TEST OF PRACTICAL KNOWLEDGE QUESTION (a) ldentify specimen E. Specimen E; (b) State four damages caused to crops by the specimen. (c) Briefly describe its l...

Answer Details

(a) Identity of Specimen E

Specimen E is a caterpillar (larva of a butterfly or moth, order Lepidoptera). It can be identified from the diagram by its long, soft, cylindrical, segmented body coloured green with black transverse bands and rows of yellow-orange spots, a distinct head capsule with chewing (biting) mouthparts, three pairs of true (jointed) legs on the thorax, and several pairs of fleshy prolegs on the abdomen. A common crop-attacking example is the larva of the African armyworm or the diamond-back / cabbage caterpillar.

(b) Four damages caused to crops by the specimen

1. It bites and chews the leaves, creating holes and reducing the leaf area available for photosynthesis.
2. It defoliates the crop completely in heavy infestations, so that only the stalks and midribs remain.
3. It bores into and feeds on fruits, pods, cobs and growing shoots, lowering yield and quality.
4. The wounds and faeces it leaves behind create entry points for fungal and bacterial diseases and contaminate the produce, reducing its market value.

(c) Life cycle

The specimen undergoes complete metamorphosis (holometabolous development) with four distinct stages:

\[ \text{Egg} \;\rightarrow\; \text{Larva (caterpillar)} \;\rightarrow\; \text{Pupa (chrysalis)} \;\rightarrow\; \text{Adult (butterfly/moth)} \]

The adult female lays clusters of eggs on the underside of leaves. The eggs hatch into the larva (the caterpillar shown), which feeds voraciously on plant tissue and moults several times as it grows. The mature larva stops feeding and turns into a pupa (chrysalis), often protected in soil, plant debris or a silken cocoon. Inside the pupa the tissues are reorganised, and an adult winged butterfly or moth finally emerges. The adults mate and the female lays eggs again, completing the cycle.

(d)(i) Stage most dangerous to crops

The larval (caterpillar) stage is the most dangerous to crops.

(d)(ii) Reason

It is the actively feeding stage: the caterpillar possesses strong biting and chewing mouthparts and feeds continuously and greedily on leaves, shoots and fruits, causing the direct destruction of crops. (The adult butterfly/moth feeds only on nectar and does no feeding damage.)

(e) Three control measures

1. Chemical control: spray the crop with an appropriate contact or stomach insecticide (for example a carbamate or pyrethroid) while the larvae are young.
2. Cultural control: practise crop rotation, early planting, deep ploughing to expose pupae, and destruction of crop residues and alternative weed hosts; hand-pick and destroy the caterpillars and egg masses in small plots.
3. Biological control: encourage or introduce natural enemies such as parasitic wasps, predatory birds and the bacterium Bacillus thuringiensis to attack the larvae.