Definition of Pest
A pest is defined as any biological factor (weed, pathogens, mammals, insects) in nature that possesses harmful attributes that interfere with man and his interests (crop, fibre, livestock, etc.) negatively within a given period and results in economic damage thus justifying the application of a control measure.
There are five key factors an organism must fulfil before it can be termed a pest:
- Biological factor;
- Possesses/expresses harmful attributes;
- Interferes with man and his interests negatively;
- Time period; and
- Economic damage.
Broadly speaking, a pest is any organism that is in competition with humans for some resources. In agriculture, groups of organisms can function as pests, e.g. plants, arthropods (insects, mites), fungi, bacteria, nematodes, viruses, viroids, mycoplasma- like organisms, rodents, birds and other vertebrates.
Pest Control Strategies
The business of production of crops and domestication of animals without proper protection of crops from pest attack will be a futile effort. Pests can reduce plant density, cause stunted growth or even death of plants, lower production capacity and consequently reduce the yield and/or quality of agricultural products.
Protection measures do not increase yield rather they remove/reduce constraints (pests) to crop production so that crops can produce at their optimum potentials. Various methods used in crop protection are cultural control, biological control, host plant resistance, chemical (agro-chemicals), etc. We will consider some of these in this chapter.
Pest control measures can be classified according to their basic nature, this list is not exhaustive, but it serves as a good introduction to a non-major agriculture student. They include:
- Physical methods of control;
- Cultural control;
- Host plant resistance to pest attack;
- Biological control;
- Chemical control;
- Organic farming; and
- Integrated pest management.
Physical Methods of Control
Physical methods deal with mechanical removal or destruction of the pest.
- Mechanical: Hand picking is a profitable method for the removal of some caterpillars from young fruit tree. Armyworms on the move can often be trapped in trenches dug across their line of marching and when trapped they are easily destroyed by burning, filling the trench with soil or spraying with chemicals. Placing of bags around large fruits to deter fruit flies from oviposition is another mechanical control. Another mechanical method is the protection of seed beds of vegetables in a cage of plastic mesh e.g. (0.5 ha). The fine netting will keep insects as small as aphids at bay.
- Use of Physical Factors: This is the use of lethal temperature either high or low for insect pest destruction. Use of cool storage in insulated stores for grain is practiced in Asia and Africa. The purpose is for the drastic reduction of development of the insects following the reduction of the metabolic rate.
- Hot Water Treatment: dipping of plant bulbs in hot water that will not destroy the plant material but will kill mites, fly larvae or nematodes.
- Hermetic (Airtight) Storage: The principle involved is that only a small quantity of air is enclosed within the sealed bin, oxygen is quickly used up by the respiration of the pests and subsequently carbon-dioxide accumulation resulting in the death of all contained pests, both arthropods and microbial. Triple bagging of cowpea grain called Purdue Improved Cowpea Storage (PICS) is a current technology practised by cowpea producers in northern Nigeria.
Cultural Control
These are regular farm operations designed to destroy pests or to prevent them from causing economic damage.
- Optimal Growing Conditions: A healthy plant growing vigorously has considerable natural tolerance to pests and diseases, both physically and physiologically. Damage by pests is more serious if the plant is suffering from water stress (drought), unfavourable temperature, imbalance in nutrients or nutrient deficiency. When crops are grown on marginal lands it predisposes the crop to pest attack.
Time of Sowing: By sowing early (or sometimes late) it may be possible to avoid the egg-laying period of a pest, or else the vulnerable stage in the plant growth may have passed the time the insect numbers have reached high levels. Early sowing is regularly practiced against sorghum midge, Contarina sorghicola on sorghum.
Deep Sowing (Planting): Some seeds are less liable to damage and pest attack if rooted deep, but if planted too deep germination may be impaired. Many root crops are also less liable to attack by pests if they are deeper in the soil. Sweet potato tubers buried deep in the soil always have fewer weevils (Cylas spp.) boring into them.
Time of Harvesting: Prompt harvesting of maize and cowpea may prevent these crops from becoming infested by maize weevil (Sitophilus zeamais) and cowpea beetle (Callosobruchus maculatus), respectively. Both pests infest the field crops from neighbouring stores. Cultivation of maize or cowpea fields far away from the nearest grain store can be an added precaution.
Early maturing varieties may enable a crop to be harvested early, before pest damage is serious. The principle underlying this is that crop development is desynchronized in relation to pest population development.
Close Season Factor: It is employed to prevent population build-up of an insect pest. All plants must be uprooted and destroyed (burnt) by a certain tain date and no seed should be planted until the following rains arrive. Uprooting and destroying plant residues help to reduce the survival of diapausing larvae which could re-infest new crop planted.
Fallow: Allowing a field to lie fallow almost invariably reduces pest populations but care must be taken to ensure that there are no volunteer crop plants or important secondary host weed species around the area. Fallowing may be done as bare fallowing when the soil surface is left bare, although this can lead to soil erosion, or flood fallowing when the field is flooded with water for a while. Also an alternative cover crop of legumes is grown as green manure which is then ploughed under.
Crop Rotation: Continuous cultivation of one crop over a long period of time depletes the minerals and trace elements in the soil and also induces pest build-up. Crop rotation separates crops both spatially and in time. However crop rotation is not effective against migratory pests or those with effective powers of dispersal. Alternation of cereals with non-cereals may be an important method for curtailing weevils in Africa.
Crop Sanitation:
Destruction of diseased or badly damaged plants in horticultural crops such as fruit orchards, flowers and some vegetables, destruction of infested branches can help to remove the loci of infestation.
Removal and destruction of rubbish-old crop remnants, fallen leaves, branches, dead trunks and also weeds. Some pests use rubbish heaps for breeding purposes. Larvae of Oryctes beetles are found in rotting palm trunks and rubbish heaps.
Removal and destruction of fallen fruits help in the control of many fruit flies and boring caterpillars because insects will continue to develop in the fallen fruits and pupate either there or in the soil.
Host Plant Resistance to Pest Attack
Resistance to pest attack is characterised by the resistant plants having a lower pest population density or fewer damage symptoms, than the other plants which are termed susceptible. Varietal resistance to pests is broadly classified into three categories: non-preference (antixenosis), antibiosis and tolerance. These categories are known as mechanisms of resistance.
Non-preference (Antixenosis): It is displayed when an insect either refuses to feed on the plant, or takes only very small amounts of food, or when an ovipositing female insect refrains from laying eggs on the plant.
Antibiosis: In this case, the plant resists insect attack and has an adverse effect on the bionomics of the pest by causing the death of the insects or decreasing their rate of development or reproduction. The resistant plants are generally characterised by anatomical features such as thick cell wall, hairy stems and leaves, thickened stem (cereals), compactness of the panicle in sorghum, tightness of husk in maize, and the tightness of leaf sheath in rice. Also, it could be biochemical components such as presence of various toxic or distasteful chemicals in the plant sap or a feeding stimulant which stimulates the insect to feed is missing or in suboptimal levels in the resistant plants.
Tolerance: It is the term used when host plants suffer little actual damage in spite of supporting a sizeable insect pest population that will severely damage a susceptible plant of the same species. Such resistant plants show compensatory growth and they heal quickly.
Biological Control
Biological control includes all types of control involving the use of living organisms, predators, parasites and disease-causing pathogens.
Predators: These animals prey and feed on insects (Mammals, Aves, Reptilia, Arachnida, Acari, etc.). Also some insects are good predators examples are in the Order Odonata (dragonflies), Mantidae (preying mantis), some Diptera (trueflies), Hymenoptera (parasitic wasps, ants), Hemiptera (true bugs) and Coleoptera (beetles).
Parasites: They belong to two large orders (Diptera and Hymenoptera) and one small order (Strepsiptera) and entomophilic nematodes. They lay their eggs on (ectoparasites) or within (endoparasites) their hosts.
Pathogens: Control by pathogens is often referred to as microbial control. Three main groups that are concerned are bacteria, fungi and viruses. Several species of Bacillus are specific to caterpillars or beetle larvae. Fungi are responsible for producing antibiotics and these acts directly as killing agents or inhibitors of growth and reproduction.
Chemical Control
Chemical control method is very quick in action, and for the majority of pest outbreaks, chemical control remains the method by which the surest and most predictable results are obtained. Insecticides can be grouped based on the mode of action and chemical composition.
The Modes of Action of Insecticides are:
Repellants: They are designed to keep insects away. usually employed against mosquitoes, example odomus, a mosquito repellent, treated bed nets, etc.
Antifeedants: These chemicals block part of the feeding responses in some phytophagous insects.
Fumigants: These are volatile substances that vapourise and the toxic gases kill insects within enclosed containers.
Stomach Poisons: They have to be ingested to be toxic, either sprayed on to foliage (for foliage feeders) or mixed with a bait to encourage ingestion. Stomach poisons act on the Central Nervous System (CNS).
Contact Poisons: This is usually absorbed directly through the cuticle. When the insecticide touches the insect body, it enters through the respiratory holes of the insect and travels to the Central nervous system.
Systemic Poisons: It is applied to the soil and after watering, the root takes its up in the plant system or sprayed on plants, or applied to the trunk. The poisons are absorbed and translocated by the plant and effective against sap-sucking insects.
Classification of Agrochemicals (according to their chemical composition):
Organochlorines (OC): DDT, ABHC
Organophosphates (OP): malathion, parathion, endosulphan, fenitothion, dichlorvos
Carbamates: carbaryl, aldicarb
Pyrethroids: Cypermethrin/cymbush, lambda- cyhalothrin (Karate)
Growth regulators: Juvenile hormones (methoprene); chitin synthesis inhibitors, etc.
Neonicotinoids: imidacloprid, nitempyram, acetamiprid, thiamethoxam,
Toxicity of Insecticides
Insecticides are toxic or lethal to insects and warm blooded animals including man. Agrochemicals should be kept away from children. They also constitute danger to the machine operators and consumers of treated produce.
There are two types of toxicity: Chronic and Acute toxicity.
Chronic Toxicity: This is the effect of small non-lethal dose received over a long time which results in cancer or brain damage.
Acute Toxicity (AT): Immediate poisonous effect of a single dose resulting in nervous symptoms. This takes effect immediately. AT is measured in LD50, i.e. the lethal dose that will kill 50% of the population. LDso is defined as the amount of insecticide in milligrams/kg weight of animal that kills half of a population of the test animal (female white rat or guinea pig). The higher the LDso value, the less toxic the insecticide. Two values commonly quoted for toxicity are: Acute oral toxicity (AOT) - nose and mouth; Acute dermal toxicity (ADT) - skin.
AOT: This is a measure of the amount of chemical to be swallowed or ingested to cause death.
ADT: This is the amount of insecticide absorbed through the skin to cause death.
The LD50 value is written on agrochemical packets.
Side Effects of Insecticides
Resistance to Insecticides: One of the undesirable side effects of insecticide usage is the development of insect resistance to insecticides. Resistance to insecticides is the development in a strain of insect to tolerate doses of toxicants which would be lethal to a majority of individuals in a normal population of the same species. That is, the insect will adjust to the insecticide and so, will not be killed by it.
Leaves toxic residues on food and forage crops.
Rapid resurgence of treated insect species.
Killing non-target organisms such as pollinators (bees) or natural enemies.
Phytotoxicity: continuous use of insecticides or higher concentrations causes phytotoxicity.
Equipment Used for Applying Insecticides
Hand sprayers
Knapsack sprayers (pneumatic - pressure retaining type & Hydraulic knapsac - non-pressure retaining)
Power driven sprayers: Tractor driven and Aircraft sprayers.
Sprayers have three component parts in common - tank, pump, spray bar and nozzle.
Natural Pesticides (Botanicals)
Botanicals are derived from natural plants and used as insecticides. The following are examples of such: Nicotine from Tobacco, Pyrethrum from Chrysanthemum sp., Rotenone derived from Derris (Leguminous plant), Azadirachtin from Neem (Azadirachta indica). Their mode of action is contact and out of all of these pyrethrum is widely used particularly for control of household, industrial and stored product pests.
Characteristics of pyrethrum include high vulnerability and susceptibility to photo decomposition, high cost, low mammalian oral toxicity, and rapid insecticidal action. Synthetic pyrethriods are Lambda-cyhalothrin (Karate, Laraforce), Raid, etc.
Integrated Pest Management (IPM)
In 1967, the FAO panel of experts on integrated pest control defined pest control as "a pest management system that utilises all suitable techniques and methods in a compatible manner as possible and maintains the pest population at levels below those causing economic injury." This definition incorporates the concept of pest management.
Originally integrated control generally referred to the modification of insecticidal control in order to protect and enhance the activities of beneficial insects. However, the definition of pest management connotes a broader ecological basis and a wider variety of options in devising solutions to pest problems.
Pest management is therefore defined as the reduction of pest problems by actions selected after the life-systems of the pests are understood and the ecological as well as economic consequences of these actions have been predicted, as accurately as possible, to be in the best interests of mankind.
The IPM concept involves three important conceptual bases:
First, IPM is all about integration. That is, it uses a combination of different strategies to achieve pest control. Second, pest control is not about elimination, it is about management. We can tolerate some damage and in fact we might need certain level of the pest in the field. The economic injury level is the level over which IPM should be implemented. Any injury below that level can be tolerated. This injury level is determined based on the crop. Third, preserving the environment by reducing insecticide use. Reducing the amount of chemicals in the environment not only reduces contamination but also preserves biodiversity.
Organic Farming
Due to increasing concerns on chemical contaminants that we eat daily as part of our overall diet, as well as water contamination and air pollution there is now a recent development worldwide in organic farming and chemical-free produce. The main feature of organic farming is the absence of pesticides and some synthetic fertilisers in crop production.
Organic farming (OF) operates on four principles: health, ecology, fairness and care. These principles encompass sustaining and enhances the health of soil, plants, animals, humans and consequently the health of the planet. Healthy soil and a diversified natural environment lead to healthy crops.
When a plant is healthy it can resist, tolerate physiological disruption and damage from pests. Organic farming basically employs natural control mechanisms, habitat management and non-reliance on externally sourced inputs (fertilisers, pesticides, etc.) to prevent pest damage to crops.
Pests of Selected Agricultural Crops
Staple crops in sub-Saharan Africa were selected from the major groups of Legumes (cowpea), cereals (maize), roots and tubers (cassava) and fruits and vegetables.
Cowpea, Vigna unguiculata
Cowpea is attacked by insect pests at every stage of its life cycle. During the vegetative stage of cowpea, aphids (Aphis craccivora) and leafhoppers (Empoasca dolichi) suck the sap in the leaves, while foliage beetles (Ootheca mutabilis) defoliate the leaves.
At the pre-flowering/flowering/early podding stage, flower thrips (Megalurothrips sjostedti) attack the reproductive stages causing browning, malformation, abscission, while cowpea pod borer, (Maruca vitrata) causes premature development or abscission of pods. A complex of pod sucking bugs feed on the developing seeds of cowpea. This results in and wrinkled seeds and pods, as well as empty pods without seeds.
Control with chemicals can be achieved by applying Monocrotophos-Nuvacron, Cypermethrin-Cymbush, Dimethoate-Rogor. In addition, PICS bags are currently used to store cowpea as a form of cultural control.
Cowpea is attacked by many diseases caused by viruses, bacteria, fungi and nematodes. These pathogens attack the underground parts, leading to pre- and post-emergence death, damping-off, seedling and leaf blight, root rot, sunken stem, leaf spot, and leaf rot. Fungicides are used in the control of these pathogens.
Cassava, Manihot esculentus
Variegated grasshopper, Zonocerus variegatus, is a leaf feeder on cassava, however when infestation is high, they also debark cassava stem. The nymph and adult stages are the pest damaging stages of Z. variegatus.
The cassava green mite causes chlorosis of cassava leaves and "candle-stick" appearance. Cassava mealybug is another main pest of cassava which sucks sap from cassava leaves and gives the plant a "bunchy top" appearance. If infestation is high, it defoliates the plants. Cassava pests can be controlled by treating stem cuttings at planting with Rogor, monocrotophos to control mealy bugs and mites.
Other chemicals that can be applied to the plant are Fenitrothion 0.1% wt/vol. a.i.; carbamates, Pyrethroids and Cypermethrin (cymbush) at recom- mended doses by the manufacturers.
Maize, Zea Mays
Maize is attacked by
Stem borers: Busseola fusca, Sesamia calamistis
Corn earworm: Helicoverpa zea, Mussidia nigrivenella
Armyworms: Spodoptera exempta, S. frugiperda
Maize weevil, Sitophilus zeamais.
Stem borer larva feeding creates patches on leaf epidermis called the 'window pane effect'. As a result of feeding in the stem, their activities interfere with translocation of metabolites in the plant and consequently the terminal shoot dies. This symptom is known as 'dead heart'.
Armyworms are defoliators on maize leaves, their reduces the maize plants to a rag (torn leaves) when there is heavy infestations (fig. 5.17). Consequently, the plant becomes stunted and produces few or no ears of corn. In the store, maize is attacked by Sitophilus zeamais.
Fruits and Vegetables
There are two invasive pests of fruits and vegetables that have ravaged horticultural crops. Most fruits and vegetables are attacked by different species of fruit flies but the most devastating is the invasive species, Bactrocera dorsalis, Oriental fruit fly.
It causes enormous yield loss to different types of commercial and wild fruits and vegetables especially mango, guava, citrus, papaya, cashew, pepper. Female fruit flies their ovipositor (egg-laying devise) to puncture the skin of a healthy fruit into which the eggs are laid. Larvae hatch from the eggs and feed on the fleshy part of the fruit, thereby cause fruit to rot.
Chemical application is the main method used in controlling fruit flies. Also some farms, bag fruits on trees with paper bags to prevent the female fruit fly from laying eggs. This is a form of cultural control practise that offers some measures of control. Also male annihilation is a current technique in controlling fruit flies.
A new invasive species, tomato leaf miner, Tuta absoluta, whose larvae lives and feed in the mesophyll layer of tomato is currently a threat to tomato production in Nigeria. They reduce the photosynthetic layer of tomato leaves as they tunnel into tomato fruits.