(a) A maize farmer noticed that stem borers had damaged several rows of crops in one section of the field. (i) State two signs that a farmer would observe o...

(a)(i) Two signs of stem borer attack on maize [2]:

1. Wilting of the growing point (dead heart) - the central shoot dies because the larva has tunnelled into the stem and destroyed the growing tissue from inside. [1]
2. Holes visible in the stem with frass (sawdust-like material) at the entry points, produced as the larva bores through the stem tissue. [1]

Also acceptable: tunnelling visible when the stem is split open.

(a)(ii) Two cultural methods to reduce stem borer numbers (method + explanation) [4]:

1. Remove and burn crop residues after harvest. Stem borer pupae shelter inside old maize stalks during the off-season. Destroying the residues by burning eliminates this refuge and breaks the pest's life cycle before the next season. [2]
2. Plant early so that the crop passes through its most vulnerable growth stage (whorl stage) before the peak emergence of adult stem borer moths. By the time moths lay eggs, the stems are tougher and less susceptible to damage. [2]

Also acceptable: intercrop maize with a non-host crop such as beans (push-pull strategy) to disrupt pest movement between plants.

(b) Why rotating maize with a legume reduces pest populations [3]:

1. Legumes are not a host plant for stem borers, so the pest cannot feed, develop or reproduce during the legume season. The life cycle is interrupted. [1]
2. The break in continuous maize cropping reduces the carry-over population of borers from one season to the next. Without a host crop, larvae and pupae remaining in the field perish. [1]
3. Legumes also fix nitrogen and improve soil fertility, producing a more vigorous maize crop in the following season. A healthier plant can tolerate some pest damage without suffering significant yield loss. [1]