The diagrams below illustrate pests of crops. Study them and answer the questions that follow. (a) Name each of the pests labelled I, II and III. (b) Descri...

How to read the diagrams. Each drawing shows the diagnostic body plan of a crop pest. Match the visible features (mouthparts, legs, wings, body shape, feathers) to the correct group before answering.

(a) Names of the pests labelled I, II and III

(The pest labelled IV is a grain-feeding bird such as the weaver bird or quelea.)

(b) Life cycle of the pest labelled I (grasshopper)

The grasshopper undergoes incomplete metamorphosis (paurometabolous development) with only three stages: egg, nymph and adult. There is no pupa.

1. Egg stage: The fertilised female uses her ovipositor to lay batches of eggs in a pod inside moist soil. The eggs remain dormant, often through the dry season, before hatching.
2. Nymph (hopper) stage: Each egg hatches into a nymph that closely resembles the adult but is smaller, wingless and unable to reproduce. The nymph feeds on plant leaves and grows by a series of moults (ecdysis), passing through several instars. Wing buds enlarge at each moult.
3. Adult stage: After the final moult the insect emerges with fully developed wings and functional reproductive organs. It flies, feeds, mates and the females lay eggs, so the cycle begins again.

Summary: \[ \text{Egg} \;\rightarrow\; \text{Nymph (several instars)} \;\rightarrow\; \text{Adult} \]

(c) Four damages caused by the pest labelled III (shield/sucking bug) on crops

1. It pierces leaves and stems with its needle-like mouthparts and sucks the sap, causing wilting, yellowing and stunted growth.
2. It feeds on developing fruits, pods and seeds, causing them to become deformed, shrivelled or to drop prematurely.
3. It injects toxic saliva at the feeding points, producing necrotic (dead, discoloured) spots and lesions on the plant tissue.
4. It acts as a vector, transmitting viral, bacterial and fungal diseases from infected to healthy plants, thereby lowering yield and the market quality of produce.

(d) Four ways of controlling the pest labelled IV (grain-feeding bird)

1. Scaring: Use scarecrows, reflective/noise-making bird scarers, or employ people to drive the birds away, especially at grain-filling stage.
2. Netting/covering: Cover maturing crops (for example seedbeds and cereal heads) with nets to physically exclude the birds.
3. Destroying nesting and roosting sites: Locate and destroy the birds' breeding colonies, nests and roosts to reduce their population.
4. Trapping or shooting: Set traps or shoot the birds where the law permits; alternatively grow bird-resistant varieties and practise uniform, early planting so the crop matures together and is exposed for a shorter period.