Crop Pests and Diseases and Their Control

Agricultural Science · WAEC and JAMB · SS2 and SS3

Examiners want precision here: the exact pest, the exact crop, the exact damage and the exact control. A candidate who writes insects attack crops and should be sprayed scores nothing, while a candidate who writes that the cassava mealybug sucks sap from the growing point causing bunchy top, and was controlled biologically by the parasitoid wasp introduced through IITA, scores the lot.

What you need to know

A pest is any organism that damages crops or stored produce and reduces their quantity, quality or value. Pests are grouped as insects, birds, rodents and other mammals, nematodes, mites and molluscs, and by stage of attack as field pests and storage pests.
Insects damage crops in three ways. Biting and chewing insects such as grasshoppers, armyworms and beetles remove leaf and stem tissue; piercing and sucking insects such as aphids, mealybugs and whiteflies drain sap and inject toxins; and boring insects such as stem borers and pod borers tunnel inside stems, pods and fruits.
The maize stem borer tunnels into the stalk, producing shot-hole damage on young leaves, dead heart in the growing point and stalks that break and lodge. Control by early planting, destruction of crop residues after harvest, crop rotation and the application of an approved granular insecticide into the leaf funnel.
The cassava mealybug sucks sap from the growing point, producing stunted bunchy top, leaf distortion and heavy yield loss. Nigeria's classic answer was biological control: the parasitoid wasp Apoanagyrus lopezi was introduced and spread through work coordinated at the International Institute of Tropical Agriculture, Ibadan, and it brought the pest under control across West Africa.
The variegated grasshopper defoliates cassava in the dry season; the cassava green mite causes mottled, reduced leaves; aphids and whiteflies transmit virus diseases. Cowpea suffers from the pod borer Maruca, the cowpea aphid, thrips in the flowers and the pod-sucking bug, which together can destroy an unprotected crop, so cowpea normally needs two to three sprays at flowering and podding.
The weaver bird, Quelea quelea, moves in enormous flocks and strips ripening rice, sorghum and millet in the north. Control by bird scaring, scarecrows, netting, synchronised planting so no single field is the only ripe one, early maturing varieties and the destruction of roosting and nesting colonies.
Rodents such as rats, cane rats and squirrels eat planted seed, growing tubers and stored grain, and they also gnaw bags and contaminate produce with droppings and urine. Control by trapping, baiting with approved rodenticide, clearing bush around the farm and store, and rat-proofing the store with metal guards on the legs.
Storage pests are the principal cause of post-harvest loss in Nigeria. The maize weevil and rice weevil bore into grain, the cowpea bruchid lays eggs on the seed coat so that larvae eat out the cotyledons, the larger grain borer reduces maize to dust, and the Angoumois grain moth attacks the germ. Control by drying to 12 to 13 per cent moisture, cleaning the store, using hermetic or triple layer bags, admixing with approved storage dust, and fumigating with aluminium phosphide only by a trained person.
Plant diseases are caused by fungi, bacteria, viruses and nematodes, and they are spread by wind, rain splash, insects, infected planting material, tools, water and human handling.
Important fungal diseases are black pod of cocoa caused by Phytophthora, which blackens and rots the pods in the wet season; rice blast, which gives diamond-shaped lesions on the leaf and neck rot that empties the panicle; maize rust and smut; groundnut leaf spot; yam anthracnose; and damping off of nursery seedlings, which collapses them at soil level.
Important bacterial diseases are cassava bacterial blight, which gives angular water-soaked leaf spots, wilting, gum exudation on the stem and die-back, and bacterial wilt of tomato, in which the plant wilts suddenly while the leaves remain green and a cut stem oozes a milky stream in water.
Important viral diseases are cassava mosaic disease, which gives yellow and green mottled, distorted leaves and heavily reduced tubers and is spread by whitefly and infected cuttings; maize streak virus, carried by leafhoppers, giving fine white streaks along the veins; groundnut rosette; and cocoa swollen shoot, which swells the shoots, mottles the leaves and kills the tree.
Cultural control is the backbone of pest and disease management in Nigeria because it costs little: crop rotation, field sanitation and destruction of crop residues, use of certified disease-free planting material such as IITA improved cassava varieties, resistant varieties, early and synchronised planting, correct spacing for air movement, roguing of infected plants, intercropping, and a closed season for cocoa.
Other control methods are physical and mechanical, such as hand picking, trapping, bird scaring, hot water seed treatment and solar drying; biological, using natural enemies such as the mealybug parasitoid, predatory ladybirds and neem-based botanical extracts; chemical, using insecticides, fungicides and nematicides at the recommended dose with observance of the pre-harvest interval; legislative, through quarantine and inspection of imported planting material; and integrated pest management, which combines all of these and uses chemicals only when the pest crosses an economic threshold.

Key terms

Pest: Any organism that attacks crops or stored produce and reduces their quantity, quality or market value.
Plant disease: A disorder of a plant caused by a pathogen such as a fungus, bacterium, virus or nematode, or by an adverse environmental factor, which disturbs its normal functioning.
Vector: An organism, usually an insect, that carries a disease-causing pathogen from an infected plant to a healthy one.
Biological control: The deliberate use of a pest's natural enemies, such as predators, parasitoids or pathogens, to reduce its population.
Integrated pest management: A control strategy that combines cultural, biological, physical and chemical methods, using pesticides only when the pest population reaches an economic threshold.
Roguing: The removal and destruction of diseased or off-type plants from a field to prevent the spread of infection.
Economic threshold: The pest population level at which the cost of control becomes less than the value of the crop that would otherwise be lost.

Worked examples

A maize farmer in Oyo State finds his crop with fine white streaks running along the veins of young leaves, with stunted plants that produce small cobs. Identify the disease, name the causal agent and the vector, and state three control measures.

Read the symptom carefully: fine continuous white or yellow streaks running parallel to the veins on young leaves point to maize streak, not to rust, which gives raised reddish-brown pustules.
Name the causal agent: maize streak virus.
Name the vector: leafhoppers of the genus Cicadulina, which transmit the virus as they feed.
Give control measures that attack both the virus and the vector: plant resistant or tolerant improved varieties from IITA or the National Cereals Research Institute; plant early and synchronise planting in the community so the crop escapes peak leafhopper numbers; destroy volunteer maize and grass weeds that host the vector; rogue and burn infected plants; and apply an approved insecticide against leafhoppers where infestation is heavy.
Note what will not work: fungicide is useless against a virus, so do not offer it.

Answer: Maize streak disease, caused by maize streak virus and transmitted by Cicadulina leafhoppers. Control by planting resistant varieties, early and synchronised planting, destruction of grass weeds and volunteer maize that host the vector, roguing of infected plants, and insecticide against the leafhopper where necessary.

State four ways of protecting stored maize grain from weevil attack without using a fumigant.

Start with the condition that makes attack possible: high moisture. Dry the grain thoroughly in the sun to about 12 to 13 per cent moisture before storage, since weevils and moulds thrive in damp grain.
Deal with carry-over infestation: clean and disinfect the store, remove and burn old infested grain and residues before the new harvest goes in.
Exclude the insect physically: store in hermetic or triple layer bags, or in sealed drums, so that the oxygen inside is used up and the insects die.
Reduce access and contamination: stack bags on pallets away from the walls, fit rat guards and keep the store dry and well ventilated.
Add a low-cost protectant: admix an approved storage dust, or traditional protectants such as wood ash or dried neem leaf powder, and store the maize on the cob in a ventilated crib where appropriate.

Answer: Dry grain to about 12 to 13 per cent moisture before storage; clean and disinfect the store and destroy old infested stock; use hermetic or triple layer bags or sealed drums; stack on pallets in a dry ventilated store with rat guards; and admix an approved storage dust or a botanical protectant such as neem leaf powder.

The mistake to avoid

Candidates confuse symptoms across diseases, offering rust for streak or fungicide for a virus disease. Match the control to the causal agent: viruses are controlled through resistant varieties, clean planting material and the vector, never with fungicide. The second trap is writing spray with insecticide as the answer to every pest, with no chemical named, no dosage mentioned and no cultural method offered, when WAEC allots most of the marks to cultural control.

In the exam

Answer pest and disease questions in the fixed order pest or disease, crop attacked, causal organism where relevant, symptoms of damage, then control measures grouped as cultural, biological and chemical. Use the proper symptom words, dead heart, bunchy top, mottling, die-back, water-soaked lesion, because the precise term scores where a general description does not. Keep three IITA or NCRI examples ready; named Nigerian institutions and improved varieties lift an average answer.