Crop Protection: Weeds, Insect Pests and Diseases
What this domain tests
Identification and management of weeds, insect pests and diseases, integrated pest management, economic thresholds, herbicide selectivity and pesticide safety.
Core rules
- Weeds are classified by life cycle (annual, biennial, perennial), morphology (grassy, broad-leaved, sedge) and habitat. The critical period of crop–weed competition is generally the first 30–45 days, and uncontrolled weeds commonly cut yield by 20–50%.
- Herbicides: selective (2,4-D in wheat, pendimethalin as pre-emergence) versus non-selective (glyphosate, paraquat); pre-emergence, post-emergence and soil-applied types. Rotate modes of action to delay resistance.
- Insect pests: chewing (armyworm, stem borer, bollworm larvae) versus sucking (aphid, jassid, whitefly, thrips). Scouting, light traps, pheromone traps and ETL-based spraying drive decisions; biological agents include Trichogramma, Chrysoperla and NPV.
- Diseases follow the disease triangle - susceptible host, virulent pathogen and favourable environment. Seed-borne, soil-borne, air-borne and vector-transmitted diseases need different tactics: seed treatment, resistant varieties, sanitation and fungicide rotation.
- Safety: read the label, wear protective equipment, respect the pre-harvest interval, never spray against the wind and dispose of empty containers safely.
Worked example
Maize plants show ragged whorl holes with moist sawdust-like frass, and larvae carry an inverted Y mark on the head. Reasoning: damage is inside the whorl, frass is moist and granular and the Y mark is present - this is fall armyworm, not stem borer, which causes a dead heart with dry frass. Action: destroy egg masses and larvae in small fields, install pheromone traps, spray only when about 10% of whorls are damaged, direct the spray into the whorl, and prefer recommended bio-pesticides at early stages.
Traps
Spraying a contact insecticide for a sucking pest sheltered under the leaf; treating uniform nutrient yellowing as a disease; applying a pre-emergence herbicide after weeds have emerged; and confusing the economic threshold level with the economic injury level.