Arthropods comprise most of the known animal species and about 800,000 of the 900,000 or so species of arthropods are insects. The others include Crustacean, Arachnida, Chilopoda, and Diploda.Insects are characterized by several easily recognized traits that set them apart from any other group of organisms. Like other members of the Phylum Arthropoda (“jointed foot”) insects possess an external skeleton, or exoskeleton, which encases their internal organs, supporting them and protecting them. An insect’s skeleton is on the outside of its body and is called an exoskeleton. It serves as a support for muscles and internal organs as well as a covering. The body wall of an insect has three layers. The outer layer is cuticle containing chitin, proteins and often pigments. This is secreted by a cellular layer beneath it called the epidermis. The inner layer is called the basement membrane. As the insect’s rigid exoskeleton cannot expand. This is secreted by a cellular layer beneath it called the epidermis. The inner layer is called the basement membrane.
Crops Insect pests
- Pests are usually defined in terms of the degree and importance of crop damage or loss. most often, the definition of a pest depends on individual and the prevailing conditions.
- A pest is any animal or plant which harms or causes damage to man, his animals, crops or his possessions, or even just causes him annoyance.
- A pest is any organism detrimental to man, whether it is an insect, disease organism, weed, rodent, or other.
- A pest is any form of animal or pathological agent injurious or potentially injurious to plant or plant products, livestock or man.
- An organism is a pest when the level of damage it causes is sufficient to warrant control measures.
- Pest is an all encompassing word that includes fungi, bacteria, insect, viruses, phytoplasmas, nematodes, mollusks, vertebrates, weeds and parasitic flowering plants (striga).
- Insects belong to phylum Arthropoda. Arthropods form a major group in the animal kingdom and constitute one of the largest and most important pests of crop plants.
- It is a group of animals with segmented bodies and hard skin
- Jointed appendages
Class Insecta have the following characteristics
- Have one pair of antennae
- Tracheal respiration
- Three pairs of true legs
- Three body regions (head, thorax(pro, meso and metathorax) and abdomen)
- Examples of organisms belonging to this class are moths and butterflies (Lepidoptera), grasshopper (orthoptera), bee (hymenoptera), beetle (coleoptera), bugs (hemiptera), aphids (homoptera), fruit flies and leaf miners (diptera) and thrips (thysanoptera)
Ways in which insects cause damage to plants
- They chew leaves, stems, bark or fruits of plants e.g. grasshoppers, beetles, caterpillars and slugs.
- They suck sap from leaves, buds, stem or fruits e.g. aphids, leafhoppers, whiteflies, scales, thrips, and mites
- They bore and tunnel into bark, stems, twigs, wood, fruits, nuts and seeds e.g. leaf miner, weevils, twig borers, root borers and caterpillars.
- They cause galls and abnormal growth on plants e.g. gall wasp and mites
- They attack the roots of plants in an of the above ways
- They lay eggs in plant tissues e.g. katylids and fruit flies
- They take plant parts for nest or shelter e.g. leaf cutting bees, some ants and bagworms
- They carry other harmful insects to plants
- They are vectors of plant diseases e.g. aphids, leafhoppers and thrips
Insects cause injury or annoyance to people and animals
- Can cause annoyance by their presence, buzzing, foul odors and excretions on foods e.g. flies and mosquitoes
- Infest fruits
- Biting, stinging e.g. mosquitoes, fleas, wasps, bees and bed bugs
- Enter the eyes ear and nose
- Lay eggs on skin, hair, and feathers
- Apply venom by biting, stinging or hairs
- Leave caustic body fluids or irritants when crushed
- Cause allergies
- Can be poisonous if swallowed
- Make their homes on or in the body as parasites, injuring the host
- Transmit disease organisms or create unsanitary conditions e.g. mosquitoes, ticks and fleas
- Damages to products and structures e.g. ticks
Categories of crop pests
- Pests are categorized according to several factors including their abundance, damage caused, etc. this is very important in determining which organisms are pests
- Very often, the degree of seriousness of damage is related to numbers. However, there are exceptions, e.g. disease transmitting organisms, which the effect of organism on crop is not directly proportional to numbers or in special quality products where a slight contamination may lead to serious financial loss
- In spite of this, the concept of economic threshold based on population levels of organism, or level of incidence of a disease, is still the most acceptable in categorizing pests
- Economic threshold is the population density at which control measures should be applied to prevent an increasing pest population from reaching economic injury level or population level of the organism or level of disease incidence above which economically significant damage or loss is caused, and below which damage or loss is negligible or the population level above it will pay the farmer to control his pests and below which is uneconomical.
- The concept of economic threshold is based on the fact that organisms over a long period of time and in a relatively undisturbed environment reach a state of equilibrium with their environment.
- This is a dynamic state of equilibrium, which means that although population densities vary from season to season, year to year or place to place, for a particular place, there is an average population level which is reasonably stable over a long period of time.
- The economic threshold and economic injury level (the lowest population density that will cause economic damage or injury that will justify the cost of artificial control measures) are usually above this average population level.
- These levels are not constant for any pest, disease or environment, but they can be worked out from an intimate knowledge of the organisms, the crops which they are attacking and other components of the environment. Economic damage is the amount of injury that will justify the cost of artificial control measures
- On the basis of the concept of economic threshold and depending on the severity of damage caused, the number of organisms involved, frequency of occurrence and the prevailing conditions, pests are categorized as follows:
- a) Key pests (major pests, regular pests)
- These are perennial pests which cause serious and persistent economic damage in the absence of effective control measures
- The population of the damaging stage stamp above the economic injury level
- E.g. the variegated grasshopper zonocerus variegates is a key pest of cassava, vegetables, citrus and many cultivated crops in west Africa.
- Maruca testulalis, the cowpea borer is a major pest of cowpea.
- Dydercus volkerii, the cotton stainer in cotton.
- Some major pests cause economic damage at low populations and are therefore called low density pests, e.g. cocoa mirids.
- Other pests like locusts and grasshoppers usually occur in very dense populations and are therefore described as high-density pests. Key pests are the main target of pest control operations.
- b) Minor pests
- Some organisms that cause economic damage only under certain circumstances in their local environment.
- Under normal conditions, their populations are low and the damage they cause is insignificant
- Examples: the cocoa- pod husk minor marmara sp is a minor pest of cocoa in Nigeria and Ghana.
- Minor pests are not the focus of crop pest operations.
- c) Occasional pests
- Populations of occasional pests are normally below the economic threshold level, occasionally rise above it.
- Examples: man lepidopteron defoliators and stem borers occur at irregular intervals and cause economic damage to crops.
- d) Potential pests
- Potential pests are those species whose population level are usually far below the economic threshold but can become highly injurious under changed cultural practices or as an introduced pest. e.g. the giant looper ( ascotis selenari reapocaria (wlk) became a major pest of coffee estates in Kenya following indiscriminate and uncontrolled use of pesticides in the agro ecosystem.
- e) Migrant pests
- These move from one area to cause damage to crops in another area. They are a special group of pests. Their control normally involves international cooperation between the member countries involved e.g. the African migratory locust is jointly tackled by the west Africans which form the OICMA organization with headquarters in Bamako, Mali.
- Army worms (spodoptera spp) are jointly monitored by the West Africa army worm forecasting programme involving many East African countries.
- The village weaver birds, Queen spp. Are also migratory pests.
Categories of storage pests
The pests may be categorized on the basis of their feeding behavior as follows:
- Primary pests; these insects are able to penetrate the outer coats of grains and seeds, and include Ephestia spp, Trogoderma, Rhizopertha, Cryptolestes and Sitophilus spp. As well as rats and mice.
- Secondary pests: these are only able to feed on grains already damaged by primary pests or physically damaged during harvest e.g. Oryzaphilus spp.
- Fungus feeders; a number of insects (mostly beetles) that are regularly found infesting stored products are usually feeding on the fungi growing on the moist product. Some species, however, may be both fungus and secondary pests’ e.g. some psocoptera.
- Scavengers: these are polyphagus, often omnivorous, casual or visiting pests. These include cockroaches, crickets, ants, some beetles, rats and mice
Factors contributing to the biological success of insect pests
- They are generally small in size. Most vary from 2mm to 3cm. this assists in dispersal, escape from predator and eat a small amount of food.
- Their life cycles are quite short, less than one year in most cases and many have a shorter span, either by design or predation. They compensate for this by producing large numbers of offspring. When large numbers of offspring are produced within a short span, mutations are therefore relatively frequent and which enhances their ability to compete for its needs or to adjust to changes in its surrounding.
- Adaptability: insects have adapted to all environments capable of supporting life other than marine plus utilize any organic material as food.
- Reproductive capability: ability to lay large number of eggs plus the short life period of generation produces a great variability that can be tested against the environment. Food resources can rapidly be exploited as they become available and there is capacity of rapid evolutionary change as shown by the development of insecticide resistant strains of insects.
- Power of flight. Most insects can fly and this aids in escape from predators and wide dispersal of species. Dispersal promotes colonization of new habitats e.g. new food. Species promotes evolution of new species.
- They have an exoskeleton containing varying amounts of chitin. It provides protection against injury. It is used as an attachment point for muscles. It is water resistant thereby inhibits water loss through evaporation. This major evolutionary adaptation allows arthropods to colonize dry land. The non elastic nature of the exoskeleton’s outer cuticle is an obstacle to growth, for in order to attain a larger size, hard shelled arthropods must first shed, or molt, their outer layer, which splits open along a genetically determined seam. Arthropods rely upon a very hard exoskeleton for defense.
Conditions which promote pests
- a) Favorable climatic conditions
- The most common way in which organisms attain pest status is simply by an increase in number.
- Seasonal increases in pest numbers are usually controlled by climatic conditions and biological pressures. These climatic conditions include temperature, humidity, rainfall and sunlight. Aphis gossypii (the cotton aphid) outbreaks commonly occur on young plants in spells of dry weather, but clear up rapidly with the onset of rains. In Great Britain, outbreak of certain aphids can be expected following a mild winter. The reverse is the case after a severe winter.
- b) Biological change
- When the environmental conditions are favorable, an ecological change can convert a harmless organism into a pest. The major ecological reasons for an organism developing pest status include;
- c) Change in cultural practices
E.g. monocultures represent a concentration of plants of the same species over a wide area and this is beneficial to the insect or organism which will thereby have little difficulty in finding its host plant.
- Change in the character of food supply
Plants grown for agriculture have normally been selected for their nutritive value and therefore more attractive to pests than their wild relatives. E.g. Sorghum and maize are more attractive to stem borers than wild grasses.
- Introduction to new environments
Insects and other organisms become established as pests when taken to countries where they did not previously exist. In the new country, the natural enemies (parasites and predators) and competitors for food are often absent, hence allowing the population of the new pests to increase dramatically e.g. icerya purachsi mass (cotton cushion scale) is a native of Australia but was introduced into California in 1868. By 1887, it had become a serious pest of citrus in its new environment. Most storage pests exist in small population in the field but increase economically in numbers and become serious pests in the favorable climatic and abundant food of a grain store. e.g. sitophilu oryzae (l) on maize cob, stotroga cerealella (olive) on sorghum and the bruchids on cowpea.
- Climate in host/natural enemy relationships
The application of pesticides on a large scale in agricultural operations generally affects natural enemies more than pests e.g. acsotis selenaria (the giant cooper) is normally a minor pest of robusta coffee in Uganda. It became a serious pest of Arabica coffee in Kenya after very frequent use of parathion in coffee plantations.
- Loss of competing species
Under monoculture conditions, there are fewer insect species than under natural conditions and many species now become pests which were not pests under natural conditions.
- Economic change
A pest may arise for purely economic reasons because of a change in the value of the crop. Damage that is not serious when prices are low can be very important when prices are high. If the crop is in short supply, consumers overlook a little damage. Generally, organisms which cause significant economic losses in quantity and/or quality of crops are widely recognized as pest and disease organisms.
- Multiplication of suitable habitats
E.g. growing maize in western Kenya, the crop is infested by weevils.
- Spread of insects and crops by man
Minimum cultivation techniques
- Farmers are using chemicals to prepare the land followed by subsequent planting of new crops in undisturbed soil. Ploughing and harrowing reduce pests by exposing them to sunlight.
- Plantations growing crops or fruits for a long time may suffer from a particular pest proble
Economic importance of insects
- Pollination of cultivated plants. Insects pollinate flowers producing fruits, seeds, vegetables and flowers. Many plants depend on insects for effective pollination. Colonies of bees can be brought in cases of less insect pollinators. Such relationship has played an important role in the evolution of insects and plants.
- Insects produce useful materials such as silk from silkworm, honey and beeswax from bees, dyes and shellac from scale insect.
- Insects are natural enemies of pest species. They are used in biological control as predators and parasites to destroy pest insects and weeds. Insects regulate the population of other insects which they predate on. Pest problems would have been severe without predator insects e.g. lady bird beetles which feed on aphids. Parasitic wasps lay eggs inside the eggs of many moths where the producing larvae consume the pest egg contents and hence prevents pest population from developing.
- Insects are food sources for some people, fish, birds and animals. Edible insects are a source of protein and fats in people’s diets e.g. termites.
- Insects act as scavengers. Insects feed on dead plants and animals carrying out the initial stage of decomposition by pre-disposing and to removal of carcasses, dead plant materials and dung. E.g. termites on wood and scarab beetle aerate the soils and thus help to improve the soil by burrowing and providing organic matter.
- Insects can act as experimental units. They are important in scientific research and genetics (fruit flies), toxicology and neurobiology.
- They can be pleasing and entertaining. Some butterflies and beetles are colorful and are collected as a hobby.
- Some insects have had some value in medicine ( such as maggots clean out wounds, honey bee stings for arthritis)
Is when a part of the plant to be harvested is the one attacked e.g. leaves (kale, tobacco), fruits (mangoes, tomatoes), and root tubers (arrow roots, sweet and Irish potatoes).
Is when the part of the plant damaged is not the part to be harvested e.g. the roots of tobacco, sugarcane, leaves of tomatoes or potatoes.
Planting stages: Insects will have an effect throughout the life stages of the plant. The pest can attack the plant on the following stages; some dormant plant materials may harbor insects e.g. potato tuber moth can be carried in seed potato into the field.
Vegetative stage: the pest can either attack;
- Roots– we have insects attacking the root which leads to loss of water and a nutrient which leads to inhibition of uptake of nutrients e.g. the larval stage of the scarab beetle an important pest for pasture crop. Most plants are able to recover from root damage by compensating through root branching.
- Stem- e.g. cutworms Argotis species which feed on the stems. Stem borer attacks crops at this stage and leads to lodging of the plant.
- Foliage- the most important pests are found on the foliage when pests are biting foliage or sucking sap. Four types of damage can be inflicted by the pests on this stage leading to;
- Reduction of the photosynthetic area caused by leaf eaters, leaf rollers, leaf miners and leaf Webbers.
- The disruption of the sap flow by insect lodging inside the leaf e.g. leaf miner tends to reduce the flow of photosynthesis into the leaf
iii. Due to fouling e.g. Aphid deposit honey dew on the leaves and also on the fruits and sometimes disease can be encouraged to grow on these honey dew resulting to secondary contamination from disease.
- Sucking or piercing insect pests which cause loss of nutrients by loss of sap which can lead to wilting e.g. aphids removing sap from the plants or premature leaf fall due to scale insects.
- Piercing can also cause entry point of pathogens into crops. Sucking or piercing insects can also inject toxins into the plant which leads to development of gall leading to necrosis due to thrips and mealy bugs.
Flowering stage: The insects can damage the floral part leading to reduction in fruit and seed setting e.g. contarinia sorghicola reduces seed set in sorghum and some insects will fed directly on the flower e.g. pollen beetles mylabris spp. Insects can transmit diseases through flowers e.g. dysedercus spp cotton stainer.
Fruit stage: during this stage the pest causes damage on the fruits where they damage the fruit and flower. Fruit flower some can feed on the apical parts of the flower to cause distortion of the plant parts. E.g. helicoverpa armigera on tobacco causes distortion of apical stage of the tobacco.
Harvesting stage: the pest feeds directly on the stored products. E.g. grain weevils feed on the soft grain in the field. Leaf miner causes damage on leaves when they mine into the leaf e.g. in tobacco
Direct -feeding directly on the grain leads to loss of nutrient value on the grain.
Indirect- down grading due o webbing, nutritional or value loss presence of excrements on the grain.
Categories of pest damage
(a) Biting and chewing
- Some insects consume part of the plant with the aid of their biting and chewing mouthparts. Grasshoppers, lepidopterous caterpillars and beetles all consume whole portions of leaves, stems, flowers, fruits or roots of plants.
- Locust and armyworms consume whole plants. The quantities of vegetation consumed can be quite high. For example, a single female locust schostocercasp can consume 1.5g of vegetation per day and a swarm of locusts covering 10km can eat up to 2000 tons of vegetation per day.
- Lepidopterous caterpillars are defoliators and can completely eat up the leaves of plants. Removal of leaves and other vegetative plant tissues interferes with growth and development of crops, reduces photosynthetic surface of plants and reduces yield damage to the flowers and fruits leads to drastic reduction of the yield of crops.
(b) Piercing and sucking
- The hemiptera and thysanoptera have mouthparts which are modified and adapted for piercing and sucking plant tissue.
- The siphuculata and some diptera have mouthparts for piercing and sucking animal tissue too.
- Piercing and sucking insects do mechanical damage to the tissues they pierce.
- They cause loss of plant sap or blood and seriously affect the growth and development of the host.
- In some cases, parts of the plants attacked may be distorted and rendered unfit for sale and human consumption.
- Sucking insects also inject toxic saliva into plant tissues and may cause death of tissues.
- When fruits are attacked, blemishes may result and this reduces the quality of the fruits.
- Thrips pierce and suck cowpea flowers, cause flower abortion, reduce fruit formation and yield.
- Piercing and sucking pests cause secondary damage through introduction of pathogenic organisms, such as fungi and bacteria. These invade wounds resulting from insect attack. For example, cocoa mired feeding lesions formed on cocoa stems are invaded by the weakly pathogenic fungus Calonectria rigidiuscula which causes die back of the cocoa stem.
- Piercing and sucking insects also directly transmit various diseases of crops. E.g. maize streak, virus on maize, leaf curl on cotton, cassava mosaic disease, ground nut rosette virus and cocoa swollen shoot virus disease.
- Tsetse flies (Glossina spp) transmit the protozoan parasite trypanosomes, which cause sleeping sickness in humans and nagana in cattle.
- Mosquitoes transmit the malarial parasite in humans.
- Some insects tunnel into stems and fruits of crops and remain inside the tissue where they consume large quantities of tissue. Such insects have mouth parts adapted for biting and chewing. E.g. larvae of most stored produce insects like Callosobruchus maculates on cowpea, Tribolium sppon cereal grains, Ephestia cautella on maize, Dermestus maculates and Necrobia rufipes on dried fish and Cylas puncticollor on sweet potato. Maruca vitrate (testularis) is a major pest of cowpea where it bores into the unripe pods, while Sesania spp are important stem borers of maize and sugar cane plants which are bored and die quickly. Boring insects reduce the quality of timber and stored produce, thus lowering the farmer’s income.
- Insects may make the plant more difficult to cultivate or harvest
- May delay growth and maturity e.g. bollworm in cotton and cereals may be distorted for dwarfing.
- Insect infestation leads to contamination and loss of quality in the crop.
- Quality loss may be due to reduction in nutritional value or marketability e.g. in stored produce.
- Transmission of disease organisms (fungal, viral, bacterial)
Damages caused by storage pests
The relative importance of the different species of storage pests depends on the nature of the damage done.
- Direct damage
This is the most obvious typical form of damage. It is often measured as a direct loss of weight reduction in volume. However, neither is accurate since although produce is eaten, there is an accumulation of frats, faecal matter, dead bodies, etc all the insects, mites and rodents are responsible for such damage.
- Selective eating
Some insects prefer the germ region of seeds and grains. Thus a fairly low level of damage will severely impair germination of the stored seeds. In stored food grains, there will be serious reduction in quality resulting from the loss of the protein, minerals and vitamins that occur in the germ region.
This preference is shown particularly by Ephestia larvae and Crptolestes.
- Heating of bulk grain
When grain or any other similar produce is stored in bulk, stagnant air trapped within the produce becomes heated by the insect metabolism and a “hot spot” develops. The moisture from the insect bodies and the stored grains condense on the cooler grain at the edge of the hot spot. The condensed water causes caking, leading to fungal development and may even cause some grains to germinate.
- Webbing by moth larvae
The pyralid (moth) larvae in stored products produce silk webbing which if present in large quantities may clog machinery and otherwise a nuisance.
For export crops and produce to be sold, the presence of insects and dead bodies, exudates, frass, faeces, urine, hairs etc causes a general loss of quality and value. Export crops are mostly destined for Europe and America where infestation control legislation is particularly stringent. Many consignments have been rejected at the point of entry owing to the presence of rodent hairs, urine or faecal matter, certain insect pests or residue of insecticides. In Nigeria, many consignments of sorghum and maize grain have been rejected by breweries for similar reasons.
Major insect pests
The major quarantine pests identified recently on cut flowers include bollworms (Helicoverpa armigera, Spodoptera spp.), leaf miner (Liriomyza sp.), white flies (mainly Bemisia tabaci), thrips (mainly western flower thrips – Frankliniella spp.), and fruit flies (Otieno, unpubl.). The greenhouse provides a modified environment which, in many ways, favours the proliferation of cut flower pests and diseases. During certain times of the year, day temperatures in the greenhouse may soar upto 26° -33°C which reduces the regeneration cycle for many pests.
Major orders in the class Insecta
Some of the orders in the class insecta include those listed below. They include;
Insects are of economic importance in the world. They cause diseases to both man and animals. Some will feed on the crops during the growth stage while other during storage. A number of insect pests cause injury. There are also beneficial insects that pollinate flowers and others feed on other insects thereby reducing on pest status.
NOTE: Not all insects are pests and they need to be treated with caution. Many of the insects that are in the various habitats are farmer friendly and thus need to be conserved