Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Pests and Diseases of Crops
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Pests and Diseases of Crops
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Subject: Biology
Class: Senior Secondary 2
Term: 1st Term
Week: 8
Theme: The Organism And Its Environment
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Identify pestsof certain cropsand in dicate the ircontrol. Describe the life cycles of some of the pests. Describesome 8 commondiseases causedby pests. Describe pestcontrol measures.
This section provides in-depth explanations of the core concepts related to pests and diseases of crops, crucial for effective teaching. 2.
1. Definition of Key Terms: Pest: An organism that causes damage or nuisance to humans, their crops, livestock, or property. In agriculture, pests are organisms that compete with, injure, or consume crops, leading to economic losses. Examples include insects, nematodes, mites, birds, rodents, and weeds.
Disease: Any abnormal condition of a plant that impairs its physiological functions, often caused by pathogenic organisms (e.g., fungi, bacteria, viruses) or environmental factors. Plant diseases lead to reduced yield, quality, and sometimes plant death.
Crop: Any plant or plant product grown and harvested for human use or for economic profit. Examples in Nigeria include maize, cassava, yam, rice, cowpea, cocoa, oil palm.
Pathogen: A biological agent (e.g., virus, bacterium, fungus, protozoan) that causes disease to its host.
Vector: An organism that transmits a pathogen from one host to another. Many insect pests act as vectors for plant viruses. 2.
2. Types of Crop Pests and Crops They Attack (PO1): Pests are diverse and cause damage in various ways (chewing, sucking, boring, defoliating, forming galls, etc.).
Insect Pests: Stem Borers (e.g., Busseola fusca, Mussidia nigrivenella): Larvae bore into the stems of cereals (maize, sorghum, millet, rice), hindering nutrient transport and causing lodging.
Maize Weevil (Sitophilus zeamais): Attacks stored maize, rice, and wheat. Larvae develop inside kernels, causing significant post-harvest losses. Aphids (e.g., Aphis fabae, Aphis gossypii): Small, soft-bodied insects that suck sap from leaves and stems of various crops (cowpea, okra, tomatoes, cotton). They also vector viral diseases. Locusts/Grasshoppers (e.g., Schistocerca gregaria, Zonocerus variegatus): Defoliate vast areas of various crops (cereals, vegetables, tubers) in swarms, causing total crop loss. Zonocerus variegatus (African variegated grasshopper) is a common pest of cassava, yam, and various vegetables in Nigeria.
Whiteflies (Bemisia tabaci): Small, winged insects that suck sap from undersides of leaves of various vegetables (tomatoes, cassava, sweet potatoes) and vector highly destructive viral diseases like Cassava Mosaic Disease. Fruit Flies (e.g., Bactrocera dorsalis - African invader fruit fly): Lay eggs in ripening fruits (mango, guava, citrus, pepper), larvae feed inside, causing fruit rot and spoilage. Leafhoppers (e.g., Cicadulina mbila): Suck sap from leaves of cereals (maize) and transmit viral diseases like Maize Streak Virus. Armyworms (e.g., Spodoptera frugiperda - Fall Armyworm): Larvae consume leaves and reproductive parts of cereals (maize, sorghum) and other crops, causing severe defoliation.
Nematode Pests: Root-knot Nematodes (Meloidogyne spp.): Microscopic roundworms that infect roots of various crops (yam, cassava, cowpea, vegetables, plantain), causing galls (swellings), stunted growth, and reduced yield.
Mammal Pests: Rodents (Rats, Mice, e.g., Rattus rattus, Mastomys natalensis): Consume grains in fields and stores (maize, rice), damage tubers (yam, cassava), and contaminate produce. Birds (e.g., Weaver birds, Village birds): Consume ripening grains (rice, sorghum, millet) in fields.
Monkeys: Damage and consume fruits (oil palm, plantain, banana, cocoa pods) in plantations.
Mollusc Pests: Snails and Slugs: Feed on leaves and young shoots of various vegetables (fluted pumpkin, okra, leafy greens), particularly in moist conditions. 2.
3. Life Cycles of Some Common Pests (PO2): Understanding the life cycle is crucial for effective pest control, as different stages may be vulnerable to different interventions.
1. Life Cycle of an Aphid (Incomplete Metamorphosis, often Asexual): Egg: Overwintering eggs are sometimes laid on host plants in temperate regions, but in tropical Nigeria, aphids often reproduce continuously without an egg stage.
Nymph: Eggs hatch into nymphs, which are miniature versions of adults but lack wings. Nymphs feed, grow, and moult several times (usually 4 instars).
Adult: Nymphs develop into adults. Adults can be wingless (apterous) or winged (alate). Winged adults facilitate dispersal to new host plants.
Reproduction: Aphids reproduce very rapidly, often asexually (parthenogenesis), where females produce live young without mating. This allows for rapid population buildup. Sexual reproduction (egg-laying) may occur under certain conditions. * Duration: The entire life cycle can be completed in as little as 7-10 days under favourable conditions, leading to many overlapping generations in a season. 2. *Life Cycle of Nymphs feed, grow, and moult several times (usually 4 instars).
Adult: Nymphs develop into adults. Adults can be wingless (apterous) or winged (alate). Winged adults facilitate dispersal to new host plants.
Reproduction: Aphids reproduce very rapidly, often asexually (parthenogenesis), where females produce live young without mating. This allows for rapid population buildup. Sexual reproduction (egg-laying) may occur under certain conditions.
Duration: The entire life cycle can be completed in as little as 7-10 days under favourable conditions, leading to many overlapping generations in a season.
2. Life Cycle of the Maize Weevil (Sitophilus zeamais) (Complete Metamorphosis): Egg: The adult female weevil chews a small hole in a grain kernel (e.g., maize, rice), lays a single egg inside, and seals the hole with a gelatinous plug.
Larva: The egg hatches into a white, legless grub-like larva that feeds entirely within the grain kernel. It undergoes several instars.
Pupa: After growing sufficiently, the larva pupates within the same grain kernel. The pupa is inactive and undergoes transformation.
Adult: The adult weevil emerges from the pupa, then chews its way out of the grain kernel, leaving a characteristic emergence hole. Adults feed on grain and can live for several months, laying hundreds of eggs.
Duration: The complete life cycle takes approximately 30-40 days under optimal conditions (warm, humid environment), leading to continuous generations in stored grains.
3. Life Cycle of a Locust (e.g., Desert Locust, Schistocerca gregaria) (Incomplete Metamorphosis): Egg: Adult female locusts lay eggs in pods in sandy soil, usually in moist areas. Each pod contains many eggs (tens to over a hundred).
Nymph (Hopper): Eggs hatch into nymphs, also called hoppers. Hoppers are wingless, smaller versions of adult locusts. They feed and grow, undergoing 5-6 moults (instars). As they grow, they may start to aggregate, forming bands.
Adult: After the final moult, the hopper becomes a winged adult. Adults feed, mature, and eventually mate.
Phases: Locusts exhibit two main phases: Solitary phase: Low population density, hoppers and adults behave individually.
Gregarious phase: High population density, hoppers form marching bands, and adults form swarms. This phase is triggered by environmental conditions and leads to rapid reproduction and migration.
Duration: The life cycle from egg to adult takes about 30-70 days, depending on temperature and food availability. Adults can live for several months. 2.
4. Common Diseases Caused by Pests / Vector-Borne Crop Diseases (PO3): This section focuses on significant crop diseases in Nigeria, particularly those transmitted or directly caused by pests.
1. Cassava Mosaic Disease (CMD): Causative Agent: Cassava Mosaic Virus (CMV), a geminivirus.
Pest Vector: Whiteflies (Bemisia tabaci).
Symptoms: Chlorosis (yellowing), mosaic patterns on leaves, stunted growth, distortion and malformation of leaves, reduced tuber yield. Highly prevalent in Nigeria.
2. Maize Streak Virus (MSV): Causative Agent: Maize Streak Virus, a geminivirus.
Pest Vector: Leafhoppers (e.g., Cicadulina mbila).
Symptoms: Characteristic yellow streaks along the veins of young leaves, stunting of plants, reduced cob formation and grain yield.
3. Cowpea Aphid-borne Mosaic Virus (CABMV): Causative Agent: Cowpea Aphid-borne Mosaic Virus, a potyvirus.
Pest Vector: Aphids (e.g., Aphis craccivora).
Symptoms: Mosaic patterns on leaves, leaf distortion, stunting, reduced pod and seed yield in cowpea.
4. Groundnut Rosette Disease (GRD): Causative Agent: Groundnut Rosette Virus (GRV) complex.
Pest Vector: Groundnut Aphid (Aphis craccivora).
Symptoms: Severe stunting, chlorosis (yellowing) or mosaic patterns on leaves, leaf distortion, reduced pod formation, and significant yield losses in groundnut.
5. Tomato Yellow Leaf Curl Virus (TYLCV): Causative Agent: Tomato Yellow Leaf Curl Virus, a geminivirus.
Pest Vector: Whiteflies (Bemisia tabaci).
Symptoms: Severe stunting of plants, upward curling and yellowing of leaf margins, reduced fruit set, and significant yield reduction in tomatoes.
6. Root-knot Nematode Disease: Causative Agent: Root-knot nematodes (Meloidogyne spp.), which are pests themselves.
Symptoms: Formation of characteristic galls or swellings on roots, stunted growth, wilting during hot periods, yellowing of leaves, reduced vigour, and poor yield in a wide range of crops (e.g., yam, cassava, vegetables, plantain).
7. Witches' Broom of Groundnut: * Causative Agent: A phytoplasma (a type a geminivirus.
Pest Vector: Whiteflies (Bemisia tabaci).
Symptoms: Severe stunting of plants, upward curling and yellowing of leaf margins, reduced fruit set, and significant yield reduction in tomatoes.
6. Root-knot Nematode Disease: Causative Agent: Root-knot nematodes (Meloidogyne spp.), which are pests themselves.
Symptoms: Formation of characteristic galls or swellings on roots, stunted growth, wilting during hot periods, yellowing of leaves, reduced vigour, and poor yield in a wide range of crops (e.g., yam, cassava, vegetables, plantain).
7. Witches' Broom of Groundnut: Causative Agent: A phytoplasma (a type of bacteria-like organism).
Pest Vector: Leafhoppers.
Symptoms: Proliferation of lateral shoots, giving a "witches' broom" appearance, small and yellow leaves, absence of flowers and pods, resulting in significant yield loss.
8. Banana Bunchy Top Disease (BBTD): Causative Agent: Banana Bunchy Top Virus (BBTV), a nanovirus.
Pest Vector: Banana Aphid (Pentalonia nigronervosa).
Symptoms: Dark green streaks on petioles and leaf veins, leaves becoming progressively smaller, stiff, and brittle, and bunching at the top of the plant. Infected plants do not produce fruit or produce small, malformed fruit. Highly destructive to banana and plantain. 2.
5. Pest Control Measures (PO4): Effective pest management relies on a combination of strategies, often integrated into an Integrated Pest Management (IPM) approach.
1. Cultural Control: Crop Rotation: Alternating different crops in a sequence to break the life cycles of pests and diseases specific to a particular crop. E.g., planting maize after yam to control maize-specific pests.
Sanitation/Field Hygiene: Removal and destruction of infected crop residues, weeds, and volunteer plants that can harbour pests and pathogens. E.g., burning maize stalks after harvest to kill stem borer larvae.
Timely Planting/Harvesting: Planting crops at optimal times to avoid peak pest populations or harvesting before significant damage occurs. E.g., early planting of maize to escape severe stem borer attacks.
Resistant/Tolerant Varieties: Using crop varieties bred or selected for their ability to resist or tolerate pest and disease attacks. E.g., CMD-resistant cassava varieties.
Trap Cropping: Planting a small area of a highly attractive crop to draw pests away from the main crop, then destroying the trap crop with the pests.
Intercropping/Mixed Cropping: Planting two or more crops together in the same field. This can confuse pests, provide habitat for natural enemies, or act as physical barriers. E.g., maize and cowpea.
Tillage: Ploughing and harrowing can expose soil-borne pests (e.g., some insect larvae, pupae, nematodes) to predators or adverse weather conditions.
2. Biological Control: Natural Enemies: Using beneficial organisms (predators, parasites, pathogens) to suppress pest populations.
Predators: Organisms that kill and consume pests (e.g., ladybirds eating aphids, spiders, birds).
Parasites/Parasitoids: Organisms that live on or in a host and eventually kill it (e.g., parasitic wasps laying eggs in caterpillars).
Pathogens: Microbes that cause disease in pests (e.g., Bacillus thuringiensis (Bt) bacteria for caterpillars, fungi for locusts).
Sterile Insect Technique (SIT): Releasing large numbers of laboratory-reared, sterile male insects (sterilized by radiation) into the wild population. When they mate with wild females, no offspring are produced, leading to population decline. E.g., used for fruit flies.
3. Chemical Control: Pesticides: Chemicals designed to kill, repel, or control pests.
Insecticides: Target insect pests.
Nematicides: Target nematodes.
Fungicides: Target fungal pathogens.
Herbicides: Target weeds (not pests in the strict animal sense, but part of crop protection).
Application Methods: Spraying, dusting, fumigation (for stored grains), seed treatment.
Types: Contact Pesticides: Kill pests upon direct contact.
Systemic Pesticides: Absorbed by the plant and translocated throughout its tissues, killing pests that feed on the plant.
Precautions: Use with caution due to potential hazards to humans, beneficial organisms, and the environment. Strict adherence to label instructions, use of protective gear, proper storage, and disposal are essential.
4. Physical/Mechanical Control: Handpicking: Manually removing larger pests (e.g., grasshoppers, caterpillars, snails) from crops.
Trapping: Using physical traps (e.g., sticky traps, pheromone traps, light traps, rodent traps) to capture and remove pests.
Netting/Barriers: Using nets or fences to prevent pests (birds, larger insects, rodents) from accessing crops.
Burning: Incinerating crop residues or entire fields on the plant.
Precautions: Use with caution due to potential hazards to humans, beneficial organisms, and the environment. Strict adherence to label instructions, use of protective gear, proper storage, and disposal are essential.
4. Physical/Mechanical Control: Handpicking: Manually removing larger pests (e.g., grasshoppers, caterpillars, snails) from crops.
Trapping: Using physical traps (e.g., sticky traps, pheromone traps, light traps, rodent traps) to capture and remove pests.
Netting/Barriers: Using nets or fences to prevent pests (birds, larger insects, rodents) from accessing crops.
Burning: Incinerating crop residues or entire fields to destroy pests, eggs, and pathogens (use with caution to avoid environmental damage).
Roguing: Uprooting and destroying individual diseased plants to prevent spread (e.g., for CMD-infected cassava).
5. Integrated Pest Management (IPM): A holistic and sustainable approach that combines various control methods (cultural, biological, chemical, physical) in a compatible and environmentally friendly manner.
Principles of IPM: Pest monitoring and scouting. Setting action thresholds (when to intervene). Emphasis on non-chemical methods first. Using pesticides as a last resort and judiciously. Protecting beneficial organisms.
Benefits: Reduces reliance on chemical pesticides, minimizes environmental impact, protects human health, and is economically viable for farmers in the long term.
This topic has profound real-life implications for individuals and the nation. Food Security and Economic Impact for Farmers: Students will understand that effective pest and disease control directly translates to higher crop yields, reduced post-harvest losses, and improved food availability. This is crucial for the livelihoods of millions of Nigerian farmers and for the nation's overall food security, especially with a growing population. For instance, controlling maize weevils in storage prevents grains from being destroyed, ensuring food supply and market stability.
Environmental and Health Awareness: The lesson highlights the double-edged sword of chemical pesticides. Students learn about the potential dangers of indiscriminate pesticide use – environmental pollution (soil and water contamination), harm to beneficial organisms (e.g., pollinators), and risks to human health (pesticide residues in food). This encourages a critical approach to agricultural practices and promotes safer alternatives like Integrated Pest Management (IPM), fostering environmentally conscious citizens. Career Opportunities and Sustainable Agriculture: Understanding pests and diseases opens up potential career paths in agriculture, entomology, plant pathology, agro-allied industries, and research (e.g., at institutions like the International Institute of Tropical Agriculture, IITA). Students gain an appreciation for sustainable agricultural practices, such as the development of pest-resistant crop varieties or biological control, which are vital for long-term productivity and ecological balance in Nigeria.