Primer lesson: crop pests

Pests are some of the most common crop problems. Crop pests can come in several different forms. A pest is any living organism that causes damage to a plant directly or indirectly by introducing pathogens. In this lesson, we will learn about the common pests, their descriptions and effects of the attack, and the methods used to control them.

Learning Objectives

By the end of this lesson, you should be able to:

Describe the meaning of a pest.
Three important definitions to categorize crop pests - General Equilibrium Position (GEP), Economic Injury Level (EIL), and Damage Boundary (DB)
Explain the classification of crop pests.
Describe common crop pests.
Explain the effects of the attack of different common pests.
Describe different control methods for different crop pests.

Crop plants are those plants that are grown specifically for human consumption or other use. Any type of creature that can damage a crop plant is called crop pest. Some of these pests may even destroy entire fields of crop plants in short periods.

1. Insects are the most common and destructive crop pests. They are found on the leaves, stems, and flowers of the plants. Many insects also eat the fruits or vegetables of crop plants, but some of these crop pests can destroy the plants before they are even able to produce any fruits or vegetables. Some insects will eat nearly any type of plant, while others only eat certain plants; cabbage worms, for instance, are often only seen in cabbage plants and other cole crops like broccoli and mustard plants. To control insects on crop plants, many gardeners and farmers use chemical pesticides. Many pesticides are also toxic, however, and some studies have shown that these substances can be dangerous, even in very small quantities. Other farmers use safer organic methods to get rid of these crop pests.

2. Animals - Several different animals can also be crop pests as well. Mice can often be found in cornfields, for instance, where they chew on corn stalks as well as the ears of corn. To get rid of mice, farmers can either trap them or poison them. Larger mammals, like raccoons and rabbits, might also be pests in many rural and urban vegetable gardens.

3. Birds can also cause an extensive amount of damage to some types of crops. They can damage plants in two ways, either by eating seeds and fruits or by hunting for insects hidden in the plants. Many birds, for example, crows, eat berries and seeds. A major area of concern for sunflower farmers is seed-eating birds. Scarecrows are sometimes effective for getting rid of crop pests like crows, but many birds are not frightened away by these stationary objects. Instead, most farmers hang CDs on strings since shiny moving objects are generally more effective bird deterrents.

Classification of Crop Pests

Crop pests can be classified according to the following criteria:

Mode of feeding or type of mouthparts

Examples include biting and chewing pests like locusts, crickets, grasshoppers, cutworms, bollworms, and armyworms. There are also piercing and sucking pests such as aphids, thrips, and mealybugs.

Part of crop attacked

Based on this criterion, there are grain or cereal feeders, root feeders, stem feeders, and leaf feeders.

Type of pest

Types of pests include insects, birds, rodents, and nematodes.

Stage of development of the pest

Pests attack crops at different stages of their life cycle. For example, moths and butterflies attack crops at their larva (caterpillar) stage while locusts attack crops at their adult stage.

Time of attack

Pests can attack crops when in the field or when in storage. Consequently, there are field pests and storage pests. Field pests include mites, maize stalk borers, nematodes, rodents, and birds. Storage pests include weevils, termites, and rats.

Some important definitions to remember

General equilibrium position (GEP) is the mean value of pest density around which the pest population tends to fluctuate as changes occur in the abiotic and abiotic components of the environment without accompanied by a permanent modification in the composition of the environment. A permanent modification of any component of the environment may alter the GEP.

The lowest level of injury where the damage can be measured is called the Damage boundary (DB) while the lowest number of insects that will cause economic damage is referred to as the Economic injury level (EIL). The EIL is also defined as a level of pest abundance or damage at which the cost of control equals the crop value gained from instituting the control procedure.

Categories of pests

Pests have been classified as key pests, occasional pests, potential pests, migratory pests, sporadic pests, and minor pests.

Key pests

They are those which are generally abundant or the kind of damage caused by the pest and the damage potential of a single individual insect is great. They are the most severe and damaging pests. The GEP lies well above DB and EIL. Human intervention in the form of control measures may bring the population temporarily below the EIL. However, it rises back rapidly and repeated interventions may be required to minimize damage. These pests present a persistent threat to the crops and are not being satisfactorily controlled with the available technology.

Major pests

They are those with the GEP close or equal to the EIL. Thus, the population crosses EIL quite frequently and repeated control measures are necessary, but economic damage is avoided by timely interventions.

Minor pests

They are those with the GEP below both EIL and DB. Under favorable environmental conditions, the population may cross EIL and DB for usually a short interval. These pests are easily manageable to available control measures and a single application of insecticides is usually enough to prevent economic damage.

Occasional pests or sporadic pests

They are those that cause economic damage only in certain places or at certain times. The population of these pests is usually negligible but in certain years under favorable environmental conditions, they appear in a virtually epidemic form crossing many times over DB and EIL. Under these conditions, the pest has to be controlled by undertaking suitable management strategies. These pests are highly sensitive to abiotic conditions and once the favorable season is over, only a residual population survives.

Potential pests

They are generally considered minor pests, cause no apparent injury to the crop under the prevailing situation. It has been termed as a potential pest because at certain times it may appear as a problem and maybe elevated to the position of key pests. Their GEP lies below the DB and does not cross EIL even under favorable conditions. Any change (cropping pattern, cultural practices) in the ecosystem may push their GEP higher and there is a danger of economic damage from these pests if control operations against the other categories of pests are undertaken in an indiscriminate manner.

Migratory pests

They are normally non-resident of any agro-ecosystem. This type of pest usually appear all of a sudden in crop area from places of their breeding, causes heavy damage to the crop, and again leaves the area. Their association with the agro-ecosystem is of transitory nature.

Integrated Pest Management (IPM)

There are four tactics under Integrated Pest Management (IPM)

1. Biological

This is a strategy when pests kill pests. Use of insecticides that contain microorganisms that are natural enemies of the targeted pest.

2. Cultural

These measures focus on reducing the pests' establishment and reproduction as well as their spread and survival. There are many practices that make the environment less favorable to pests. Examples include cultivation of alternate hosts, plant rotation, selection of planting sites, trap crops, and adjusting the timing of planting

3. Physical

These include hand-picking of pests, sticky boards, or tapes for control of flying insects in greenhouses and various trapping techniques such as traps for rodents.

4. Chemical

When all other methods fail, use synthetic pesticides sparingly. Find a spray that is safe for the air, soil, water, humans, and pets.