AgriGuide

Different types of pests

Crop pests are any organisms threatening the quality and yield of food and cash crops. Pests can be small mammals, such as rats and mice or birds. More often, these pests are small living organisms, such as insects, mites, nematodes (microscopically small worms) or snails. Microorganisms, such as fungi, bacteria and viruses, can also cause harmful plant diseases. Special mention goes to the higher plants, acting as weeds, which can also be classified as pests. However, the mere presence of these organisms on the farm does not qualify them as pests. In principle, they are not pests as long as they do not reach a threshold. Plants can be especially bothersome in one situation, but quite useful in another. Plants growing wild on a field are often weeds, but in another situation they can be a useful source of animal feed or compost. Seeds, bulbs or roots left on a field after a crop has been harvested can grow into bothersome weeds for the following crop.

Not all plants and animals found on the farm can develop into pests. All potential crop pests share the following characteristics:

• Can damage individual plants in a crop;
• Under favorable conditions can multiply very rapidly;
• Harm the farmer because the damage they cause reduces the yield or quality of the harvested product, or can only be controlled at great expense.

Pests which damage individual plants in a crop

Pests differ in the way they damage crop plants. Three groups of pests are presented as: insects, micro-organisms and weeds.

Insects
Insect pests either feed on plants or plant parts, or they pierce the plants and feed on their juices.

Micro-organisms
Micro-organisms can be a pest as they cause plant diseases. These are called disease-causing or pathogenic organisms. The symptoms of such diseases can include malformation, spots on the plants’ leaves, or rotting stems, fruit or roots.

Weeds
Most weeds are harmful as they compete with the crop plants for light, water and nutrients. This infestation slows down the crop’s growth. Some plants are considered to be weeds as they are parasitic. They live on the roots of plants and through direct contact, they extract nutrients and water from the plant. Other plants are weeds as they host pest insects or disease-causing microorganisms.

A common weed in African farms is Striga (Striga hermonthica), a parasitic weed of grass cereals, specifically sorghum and finger millet.

Most commonly occurring on low nitrogen-fertility soils and under low rainfall ecologies, as few as three plants per square meter can completely inhibit grain production. Attaching to the host root and transpiring at three times the normal rate, water and nutrients are shunted to the parasite. Striga also alters the hormone balance of the host, stimulating the crop to reduce shoot growth and extend root growth. Producing as high as 20,000 seeds per plant, and remaining viable in the soil for up to 20 years, Striga infested soils loose their productivity and become characterized by masses of purple flowers.

Crop damage

The pest organisms in an infestation can eventually have an impact on the yield and quality of a crop. The damage is felt by a farmer in the form of a smaller crop yield or a lower-quality product that will have to be sold at a lower price.

To prevent such damage, the farmer can take measures to control pests. But these measures cost money, so it is not a good idea to implement them automatically. The decision as to whether or not to take action has to be based on regular inspections of the crop. Weekly inspections will be sufficient in most cases.

The purpose of the inspections is to identify which pests and how many are present in the crop, and to determine whether they are increasing in number.