Insects belong to the phylum arthropods. 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. The following classes under which are other minor divisions are identified:
Class insect: These have one pair of antennae, tracheal respiration, and three pairs of true legs, three body regions (head thorax and abdomen). Examples of animals belonging to this class are grasshopper, bee and beetle.
Classarachnida: These are characterized by possession of two body regions (cephalothorax and abdomen), four pairs of legs, absence of antennae, and respiration by means of tracheae, diffusion through the body walls. Examples include the spider, mites and ticks.
Classchilopoda: Insects in this group have elongated body with 15 or more pairs of legs with only 1 pair occurring on each body segment e.g. the centipede.
Classdiplopoda: These have elongated body with many legs usually two pairs per body segment excet for the first few at the back of the head. E.g. millipede.
Classcrustacean: These are usually aquatic, possesses gills and have five or more pairs of legs. Examples include crabs, crayfish and lobsters.
Damage Caused By Insect Pests
Damage by insects can be grouped into three major categories:
1. Biting and chewing
Some insects consume part of the plant with the aid of their biting and chewing mouth parts. Grasshoppers, lepidopterous caterpillars and beetles all consume whole portions of leaves, stems, flowers, fruits or roots of plants. Locust and army worm consume whole plants.
The quantities of vegetation consumed can be quite high. For example, a single female locust Schostocerca sp. can consume 1.5g of vegetation per day and a swarm of locusts covering 10km2 can eat up to 2,000 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 the crops, reduces photosynthetic surface of plants and reduces yield. Damage to the flowers and fruits leads to drastic reduction in the yield of crops.
2. Piercing and sucking
The Hemiptera and Thysanoptera have mouth parts which are modified and adapted for piercing and sucking plant tissue. The Siphuculata and some Diptera have mouth parts 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 for human consumption.
Sucking insects also inject toxic saliva into plant tissues and this may cause death of such tissues. When fruits are attacked, blemishes may result and this lowers the quality of that fruits. Thrips pierce and suck cowpea flowers, cause flower abortion, and reduce fruit formation and yield.
Piercing and sucking pests cause secondary damage through the introduction of pathogenic organisms, such as fungi and bacteria. These invade the wounds resulting from insect attack.
For example, cocoa mired feeding lesions formed on cocoa stems are invaded by the weakly pathogenic fungus Calonectriarigidiusculawhich 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 of 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.
c. Boring
Some insects tunnel into the stems and fruits of crops and remain inside the tissue where they consume large quantities of the tissue.
Such insects have mouth part adapted for biting and chewing. E.g., larvae of most stored products insects like Callosobruchus maculates on cowpea, Tribolium spp on cereal grains, Ephestia cantella on maize, Dermestus maculates and Necrobia rufipes on dried fish and Cylas puncticollor on sweet potato.
Marucavitrate(testulalis) 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 died quickly. Boring insects reduce the quality of timber and stored produce, thus lowering the farmer’s income.
Important Insect Pests of Major Crops
1. Insect Pests of Cereals
Maize Aphid (Rhopalosiphum maidis Fitch) Importance:
The aphid is a common and serious pest of maize. Young plants are most at risk. The pest can also be found on sorghum, millet, sugar cane, wheat and on numerous associated wild hosts.
Aphids build up large numbers in colonies, on leaves and tassels. New growth becomes distorted, chlorotic and stunted. Heavily infested tassels may become sterile.
Honey dew secreted by the aphids encourages growth of sooty moulds and covers the seeds in a strictly residue which makes processing difficult. These aphids transmit virus diseases of maize, such as leaf fleck and sugar cane mosaic.
The aphids vary in colour from yellow – green to dark – blue – green. They may also be covered with a thin layer of white wax or shed skins. Both winged and wingless forms may be found on the same plant.
These insects reproduce asexually (pathogenesis). Pathenogenic females give birth to living young and a generation can be completed in 8 years. They reproduce continuously throughout the year.
Non chemical control: Vigorous plants are usually tolerant of aphid attack. Natural enemies may provide sufficient control and should be encouraged.
Chemical control: Very heavy infestations may be controlled by applying an aphicide, such as pirimcarb or a systemic general insecticide such as dimethoate.
Maize stem borer (Buseola fusca (Fuller)
Buseola fusca is probably the most important pest of maize, particularly in the drier savanna zones. The primary hosts are sorghum and maize, but it may also attack millet, sugar cane and rice to a lesser extent.
Larvae feed on leaves in the funnel of young plants. They scrape the inner layer of tissue away from the leaf, leaving holes while the outer layer of the leaf is left intact. In some attacks, the central leaves may die, giving the “dead heart” effect.
Larvae bore into the main stems of the mature plant and later generations bore into the maize cobs and tassel. One or two larvae per maize can reduce yield by as much as 25%.
The adult moths are brown, with a wing span of 30 – 35mm. the moths are not usually seen during the day. Larvae are variable in colour, usually cream to pink, with black spots along the lower sides of the body.
At maturity, they are about 30mm long. Female moths lay 30 – 150 eggs on the inner surface of the leaf sheath. Eggs hatch in 10 days and remain in clusters under the sheath. They disperse the following night to feed on young leaves in the whorl.
There are 6- 7 intars and the larval stage lasts for 35 or more larvae migrate into the plant stem where they pupate. Before pupation, they cut an exit hole ready for emergence of the adult. They can be up to 3 generations per season.
The larva of the final generation goes into diapause (resting stage) at the start of the dry season. Diapause is broken by the rains, the larvae pupate and adults emerge about 2 weeks later.
Non–chemical control: Diapausing larvae can be killed by partial burning of harvested stalks before storing for fuel wood use. This burning can be done as follows: cut stalks with attached leaves and pile in the field after harvesting.
The stalks are set alight and burnt for 5 – 10 minutes until the stalks become charred and cured. This can reduce diapausing population by up to 90%.
Complete destruction of dry stalks by burning kills off diapausing larvae. Ploughing field after harvesting can also help reduce population while early sowing can reduce infestation.
Chemical control: Economic threshold level is 10% of plant with visible leaf whorl damage in maize. Treatment with carbofuran gramille (systemic) (wp) and deltamethrin can effectively control young larvae when used at 20 and 40 days after seedling emergence.
Lambda – cylalothrin and monocrotophos are also effective when applied 5 – 7 days after eggs are found on 5% of the plants. This will allow the chemical to reach the young larvae before they enter the stem. Carbofuran must not be applied late in the season because it would residues in the grain.
Leafhopper (Cicadulina spp.)
Leafhoppers are important as vectors of maize streak virus (MSV). Host plants include maize, sorghum, pearl millet and annual grasses. Cicadulinais also a vector of sugar cane streak virus.
Nymphs and adults pierce plant tissue and feed on sap. High numbers of feeding insects may cause withering of the plant and leads to growth malformation. The plant may also suffer damage if females use their ovipositor to place eggs within the plant tissue.
All adults and nymphal stages are able to transmit MSV. Population of leafhoppers is highest at the end of the growing season when adult migrate from older plants and wild grasses onto newly planted crops and seedlings.
Adults are 2 – 4 mm long. The head, thorax and abdomen are yellow or gold coloured with a pair of round black spots on the anterior margin on the crown.
Adults are commonly found resting on the upper surface of young maize leaves. The larvae are grayish with black spots all over the body. Diapausing larvae do not have these spots.
Eggs are laid along the mid – veins of the leaves of the host plant. Each female lays 100 – 200 eggs. Egg development takes 9 – 21 days, depending on temperature. There are 5 nymphal stages lasting about 23 days.
Non–chemical control: Avoid repeating planting of maize on the same plot as this leads to a build up of the pest and MSV.
Chemical control: Treat with insecticide when 10% of plants at the edge of a row are visibly infected. A spray of carbaryl or fernitrothion and endosulfan may be effective. Application of carbofuran gramilles in the soil also protects seedling against the leafhopper.
Millet stem borer (Pennisetum americanum)
This is a major pest attacking millet stems through all stages of plant growth.
Larvae bore directly into the stem without feeding on leaves. If the population of larvae is high, plants that are attacked early may collapse and die without giving any yield. Larger plants may continue to grow but may fail to set seed. Effects are most sense on late millet.
The forewings of adult are pale yellow with dark orange lines along the length. Hind wings are white and shiny. Larvae have black oval spots on each segment. About 200 eggs are laid in batches between the leaf sheath and the stem and hatch in 9 – 12 days.
Larvae bore directly into the stem where they feed. They do not feed on leaves. The life cycle is completed in 2 months and there may be 3 generations a year. At the start of the dry season, some second or third stage larvae may enter diapause within millet stem. Adults emerge about 4 weeks after the start of the rains.
Non–chemical control: Destroy all old millet stems to kill diapausing populations and clear stubbles from the field. If old stems are to be kept for fuels or building purposes, first lay them out thinly in the field in full sun to kill off diapasusing larvae.
Plant gero crops early and dauwa crops later to avoid borer attack. Apply nitrogen fertilizer to gero crops once, 4 WAP to reduce stem borer damage. Natural pest control by parasites and predators is not sufficiently effective.
Chemical control: Apply granular carbofuran to the soil at the time of planting and a side dressing 6 weeks later. Spray Lambda – cyhalothrin 5 – 7 days after eggs are found on 5% of the plants and before the larvae have bored into the stalks.
Millet head miner (Helioceilus albipunctella de Joannis)
This is a serious pest of millet in the dry, Sudan & Sahelian Savannah zones of Nigeria and can cause 20 – 50% crop loss.
Larvae feed inside the millet heads on flowers and developing grains. Damage is seen as characteristic tracks over the millet head as larvae feed from the bottom of the head upwards.
Adults have a 22 – 28 mm wingspan. The forewings are red/brown in colour with a row of mall white dots near the front edge.
The forewing of the male is slightly different, having 2 transparent patches near the front edge. Mature larvae are variable in colour, being dark green/brown to pale brown or green with darker dorsal and ventral stripes.
Eggs are laid on the millet floret. Larvae feed on the florets and grain, reaching maturity in 3 – 4 weeks. They then drop to the ground and burrow into the soil where they pupate and diapause through the dry season. Adult emerge at the start of the next rains.
Non–chemical control: Late planting of early – maturing millet varieties can reduce infestation. The varieties with the best resistance are those with compact heads and closed panicles.
Chemical control: Suggested economic threshold is 2 larvae per head, Deltamethrin, cypermethrin, paramethrin and carbaryl applied at 50% head emergence and again one week later can give effective control.
Sorghum (Sorgum bicolor) shoot fly (Altherigona soccata)
This is prominent in the grassland areas. Larvae enter the plant by crawling into the funnel between the outer leaves and the new curled leaves. They feed around the whorl, severing the growing point, giving the typical dead heart symptom.
The dead heart can be easily pulled out and, at the base, emits a bad smell. The young whitish yellow maggot feeds only on the decaying tissue, normally, damage occurs from 1 month after emergence. If attack occurs later, plants may produce side tillers that may also be attacked.
The adult flies are 4 – 5 mm long and resemble a common housefly. The head and thorax of the female is pale grey and the abdomen is yellow or pink with three parts of dorsal brown/black spots. The eggs are white and cigar – shaped. The larvae are 6 – 8mm long.
They are white to pale – yellow in colour. Eggs are laid singly on the undersurface of the leaves. Larvae hatch out in 2 – 3 days and feed on the stem. Larvae development is complete within 7 – 12 days. Pupation occurs either at the base of the decaying plant or, mile rarely, in the soil. The pupae are dark brown just before emergence.
Non–chemical control: Use high – yielding resistant varieties and plant early at the start of the rainy season to reduce likelihood of infestation. Also, cut back wild sorghum which acts as an alternative host for the pest.
Chemical control: Cypermethrin application is effective. Seed dressing with carbofuran will protects the plant for up to 28 days after germination.
African migratory locust (Locusta migratoria migratorioides)
In the gregarious phase, adult locusts swarm together and become a very serious, though sporadic pest. In Nigeria for instance, the most likely outbreak area for a swarm is in the Lake Chad region, with further invasion into neighboring states.
Locusts feed upon green vegetation with a preference for grasses and cereals. Serious outbreak at migratory phase causes complete defoliation and crop loss. The solitary phase does negligible damage.
They are short – horned grasshoppers. Adult females are 35 – 40mm and males are 40 – 50 mm long. The immature solitary phase is green or brown with an arched pronotum. The migration phase is yellow – brown or grey in colour with fine dark stripes over the abdomen and a flatter pronotum. As adults mature, their general colour becomes darker.
The main breeding site is the flood plains of middle Niger and Mali, from where past plagues of gregarious locust have originated. In certain countries, the solitary phase of the locust is usually found, and has 2 generations per year – eggs are laid in pods in bare ground.
There may be 18 – 72 eggs per pod and each female may produce 4 – 11 pods. The number of eggs laid varies with the phase of the locust. Eggs hatch in 10 – 25 days. There are normally 5 nymphtal in stars.
Effective control is based on regular surveys, prompt reporting and implementation of appropriate control measure. Egg laying sites should be identified, there are sites of a possible outbreak and should be observed closely. Egg masses can be dug up and exposed to the sun.
Chemical control: Lambda – cyhalothrin sprayed at the first appearance of the pest is effective.
Sorghum midge (Contarinia coquillte)
This is a wide spread and common pest which can be serious when high population densities occur. It is a major problem in certain regions, especially in the early season seeding stage of the plant. It is a pest of both cultivated and wild sorghum.
Larvae feed on and destroy the developing seeds. The sorghum head becomes flattered and the seeds shrunken. A typical symptom of damage is the presence of empty spikelets and red or brown glume. One midge larva per spikelet may be sufficient to cause loss of the grain.
The adult is 1.3 – 1.6 mm long, with a white head, long brown antennae and legs, dark orange thorax and abdomen, and grey wings. The female has long ovipositor. Newly hatched larvae are white but become dark orange as they mature. They are found feeding within the grains.
Females lay 30 – 120 cylindrical eggs, 0.1×0.4 mm in size, on the flowering head, usually near the spikelet tip. Eggs hatched after 2 – 4 days and the larvae burrow into the development grain.
The larval stage lasts for 9 – 11 days and pupation takes place beneath the glume. Shortly before adult emergence, the pupae case is often still visible on the spikelet of infested plants. The short life emergence, the pupae case is often still visible on the spikelet of inferred plants.
The short life cycle means that up to 9 – 12 generations can be completed in one growing season, especially where the flowering times are extended by the use of a wide range of planting dates and the maturities. Diapause occurs during the larval stage in a cocoon within a spikelet.
Non–chemical control: Early and uniform planting of sorghum varieties with similar maturity dates over a large area is an effective control measure.
Infested or deformed hears should be destroyed by burning, burying or compositing to kill diapausing larvae. Wild host sorghum plants surrounding the crop should be cut back. Resistant varieties are available.
Chemical control: Multiple insecticide application directed at adults is used primarily to reduce losses in late planting. The economic threshold level is 10 adults per panicle.
Sprays or monocrotophos and endosulfan and dusts of endosulfan and phospsalone are effective.
Oryza sativa and O. globerrima white stem borer (maliarpha separatella)
Widely distributed in all rice growing regions; it is the most important pest of upland and irrigated rice. Attacks only rice and is not found on other crops.
Rice plants are attacked at the full filtering stage. Larvae penetrate into and feed on the stem. They also migrate into the filters. Feeding impairs panicle ripening, leading to “white heads” and causes breakage of stems.
Adult females are about 18mm long. But the males are slightly shorter. Wingspan is 23 – 29 mm. both males and females have long pale gold – coloured forewings with a red – brown line along the costal vein, which is more pro9nounced in the male.
The hind wing are shinny while. When at rest, the wings slightly overlap along length of the body. Larvae are initially white with dark brown head. Mature larvae are about 18mm long, yellow in colour and falter than the earlier stage.
Eggs are laid on the upper surface of the leaves. They are attached by cement which, on drying, contracts and causes the leaves to curl and envelop the eggs in leg tissue. Eggs are laid close together in a mass up to 1.5cm long and containing about 50 eggs.
At first, they are pale, becoming darker and brown – black later. They hatch in about 7 days. Larvae then penetrate fillers just above a node. Larvae pupate in a loose cocoon at the base of the dried stubble at the end of the rice season.
They may be 3 – 4 generations a year. Larvae enter a dry season disapause which can last up to 20 weeks. At the end of the dry season, pupation occurs and the adults emerge to lay eggs on the next season’s crop.
Control Methods
a. Cultural control
Use early maturing varieties and plant early in the season so that plants mature before pest attack.
Remove weeds which provide alternative hosts.
Timely fertilizer applications order the growing period to discourage over – rapid development of the plant.
Destroy stubble after harvest by burning or flooding the field. This will destroy the diapausing stages of the pest thus reducing infestation in subsequent season. Avoid buying and selling seedlings for transplanting as these will often carry potential infestations that are not apparent at the time of sale.
Do not plant in stages (stagger), as this provides unlimited food supplies for pest population development and buildup of population.
At harvest, do not leave tall stems in the field, particularly in early maturing varieties. Cut the stack down to the base to destroy larvae and pupae in the stems.
Synchronize planting over a large area to diffuse incidence of damage and severity.
Host plant resistance. Use varieties that are resistant to or tolerant of stem borer attack.
Read Also : Definitions, Categories and Importance of Crop Pest
b. Chemical control: the economic threshold is 2 egg mass per 20 hulls from seedling to panicle initiation or 2 egg mass per 20 hulls after panicle initiation to ripening. Chemical control is difficult, as the pest is protected inside the stem from contact with the insecticide.
Systemic insecticides are necessary, but are costly. Foliar application of insecticide may kill the pest, but will also reduce population of natural enemies, leading to pest/enemy system in balance and possibly pest resurgence.
Application of granola formulation can be made necessary. Recommended chemicals include monocrotophos, chropyrifo and carbofuran.
Stalk–eyed fly (Diopsis spp.)
An important, common pest of lowland rice but can also be a problem on upland rice where humidity is high. It is an early season pest which causes death of fillers. Crop losses can range from 2 to 17%.
Adults invade nurseries early in the season. They lay eggs on leaves and steams of seedlings and transplants.
The larvae move inside the leaf sheath and feed on the stem tissues below the growing point, cutting off the apical part from the rest of the plant and causing dear heart symptoms. They move fresh tillers as attached ones start to decompose. Later larval stages feed on flower heads.
Diopsis is a very destructive insect. Adults are about 9 mm long with an orange head. The eyes are red – black and are held on long stalks away from the side of the head. The thorax is shiny black 2 spines projecting backwards from the posterior end.
The wings appear glass – like and the abdomen and legs are red. Larvae are about 18mm long with very small heads. They are cream in colour with yellow makings on the last body segment.
The insect lives near water and prefers to feed on aquatic plants. Females lay eggs individually on the upper surface of young leaves, usually near the midrib.
Each female can lay eggs out am period of 10 days. On emerging, the larvae move down the inside of the leg sheaths and feed on the developing leaves. The larval stage lasts for 25 – 33 days.
Chemical control: The economic importance of the pest is not clear and the application of pesticide may be justifiable only in severe cases.
African rice gall midge (Orseolia oryzae)
It is a sporadic, but serious pest affecting lowland rice in Nigeria including Guinea, Sudan savanna and forest zones. Early and severe infestation can cause total loss of crop.
It survives on wild grass alternative hosts.
Attack occurs at the early filleting stage. Larva move down until they reach the apical or lateral buds to swell and form a gall. Galls are hallow, elongated tube – like structures. Which are silvery in colour.
This formation is often called onion leaf or silver shoot. Infected crops show non – uniform flowering and maturing dates. Rice is most susceptible during its growth phase before panicle development. Tillers bearing galls do not produce panicles.
Adults are tiny (3 – 3.5mm long), and mosquitoes – like with strong long legs. Females have an orange – coloured abdomen. While the males have a slender brown abdomen and are generally smaller in size. Adults are nocturnal and are active in humid condition.
The midge spends the dry season on rice and grasses in wetter areas or in ratoons. At the onset of the rains, the insect population increases on the wild hosts from where transfer to cultivated rice shortly after it is planted.
Females are fertilized and can lay eggs within a few hours after emergency. Eggs are laid singly on the underside of leaf sheaths and ligules. Each female can lay 100 – 300 eggs which hatch in 3 – 4 days. The young larvae creep down the leaf sheath on a film of water.
At the base of the sheath, they enter the apical or lateral buds where they feed and remain until they pupate after 15 – 20 days.
Adults emerge after 2 – 3 days and may live for 1 – 5 days.
Non–chemical control: Avoid damage by planting rice as early as possible before the rains. The crop will then be mature and unsuitable as host when the pest is present. Remove alternative host plants from around the crop to prevent the initial increase in pest population.
Chemical control: Systemic insecticides, such as monocrotophos and carbofuran are effective against this pest.
Sugar cane borer (Eldona saccharina)
This is an important pest of sugarcane. Young larvae feed on the leaf sheath while older ones bore into the base of the stem to feed. This causes dead hearts in young plants. The newly hatched larvae feed on the leaves, usually boring into the midrib.
Fully grown larvae bore into the stem and destroy the plant tissue, causing dead heart symptom. They then move to another plant shoot or another plant. Presence of frass on the outside of stem is a characteristics sign of the presence of feeding larvae.
Larval are usually found in the lower parts of the stem. They can be dispersed by wind from plant, by hanging from the leaves in silken threads. Stalk rots are more common on stems damages by this stem borer.
The wingspan is 40 mm in adult females and 30 mm long in males. Both have pale brown forewings with 2 small dots in the centre of each. The hinderings are white. Larvae are 20 – 25 mm long.
Eggs are oval and yellow. They are laid in batches of 10 – 15 in the soil surface or at the base of leaves and in cracks on stalks. Females lay 100 – 500 eggs and start laying on their second night after emergence.
Eggs hatch after 5 – 6 days. The larval period is 30 – 35 days. Pupation takes place in the stem on the leaf sheath. Adults emerge after 7 – 14 days and live for 3 – 8 days.
Non–chemical control: Stubble should be burnt and crop residues buried to prevent insects from laying eggs in the dead leaf litter. Volunteer plants and other alternative host plants should be removed.
Chemicalcontrol: Endosufan or carbaryl can be effective in the early stages of attack.
iii) Leaf feeding beetle (OothecasppColeoptera): Chrysomelidae
It is a major pest of cowpea. It is the most damaging of cowpea leaf feeders. It is a polyphagous feeder and feeds on many other leguminous crops, coffee and cocoa.
Adults feed between leaf veins, making small irregular holes. Yield is reduced if attack is heavy or occurs on young plants.
Heavy infestation can lead to complete defoliation of cowpea seedling and the death of the plant. Adults also cause indirect damage by transmitting cowpea yellow mosaic, cowpea mottle virus and southern bean mosaic virus.
Larvae feed on the plant roots but are minor pests compared to the adults.
Adults are about 6mm long, oval in shape & shiny black head and legs. The body colour may vary from brown to light brown or orange. Larvae are 6 – 12 mm long and are white with brown head.
There are two generations a year. Adults emerging from the dry season diaspause usually starts to lay eggs in March or April, 15 – 20 mm below the soil surface. Each female lays 200 – 500 pale yellow eggs. Hatching occurs in about 13 days.
They go their 3 intars in the soil in 30 days. This is followed by a non–feeding pre–pupae stage which last for 8 – 18 days, finally developing into a yellow pupa. Adult begins to appear in July and August. There then produce a second generation which diapause in the soil until the following March.
Non–chemical control: Population are higher on cowpea that is intercropped with maize than when cowpea in grown as a sole crop.
Chemical control: One or two sprays of endosulfan, methomyl or chlorpyrifos may be effective.
Insect Pest of Legumes
a. Pod borer Maruca testulalis
The larvae are a regular pest of cowpea. It damages both flowers and pods wherever cowpea is grown. Uncontrolled infestation can reduce yield by 70% the main hosts are beans and cowpeas. Alternative hosts include groundnut, castor, tobacco, rice and Hibiscus spp.
Eggs are laid on the flower buds and young leans. The young larvae bore into the flowers, feeding inside and causing the flower to drop. Young stems, terminal shoots and peduncles are also damaged.
Signs of larval feeding include the webbing of flowers, leaves and pods, and the presence of frass on the shoot tips and pods. Several flowers may be attacked by one larva. Later larval stages are highly mobile and cause damage throughout the reproductive cycle of the crop. Feeding on flowers and boring into green pods to feed on the developing seeds.
Larvae are active at night; during the day they sheltering flowers, pods, stems and leg debris beneath the plant. Damage is more severe to pods which are located in the leaf canopy on short peduncles or those touching other parts of the plant.
The adult moth has a span of 16 – 27 mm. the following are light brown. With white markings and hind wings are white with brown margins. Mature larvae are 16mm long, whitish in colour with a black head. They have characteristic dark spots on each body segment.
Female lay over 200 oval yellow eggs on flower, terminal shoots, leaves and pods. Each hatch in 2 – 3 days and there are 5 larval stars, lasting 8 – 14 days. Pupation occurs in the pod or in a cocoon in then leaf debris beneath the plant. Adults emerge after 5 – 10 days and may live for 5 – 15 days.
Non–chemicalcontrol: Cowpea varieties with long pod peduncles and tougher pods are less susceptible. Mono–cropping should be avoided.
Chemical control: The action threshold is when 60% of flowers are damaged. Dimethoate combined with cypermethrin gives effective control. Mixture of deltamethrin + dimethoate and lambda – cyhalothrin + demethoate are also effective.
b) Groundnut aphid (Aphis craccivora koch)
A widespread pest, but more common in the north, especially during dry spells when population can increase rapidly. Direct damage by feeding insect is not usually serious but the insects are important vectors of groundnut rosette virus.
The aphids primarily attack seedlings and feed on the undersurface leaves. Sap – sucking by large number of aphids may cause plant to wilt and due in hot weather and reproduction in yield.
Leaves and shoots may become distorted. Sticky honeydew is also produced, encouraging the growth of moulds. Plants infected with rosette virus are stunted yellow or dark green molting on the leaves.
Adults can be winged (alate) or commonly wingless (apterous). The aphids are shiny blackish, grayish–green or brown. The cauda and sipunculi are black and the antennae are 2/3 the length of the body. They are gregarious insects, forming clusters on buds, flowers, green pods stems and the underside of leaves.
Aphids can produce either sexually or asexually. Adult females produce up to 100 young and between 2 and 20 daily. They are 4 nymphal stages, and the life cycle takes 6 – 8 days. Adults live for 5 – 15 days.
When food supply is abundant, apterous forms develop in response to overcrowding and lack of food. There forms are responsible for the migration of the aphids between crops.
Non–chemical control: Early sowing and close spacing of groundnut can help prevent infestation.
Chemical control: Aphid – specific insecticides, such as pirimicarle, should be prepared since they kill only aphids and allow natural enemies to survive. Other effective insecticides include lambda – cyhalothrin and monocrotophos.
c) Termites (various species): Amitermis sp., Odontotermes spp, Microtermis ssp.
Termites build nests around the root of the plants, feeding on the roots and may also attack foliage. They attack plants of any age but usually established plants. Infestation intensifies as the plant matures. Attacks are more common where plants are under water stress.
Crop losses due to termites can be as high as 10 – 40% microtermes spp are a more serious pest in the Sudan savanna zone, particularly in well drained sandy soils. Damage is caused by termites penetrating into the root, stem and pegs, destroying e plant tissue which prevents transportation of water up to the plant.
This causes the plant to wilt. They also scarify the pods, making them liable to crack when harvested and this may allow the entrance and growth of toxic fungus (Aspergilus) is aflatoxin) termites cause the plant to collapse at chewing into the main stem so weakening the plant.
Non–chemical control: Crops which mature early and are harvested early may escape attack. Seed of a uniform variety should be planted to ensure even ripening and a single harvest date.
Repeated mechanical cultivation and burying of crop residues may reduce termite population. Rotation with other crops may help prevent termite population build– up.
Chemical control: Kerosene may be poured around the base of the plant to destroy the nests. A mulch of neem cake is effective at preventing damage when groundnut is being dried in the field. The groundnuts are either mixed with the mulch or placed on top of it.
d) Pigeon Pea Blister Beetle Mylabris spp.
This occurs wherever pigeon pea is grown. Adult beetles feed on the flowers and greatly reduce the number of pods that are set.
MylabrispostulateThunberg in about 25mm long. Adults have obvious black and red coloration. Other genera and species are of varying sizes but are conspicuously coloured.
Their name derives from the blisters on human skin caused by the exudates (containing cantharidine) which is produced by the beetles when they are disturbed.
Eggs are usually laid in the soil. The larvae of most species are generally beneficial because they feed on insects in the soil (e.g. grasshopper eggs).
Non–chemical control: Blister beetle cause little damage in areas where pigeon pea is grown over large areas, because they spread across the crop.
However, in small pigeon pea plots that are in the flowering stage during the period of peak adult activity most of the flower may be eaten by pest and crop loss may be substantial.
The beetle can be controlled manually by picking them by hand or collecting them by insect net and crushing them, since they are slow moving. Care should be taken to protect the skin.
Chemical control: Most insecticides are not effective against these beetles, but synthetic pyrethroids work reasonably well.
e) Pod–sucking bugs (Clarvgralla tomentoscicollis stal)
This is sporadic but serious pest. It is the most important pod – sucking bug on pigeon pea. Other are plant hosts include cowpea and other pulses.
Adults and nymps suck developing seeds through the pod wall. The seed become shriveled with dark patches. Such do not germinate and are not acceptable as human food.
Adults are about 10mm and brown – gray. They are hairy and have piercing and sucking mouthparts. There are 2 lateral projecting spines on the prothorax. The wings are held flat across the back.
Each female lays about 200 eggs in clusters on host weed plants. They hatch in 6 days. There are 5 nymphal stages and develop from egg to adult takes 20 – 25 days.
Non–chemical control: Since this insect breeds on many weed hosts on which it lays eggs, weed management is essential for its control. A solution of 9% neem seed or kernel extract (wt/vol) decreases insect survival and give effective control.
Chemicalcontrol: This is done when cowpea economic threshold level is 2 insects per 10 plants at the flowering stage, and 4 insects at the podding stage. This is yet to be determined for pigeon pea. Application of cypermethrin and dimethoate are effective.
f)Pod borers Helicoreepa (Heliothis) armigera Hubner
Helicoreepa is widely distributed on many host plants. This insect destroys buds, flower and pods. If flowers and pods are not available, they feed on leaflets, leaves and veins.
On pods, conspicuous holes are made by the entry of the larvae. Usually, developing and partly matured seeds are eaten completely. At times, apportion of the seed and testa remains.
The large larvae are yellow, green and pink, orange, brown or black but all have characteristic light and dark stripes along each side. Adult moths are up to 19mm long with a wing span of 44mm. female insects have pale brown wings with dots near the outer margins and males are use. They are active at night.
Adults lay small eggs usually singly, on the upper and outer surface of leaves, flowers, pods, and stems. The young larvae feed by scraping green tissues and the older larvae chew voraciously into buds, flowers and pods, leaving characteristic round holes. Pupation is normally in the soil or in plant debris. One generation can be completed in 4 weeks under favourable conditions.
Non–chemical control: Natural enemies sometimes give adequate control. Some pigeon pea genotypes have considerable tolerance to the pest 80 host – plant resistance can be a useful method of avoiding damage.
Chemical control: Several insecticides including endosulfan and synthetic pyrthoids give good control especially if applied soon after the eggs hatch.
Important Insect Pests of Root and Tubers
a) Cassava variegated leaf hopper Zonocerus variegates
Occasionally, a severe pest of cassava can cause complete defoliation. Damage usually occurs between November and March. Alternative hosts include cocoa, coffee, sweet potato, millet, most vegetables and cotton which are attacked at the seedling stage.
Nymphs and adults eat the leaves. They can also feed on green stems by stripping the bark and leaving only bare wood. Their feeding can kill plants and reduce yield in older plants.
Large locusts are about 40mm long. The thorax and body are yellowish – brown. The head and legs are marked with red, black and yellowish – brown. The insects have an unpleasant smell when handled.
Females lay eggs in the soil during the rainy season, usually in shady areas near cassava. There is a 6 month egg diapause followed by 6 nymphal stages which last about 100 days. In the drier regions, eggs are laid at the end of one rainy season hatch at the start of the next rainy season, eggs hatch at the dry season and the instars develop over the first half of the dry season.
Non–chemical control: eggs pods should be dug up to expose the eggs and allow them to dry out and die. This must be done over large area to be effect. Flooding of egg sites will also kill the eggs.
Migration of the grasshoppers into the crop can be prevented by planting physical barriers, such as trees around the field. The borders of the field should be cleared of weeds; especially chromolaena spp. locally prepared neem products are effective when applied in the dry season.
Chemical control: Insecticides such as dimethoate and fenitrothio can be used on young nymphs in November/December. Baits of bran (wheat or other cereals) mixed with mude (e.g pupoxur dust) can be lid between the rows to attract the nymphs.
b) Cassava mealy bug Phenacocus manohoti
A very severe dry season tuber loss can be up to 75%. Alternative hosts include sweet potato, eggplant and tomato.
The mealy bugs initially feed on young apical cassava shoots. They inject toxic saliva which may cause deformation of the growing tissues and stunting. As the new leaves on the shoot die, infestation spread to the older leaves down the stem. Eventually, the plants show a candle stick appearance.
Adults are 1 – 3mm long and oval in shape. They are pale pink in colour but are covered in a white waxy layer.
Male insects are rare. About 400 eggs are laid together in a cottony sack. One generation takes about 22 days to complete. Adults live for about 20 days. Newly hatched larvae crawl. The insects can also be spread by the movement of infected planting material.
a) Non–chemical control:
Cultural control: Cassava plants over 7 months old are more tolerant to damage. Therefore, planting at the beginning of the rainy season will allow plants time to grow sufficiently to withstand attack.
Fertilizer should be applied at the recommended rates to encourage healthy plant growth. Cuttings should be dipped for 10 minutes in hot water (25oc) before planting to kill any infestation.
Biological control: Biological control with an introduced parasite Epidinocarsos lopezi (De santis) is now possible in Nigeria. The parasites are available at the International Institute for Tropical Agriculture (IITA) Ibadan. Local natural enemies, such as the coccinellid Hyperaspis pumila also play a role in the biological control of the mealy bug.
b) Chemical control
Before planting, cuttings can be dipped in dimethoate or methidathrion solution for 1 minute to kill any larvae which may be present.
Other important pests of cassava are:
Whitefly – Benisia tubaci genn homoptera: Aleyrodidae which transmit the African cassava mosaic virus and causes yield loss of 20 – 60%.
Cassava green spider mite monoychellus tanajoa Aeuia:L Tetrnychidae which causes 15 – 80 % yield loss.
In summary, insects constitute one of the largest and most important pests of crop plants. They cause damage to crops through Biting and chewing, piercing and sucking as well as boring. Damage caused by insects can be minimized or controlled through chemical and non–chemical means.
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