3 A farmer harvests 500 kg of maize grain and stores it in a traditional granary made from wooden poles and a grass roof. After three months, the farmer ope...

Labelled answer diagram:

(a) Two signs of insect attack in stored grain:

1. Small round holes (exit holes) visible on the surface of individual grains, where adult insects have emerged after completing their development inside the kernel. [1]
2. Fine powdery dust (frass) found at the bottom of bags or containers, produced as insects feed on and bore through the grain. [1]

Other acceptable signs: presence of live or dead insects; webbing on the grain surface (from grain moths); unpleasant musty smell; grain feels warm to the touch due to insect metabolic activity.

(b) Two common insects that attack stored grain:

1. Maize weevil (Sitophilus zeamais), one of the most destructive pests of stored maize in tropical regions. [1]
2. Larger grain borer (Prostephanus truncatus), which bores into grain and can cause very heavy losses. [1]

Other acceptable answers: rice weevil (Sitophilus oryzae), flour beetle (Tribolium spp.), grain moth (Sitotroga cerealella).

(c) Why grain must be dried to low moisture before storage:

1. High moisture encourages mould (fungal) growth and creates warm, humid conditions inside the grain bulk that favour rapid insect breeding and egg development. [1]
2. Grain dried to below about 13% moisture content is too dry for insect eggs to develop successfully and the rate of grain deterioration (through fungal decay, heating and germination) is greatly reduced. [1]

(d) Two physical methods to protect stored grain from insect attack:

1. Use hermetic (airtight) storage bags or containers: sealing the grain removes oxygen and builds up carbon dioxide from the respiration of any insects present, suffocating them without chemicals. [1]
2. Sun drying and heat treatment: exposing grain to direct sunlight on a clean surface raises the temperature enough to kill insect eggs and larvae within the grain. [1]

Other acceptable methods: sieving and winnowing to physically remove insects; mixing grain with inert dust (ash, sand) to block air spaces and abrade insect cuticles; raising storage containers off the ground.

(e)

Advantage of fumigants: they can kill all life stages of insects (eggs, larvae, pupae and adults), including those hidden deep inside the grain kernels, and can treat large quantities of grain quickly. [1]

Disadvantage of fumigants: they are highly toxic to humans and animals and require careful handling with specialised equipment and training; improper use can cause serious poisoning. [1]

Other disadvantages: they leave no residual protection so grain can be re-infested after treatment; they may leave chemical residues on the grain.

(f) Two reasons rodents are a serious problem in grain stores:

1. Rodents eat and contaminate large quantities of grain with their droppings and urine, making it unfit for human consumption and causing significant economic losses. [1]
2. They damage storage containers and building structures by gnawing through sacks, wood and even thin metal, creating entry points for further infestation. [1]

Other acceptable answers: rodents carry and spread diseases (e.g. leptospirosis, salmonellosis); they reproduce rapidly so a small infestation can grow quickly into a large one.