CARA'S TIPSCrop pests, diseases and weeds and their control
JAMBWAECNECO
What the syllabus asks
JAMB: B12 crop pests, B13 crop diseases, B14 weeds
WAEC: G Crop protection: insect pests (classification, life cycles, damage, control), other pests (nematodes, birds, rodents, monkeys), crop diseases (fungal, bacterial, viral; causes, symptoms, transmission, control), storage pests, weeds (characteristics, classification, effects, control), pesticides and safety; Practical C2, C7 (identification of pests, diseased specimens and weeds)
NECO: G Crop protection: insect pests (classification, life cycles, damage, control), other pests (nematodes, birds, rodents, monkeys), crop diseases (fungal, bacterial, viral; causes, symptoms, transmission, control), storage pests, weeds (characteristics, classification, effects, control), pesticides and safety; Practical C2, C7 (identification of pests, diseased specimens and weeds)
What you will learn
- Define pest and classify insect pests by mouthparts and life cycle.
- Describe damage and control of major field and storage pests of Nigerian crops.
- Describe non-insect pests: nematodes, birds, rodents, monkeys.
- Name causal organisms, symptoms, transmission and control of major crop diseases.
- Explain the methods of pest and disease control, including IPM.
- Define weeds, state their characteristics and effects.
- Describe methods of weed control including herbicide types.
- State precautions in pesticide use.
Key points
- Pests: insects, mites, nematodes, birds, rodents, monkeys, snails.
- Chewing insects: grasshoppers, beetles, caterpillars, termites; sucking: aphids, whiteflies, mirids, mealybugs, cotton stainers.
- Complete metamorphosis: egg–larva–pupa–adult; incomplete: egg–nymph–adult.
- Storage pests: weevils, bruchids, grain moths, larger grain borer, rats; control by drying, cleaning, airtight storage, approved protectants.
- Disease agents: fungi, bacteria, viruses, nematodes; viruses spread by vectors and infected material.
- Maize streak–leafhopper; cassava mosaic–whitefly; groundnut rosette–aphid; swollen shoot–mealybug.
- Black pod of cocoa: Phytophthora; remove pods, copper fungicides, prune.
- Control methods: cultural, physical, biological, chemical, resistant varieties, quarantine; IPM combines them.
- Biological control example: Anagyrus lopezi wasp against cassava mealybug.
- Pesticide safety: label, protective clothing, wind, storage, pre-harvest interval, container disposal.
- Weeds: many dormant seeds, vegetative spread, fast growth; compete, harbour pests, raise costs.
- Spear grass (rhizomes), nut sedge, Siam weed, Striga (parasitic), water hyacinth (aquatic).
- Herbicides: selective/non-selective, contact/systemic, pre-plant/pre-emergence/post-emergence.
- Aflatoxin from Aspergillus flavus: dry and sort produce promptly.
Start here: Pests: meaning, classification and life cycles
Each season farmers lose part of their harvest to pests in the field and in store. A pest is any organism that damages crops or stored produce and causes economic loss. Pests include insects, mites, nematodes, birds (quelea, village weaver), rodents (cane rats/grasscutters, rats), monkeys, snails and even wild pigs.
Classification of insect pests
By mouthparts:
- Biting and chewing insects cut and swallow plant tissue: grasshoppers, locusts, beetles (yam beetle), weevils, caterpillars (larvae of moths such as stem borers and armyworms), termites.
- Piercing and sucking insects pierce tissue and suck sap: aphids, whiteflies, mirids (capsids), cotton stainers, mealybugs, leafhoppers, pod-sucking bugs.
By place of attack: field pests (stem borers, aphids) and storage pests (weevils, bruchids, grain moths). By part attacked: root pests (termites, nematodes), stem borers, leaf feeders (grasshoppers), flower and pod pests (thrips, Maruca), fruit pests (fruit flies).
Life cycles
| Complete metamorphosis | Incomplete metamorphosis |
|---|---|
| Egg → larva → pupa → adult | Egg → nymph → adult |
| Larva looks different from adult | Nymph resembles a small wingless adult |
| Examples: moths and butterflies (stem borers, armyworm), beetles and weevils, flies | Examples: grasshoppers, locusts, aphids, mirids, cotton stainers, termites |
| Usually the larva does the damage | Both nymph and adult feed and damage |
Damage caused by pests
- Reduce yield by eating leaves, stems, roots, flowers or fruits.
- Lower quality and market value (holes, stains, frass).
- Transmit diseases (vectors), e.g. whitefly spreads cassava mosaic virus.
- Create wounds through which pathogens enter.
- Increase production cost through control measures.
- Destroy stored produce, causing food shortage.
Why is it important to know the mouthparts of an insect pest before choosing an insecticide?
Chewing insects eat plant tissue, so stomach poisons and contact insecticides sprayed on the surface reach them. Sucking insects take sap from inside the tissue, so a surface stomach poison will not reach them; a systemic or contact insecticide is needed. Matching the insecticide to the mouthparts makes control effective and avoids waste.
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Practice questions
- grasshopper
- stem borer
- yam beetle
- bruchid
Show answer
A: grasshopper
Egg → nymph → adult.
- grasshoppers
- beetles
- caterpillars
- aphids
Show answer
D: aphids
They suck plant sap and may spread viruses.
- mealybugs
- beetles
- whiteflies
- aphids
Show answer
C: whiteflies
Infected cuttings also spread it.
- grasshoppers
- mealybugs
- whiteflies
- mirids
Show answer
B: mealybugs
Mirids (capsids) cause direct damage.
- aphids
- whiteflies
- termites
- weevils
Show answer
A: aphids
Aphis craccivora is the vector.
Key words
- Pest
- Any organism that damages crops or stored produce and causes economic loss.
- Vector
- An organism, usually an insect, that carries a disease agent from one plant to another.
- Pathogen
- An organism that causes disease, e.g. a fungus, bacterium or virus.
- Metamorphosis
- Change of form during an insect’s development (complete or incomplete).
- Larva
- The immature, feeding stage of insects with complete metamorphosis, e.g. a caterpillar or grub.
- Nymph
- The immature stage of insects with incomplete metamorphosis; resembles a small wingless adult.
- Systemic pesticide
- A pesticide absorbed and carried in the plant sap, killing sucking insects that feed on it.
- Contact pesticide
- A pesticide that kills on contact with the pest’s body.
- Stomach poison
- A pesticide eaten with plant tissue by chewing insects.
- Fumigant
- A pesticide used as a gas, e.g. for stored grain.
- Biological control
- Use of natural enemies (predators, parasites, pathogens) to reduce pest numbers.
- Integrated pest management (IPM)
- Combining several compatible control methods to keep pests below damaging levels with minimal pesticide use.
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