles and reduce population build-up.


3. Hand-picking or mechanical removal of visible pests.


4. Biological control using natural predators like ladybugs, birds, or parasitic wasps.


5. Chemical control using pesticides like insecticides, rodenticides, and acaricides.


6. Preventive measures such as proper storage, farm hygiene, and quarantine to prevent pest spread.

1. Eating or destroying plant parts (leaves, stems, roots).


2. Piercing or sucking sap, allowing entry for diseases.


3. Reducing yield and quality of crops.


4. Transferring diseases from infected to healthy plants.

1. Chemicals may pollute soil and water or kill beneficial organisms.


2. Biological control may affect non-target species.


3. Mechanical control can be labor-intensive and less effective for large infestations.


4. Cultural methods alone may be insufficient if pest populations are high.

Combining multiple methods (cultural, mechanical, biological, chemical) is more effective and sustainable than using a single method.

1. Both weeds and pests compete with crops and reduce yield.


2. Many control methods are similar (cultural, mechanical, chemical, biological).


3. Early intervention is important in both cases.


4. Integrated management methods are more effective and environmentally safe.

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REVISION QUESTIONS

1. Define a weed and a pest.


2. List four characteristics of weeds.


3. Give five examples of weeds in Nigeria.


4. Explain four methods of controlling weeds.


5. State three harmful effects of weeds on crops.


6. Classify pests and give examples.


7. Describe the damage caused by insect pests.


8. Explain three methods of pest control.


9. What are the possible side effects of chemical control on the environment?


10. Why is it important to combine different control methods for weeds and pests?