Tuesday, April 16, 2019

Continuation on ECONOMIC ENTOMOLOGY

The lepidopterous borers of economic importance in Africa are Busseola fusca, Sesamia spp., Eldana saccharina, Chilo spp. and Maliarpha separatella. The dipterous species of concern is Diopsis thoracica.
STEM BORER LIFE CYCLE
The stem borers feed inside the plants and are therefore protected from parasites, predators, parasites and also contact insecticides when they are inside the plant. There are two important families under this group
I- Family Noctuidae
II- Family Pyralidae
The noctuids are very large moths while the pyralids are smaller in size. In Nigeria, particularly in cereal crops such as sorghum and maize, there are two important genera in the family noctuidae and these are:-
Genus Busseola
Genus Sesamia
They both attack cereals particularly maize and guinea corn. The adult of B. fusca is a dark moth, night flying with a wingspan of 35mm. The moth is widespread in maize growing areas of tropical and sub-tropical Africa. Mature and mated females lay their eggs under leaf sheaths in a long column stretching up the stem. The eggs are white when freshly laid but darken before eclosion. The eggs hatch in about 7-10 days. The larvae are pink to buff in colour characterized by one or more distinct black spots along the body. The larval period lasts between 33-35 days with 6-7 instars. The mature caterpillar cuts a hole in the side of the stem before pupating within the stem and tunnel. The pupa is dark brown and the pupa stage lasts for about 10 days being temperature dependent. The adult emerges through the hole in the stem prepared by the matured caterpillar.
When the eggs hatch the young larvae migrate to the leaf funnel and attack the young leaves leaving the holes or windows. In severe attack the central leaves dies. The larvae bore into the stem and cobs. The attack on the stem weakens the stem, hinders water and mineral nutrient transport and this results in poor plant growth and yield. The stem borer damage is as a result of their feeding on the leaves and in the leaf whorl and boring into the stems and fruit head causing the characteristic symptoms called dead hearts, chaffy-heads and whiteheads. Severe damage kills young plants while damage weakens older plants by borer larvae feeding within the stems. Usually more than one species can be present in the same stem, and stem may harbour several larvae. Borer infestations are carried over from one season to another by diapausing larvae, which remain in the cereal stalks, stubbles, cobs or in wild poaceous plants like Pennisetum spp.
Very few of the larvae which hatch from the eggs of B. fusca eventually gets into the stem because some are blown off by wind, some are fed on by predators or parasitized while wandering all over the plant unlike in Sesamia whose larvae enter directly into the stem. There is therefore no funnel damage in the case of Sesamia. This means that Sesamia is more difficult to control than B. fusca because it goes directly into the stem, however larvae of Sesamia do not go into diapause during the dry season.

Life cycle of Busseola fusca

Non-Chemical Control: Diapausing larvae can be killed by partial burning of harvested stalk before storing for fuel wood use. This can reduce diapausing larvae populations by up to 90%. Ploughing fields after harvesting can also help reduce populations.
Chemical Control: Treatment with carbofuran as granules, carbaryl and deltamethrin (as an emulsifiable concentrate) can be effective for control of young larvae when used 20-40 days after emergence. Lambda-cyhalothrin and monocrotophos are also effective 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 migrate to the stem.



COWPEA (Vigna unguiculata L. Walp)
Cowpea commonly referred to in Nigeria as beans is the most important grain legume crop throughout the tropical belt covering Asia, the far East, Africa, central and southern America and in the southern USA. It provides a major source of protein in human diets.
At the seedling stage cowpea is attacked by aphids (Aphis craccivora) beanflies (Ophiomyia phaseoli), leafhoppers e.g. Empoasca spp, leaf beetle, Ootheca mutabilis, and the arcticid defoliator, Amsacta moloneyi. Insects that decimate the crop at the early reproductive stage include the flower thrips e.g. Megalurothrips sjostedti while at the podding stage the legume pod borer Maruca testulalis and the bug complex viz Clavigralla tomentosicollis, Anoplocnemis curvipes, Mirperus jaculus, Aspavia armigera are the major pests of the crop
Pod sucking bugs cause considerable damage to cowpea. They usually migrate in large numbers to cowpea farms at the podding stage. Both the adults and nymphs suck sap from developing pods. Pods that are attacked dry up prematurely and seeds are poorly formed.

LEGUME POD BORER (Maruca testulalis) (Lepidoptera: Pyralidae)
The larvae of this insect are a regular pest. This borer damages both flowers and pods, and is found wherever cowpea is grown. Uncontrolled infestations can give a yield loss as high as 70%. The eggs are laid on the flower buds and younger leaves. The young larvae bore into the flowers feeding inside and causing the flowers to drop. Young stems, terminal shoots and penduncles are also damaged. Signs of larval feeding include webbing of flowers, leaves and pods and the presence of frass at the shoot tips and pods. Several flowers may be attacked by one larva. Larvae are active by night, during the day they shelter in flowers, pods, stems and leaf debris beneath the plants. Damage is more severe to pods which are located in the leaf canopy on short penduncles or those touching other parts of the plants.
The adult moth has a wingspan of 16-27 mm. Mature larvae are 16 mm in length, whitish in colour with a black head. They have characteristic dark spots on each body segment which form longitudinal dorsal and ventral rows along the length of the body.
Females may lay over 200 oval yellow eggs in batches of 2-16 on flowers, terminal shoots, leaves or pods. The eggs hatch in 2-3 days and there are 5 larval instars. The larval period lasts between 8-14 days depending on climatic conditions and food availability. Full grown larvae usually pupate in the pod but may also drop from the plant and pupate in a cocoon in the leaf debris beneath the plant. The larvae are active at night and during the day hide in flowers, pods and stems or in the soil around the plants. Adults emerge after 5-10 days and may live for 5-15 days.


Adult Maruca testulalis Larva of Maruca testulalis


Non-Chemical Control: Varieties of plant with long penduncles and tougher pods may be available. Monocropping should be avoided.
Chemical Control: Dimethoate combined with cypermethrin gives effective control in field trials. Mixtures of deltamethrin combined with dimethoate and lambda-cyhalothrin combined with dimethoate are also effective.


COTTON (Gossypium spp.)
Cotton is an important cash crop in Nigeria. It is a seed fibre and the lint is used in the manufacture of textiles. It is attacked by a lot of pests e.g. Earias insulana (Spiny bollworm), Aspavia armigera (Shield bug), Cosmophila flava (Cotton Semi-looper), Bemisia tabaci (Whitefly), Sylepta derogata (Cotton leaf roller), Dysdercus spp (Cotton stainer) and Helicoverpa armigera (Cotton Bollworm).

Helicoverpa armigera (Lepidoptera: Noctuidae) (Cotton Bollworm)
This is a very serious sporadic pest of cotton, legumes, maize, sorghum, tomatoes and okra. It is a minor pest on some fruits and it is widespread throughout Nigeria.
Larvae feed within the plant pod and fruits, eating the contents either partially or completely and causing damage that allows the entry of fungal pathogens. The larval feeding leaves characteristic circular holes surrounded by frass on the surface of the fruit or pod. On cotton, young larvae feed within flower buds and terminal buds, while older larvae feed on larger green bolls eating the contents and leaving characteristic circular holes and expelled frass on the surface. A single larva can feed on and destroy several bolls. Damages cause the bracteoles of the bud to open out leading to a symptom called ‘flared squares’. The larvae feed characteristically with the rear part of their body exposed outside the fruit.
The adult moths are up to 19 mm in length, stout bodied with a wingspan of about 40 mm. Female insects have pale brown wings with paler dots near the outer margins and males are usually more green-grey in colour. They are active at night. Young larvae are yellow-white to red-brown in colour and spotted with dark brown or black areas. Older larvae are up to 40 mm in length and may vary considerably in colour from brown or green to pale yellow and pink. There is a dark dorsal band along the length of the body and either side of this, a light, dark and another light band. This last pale band running along the side of the larvae is the most noticeable.
The eggs are spherical, 0.5 mm in diameter and yellow in colour at first, becoming brown near to hatching. Up to 500 eggs may be laid in a single night by the female moth, usually on the upper part of the plant canopy. Eclosion occurs 2-4 days and the first instar roam over the plant in search of food. There are six larval instars which last a total of 14-24 days. Fully grown larvae drop from the plant, burrow into the soil up to a depth of 25-175 mm and pupate in a lined cavity. Pupation lasts for about 10-14 days.
Non-Chemical Control: Numerous natural predators exist but generally they are not capable of preventing a pest outbreak
Chemical Control: Sprays of cypermethrin, lambda-cyhalothrin and endosulfan. Field scouting for eggs is recommended to enable chemicals to be applied at the correct time to kill first instar larvae.

Life cycle of Helicoverpa armigera



COCOA (Theobroma cacao)
Cocoa is an important export crop in Nigeria. 95% of the cocoa produced in Nigeria comes from the south western area of Nigeria. The crop used to account for 20% of the value of Nigerian agricultural exports. The name cacao is used to describe the tree, while cocoa refers to the fruit or crop and the processed products.
Production of cocoa is affected by the following pests vis Mesohomotoma tessmanni (Psyllid), Sahlbergella singularis, Distantiella theobroma (Cocoa mirids or capsids), Helopeltis bergrothi (Cocoa mosquito), Earias biplaga (Spiny bollworm) etc.

Sahlbergella singularis (Brown cocoa mirid); Distantiella theobroma (Black cocoa mirid) Cocoa Mirids (Capsids)-Heteroptera: Miridae
This can be a very serious pest of cocoa. Attacks by mirids can lead to 20% loss in yield. Sahlbergella singularis also attacks kola. D. theobroma also attacks citrus and is prevalent in western Nigeria. Damage mainly results from feeding on stems and pods. Mirids feed intensively on cocoa pods, pod stalks, shoots and stems causing considerable damage including death of the terminal shoot and consequently loss in yield. The damage takes the form of large dark lesions on the cocoa pods and stems. A few lesions can girdle a stem. Stem lesions are prone to fungal attack, causing considerable die back. Mirid attack on pods can cause pod deformation, distortion or bean decay depending on the severity of attack. Attacks lead to the depletion of the tree canopy. This allows ‘mirid pockets’ to develop in areas with relatively more shade.
Adult mirids are 7-12 mm in length. The thorax is distinctively lumpy in form. The eye protrudes and the antennae are clubbed. It completes its life cycle in 4-5 weeks. The female inserts eggs into the host stem or pod tissue. The slender filaments at the end of the egg project out of the plant stem and are just visible by the eyes. Up to 200 eggs may be laid. Eggs hatch in 13-18 days and the nymphs feed by piercing the host plant tissues to suck sap. There are five nymph instars over a 4 week period.



Sahlbergella singularis (left): geographically the more widespread species. Right: Distantiella theobroma

Cocoa mirid and damage to pod

Non-Chemical Control: Pruning of infested tissue and shade management may help to reduce the severity of mirid attack. Do not use Kola as a shade plant as these are alternate hosts for cocoa pests.
Chemical Control: Spray gamma HCH during the period of high populations. Spraying against mirids on young cocoa trees should start in June and continue at monthly intervals until February.

Earias Biplaga (Spiny bollworm also called Cocoa Bollworm) Lepidoptera: Noctuidae
This is a serious pest, particularly in the dry season. Trees younger than three years old are normally attacked. Attack is more severe on poorly shaded plants. The moth lays eggs on the apical buds and stems. On hatching the larvae bore into the buds and may feed within the stem and on the leaves. Destruction of the buds can delay or prevent formation of jorguettes and stunt plant growth.


Larvae of Earias biplaga


Non-Chemical Control: Pest incidence can be avoided to some extent by provision of adequate shade during the first three years of plant growth. Shade should be already established before the cocoa is planted.
Chemical Control: Control with dicrotophos and monocrotophos is effective. Chemical control in combination with release of reared parasites may in the future be the best control option.

CASSAVA (Manihot utilissima)
Cassava is one of the most important food crops of West Africa. It is a root tuber and it also produces latex, depending on the variety. The tuber is processed into garri, tapioca and cassava flour for human consumption. They may be fed raw or boiled to goats, pigs, horses and cattle. The main industrial use of cassava is in the manufacture of alcohol and starch.
Cassava is attacked by a lot of insect pests namely termites, green spider mites, whitefly, green cassava mealybug, variegated grasshopper etc. These insects bring about a reduction in yield of cassava.
CASSAVA GREEN SPIDER MITE (Mononychellus tanajoa) (Acarina: Tetranychidae)
This is a very serious pest which may cause 15-80% loss in tuber yield. The mites can be wind-dispersed from plant to plant. It prefers a humid environment and therefore occurs more commonly in the humid southern regions of Nigeria.
The mites feed by sucking fluid from plant tissue. Young cassava plants between 2-9 months old are most vulnerable to mite attack when young shoots and leaves may be severely damaged. The very young leaves become stunted and deformed as they grow. Young expanded leaves show varying degrees of yellow spotting which may be mistaken for the symptoms of cassava mosaic virus disease. Leaves eventually dry out and defoliation occurs progressively as new leaves appear. Older leaves remain intact. The leaves become covered in webbing produced by the mites. Older plants are less likely to be attacked.
The mites are green in colour when young becoming yellow as they mature. They are 0.2 mm in length and usually found feeding on new shoots and the underside of leaves. The eggs are laid individually on the underside of leaves. The mites live for 3-4 weeks and the females may produce 20-90 eggs during this period. The egg to adult stage lasts about 12-14 days. Populations vary with the seasonal rainfall pattern, probably in response to the availability of young foliage. Females disperse in the wind by floating on silken threads.

Egg and larva of cassava green spider mite


Cultural Control: Mite mortality due to rainfall is sufficient to suppress populations and maintain them at low levels. Cassava planted early in the wet season can remain mite free for several months. Declining rainfall later in the season allows the mite population to increase.
Chemical Control: Unless attack is extremely severe, chemical control is not recommended. Cuttings which are to be used for planting should be treated with insecticide to kill the mites. Chemical control is possible with methidathion or dimethoate.

CASSAVA MEALYBUG (Phenacoccus manihoti) (Homoptera: Pseudococcidae)
It is a very severe pest of cassava. It occurs in the dry season with the peak infestation in February/March. It is more important on young, less established or drought stressed plants. Its alternative hosts include sweet potato, eggplant and tomato. In the early stages of infestation the mealybug feeds on the young apical cassava shoots. During feeding, it injects toxic saliva into the plant which may cause deformation of growing tissues. As mealybug population increases the shoot becomes stunted and the stem twisted.
New growth becomes retarded and the shoot develops a ‘bunchy top’. Eventually all the new leaves on the shoot die and the infestation begins to spread to the older leaves lower down the stem. In the final stage s of attack the plant shows a candlestick appearance. Economic damage is due to loss of tuber yield.
The adult insects are 1-3 mm in length, oval in shape with short lateral and caudal filaments and are wingless. The living insects are pale pink in colour but this is not always obvious as they are covered in a white waxy layer. Male insects are rare. 400 eggs are laid together in a ‘cottony’ sack. One generation takes about 22 days to complete. Therefore infestation with one individual is sufficient to establish a new colony in the field. Adults live for about 20 days. Newly hatched larvae crawl to the tips of the plants where they are dispersed by the wind. The insects can also be spread by the movement of infected planting materials.

Predators feeding on cassava mealybug Adult mealybug



Cultural Control Cassava plants over 7 months old are more tolerant to damage. Hence 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 (520C) before planting to kill any infestations.
Chemical Control: Cuttings can be dipped before planting in dimethoate or methidathion solution for one minute to kill any larvae which may be present.

CITRUS (Citrus spp.)
Citrus fruits are grown in tropical areas of the world. Juices from grapefruits and sweet oranges are canned in small quantities in Nigeria both for export and local consumption. A lot of insects feed on citrus, causing a lot of damage. The most important insects are scale insects (Coccus viridis), mealybugs (Planococcus citri), fruit-flies (Drosophila spp.), aphids (Toxoptera aurantii and Toxoptera citricidus), citrus False Codling moths (Thaumatotibia leucotreta) and the larvae of swallow tail butterflies (Papilio demodocus).

FALSE CODLING MOTH (Thaumatotibia leucotreta) Lepidoptera:Tortricidae formally known as Cryptophlebia leucotreta
This is a very serious pest. It is widespread throughout Nigeria. Grapefruit and navel oranges are usually attacked. It is also a pest of avocado pear, guava, wild fruits, maturing maize ears and cotton bolls.
The larvae bore holes, usually in the base of mature fruit, where they enter and feed within. Larval damage allows secondary rots to attack the fruit and holes made by the larvae become surrounded by rotten tissues. The rotting spreads and the fruit drops before it is ripe. If the fruit is cut open the pink larva may be seen feeding inside the fruit surrounded by frass.
The adult moth has a wing span of about 16 mm and a body length of 7-8 mm. The forewings are mottled brown in colour with a prominent silver-white dot in the centre of each. The hind wings are paler with no markings and are fringed at the hind margin. Adults are nocturnal. The young larvae are white with dark spots and a black head capsule. When fully grown they are about 15 mm long and become pale pink in colour with a darker dorsal surface. The female can lay 100-400 eggs over the week long life span. Usually about 8 eggs are laid on the surface of each maturing fruit. Hatching occurs between 3-6 days. The young larvae spend some time moving about over the surface of the fruit before they penetrate. The larval period lasts for 17-19 days and there are five larval instars. Infected fruits will drop. The fifth instar larvae pupates in the fruit, in debris on the top of the soil or within the soil in a cocoon made from silk and soil particles. The pupal period lasts from 8-12 days.
Non-Chemical Control: Orchard sanitation is the main control method. Fallen infested fruits and the plant debris beneath should be gathered at weekly intervals and buried in a hole at least 50-100 cm deep, or burnt.
Chemical Control: Not economical



Adult female Thaumatotibia leucotreta Larva showing damage to orange fruit



FRUITFLY (Drosophila spp.) Diptera: Drosophilidae
The larvae and adults attack fruits at ripening, causing fruit rot and fruit drop. Stored oranges are also attacked.

Adult Drosophila



BANANA (Musa spp.)
Banana is generally eaten in the fresh form, because of its high sugar content, but it can be dried and made into a highly nutritious flour.
It is attacked by a number of insects among which are Banana weevil (Cosmopolites sordidus), Banana aphid (Pentalonia nigronevosa), Banana thrips (Hercinothrips bicinctus), Coconut scale (Aspidiotus destructor) and the Fruit piercing moth (Achaea lienardi).
BANANA WEEVIL (Cosmopolites sordidus) Coleoptera: Curculionidae
This is a major pest of banana and it is found in all banana producing areas. Records show it is only a pest of banana. The larvae cause damage by tunnelling and feeding within the corms. This makes the plant weak and susceptible to secondary attack by other insects or micro-organisms which cause the corm to rot. Death of seedlings may occur if the larvae penetrate the growing region of the plant. Tunnelled corm sometimes break, but as the insects do not attack the roots lodged plants are not usually associated with this pest. The leaves turn yellow and die.
Adults are about 13 mm in length with a long curved proboscis. When newly hatched they are brown in colour becoming black after a few days. They are winged but rarely fly. They are very slow moving and are nocturnal. The larvae are about 20 mm long, legless with a white body and brown head. The adult female bites a small hole in the base of the corm and lays a single egg. The eggs are white, 3 mm long, elongate and oval. Oviposition is continuous throughout the year but it is most common in the rainy seasons. Each female may lay a total of 10-50 eggs. Hatching occurs in 1-3 weeks and the larval stage lasts for 2-6 weeks. Larvae bore holes in the corm where they feed and pupate. Adults emerge within 1-3 weeks and live in the soil feeding on banana plant material and visiting growing plants to lay their eggs. The adults live from a few months to 2 years and can survive several months without feeding.
Non-Chemical Control: Destroy the sheltering and feeding places for adults; old stems should be cut to the ground level and covered with packed soil and weeds around the banana plants should be destroyed. Infected land should be left fallow. Pest free planting materials should be used.
Chemical Control: HCH, isofenphos, aldicarb and carbofuran are effective in the control of this pest.


Adult Banana weevil


Mature larva of Banana weevil

FRUIT PIERCING MOTH (Achaea lienardi) Lepidoptera: Noctuidae
Adult moths feed by piercing the skin of ripening fruits to suck the sap. This allows the entry of pathogenic organisms and encourages fruit rotting.

Adult Achaea lienardi


3.3. CONCLUSION
Insect pests are very destructive and cause a lot of damage if not properly controlled. The knowledge of the life cycle of most insects helps in the control of these pests.
4.0. SUMMARY
It is expected that at this stage you would be able to mention some pests of some economic crops and their control. You should also be able to use the life cycle of insects in their control.

5.0. STUDY QUESTIONS
1. What are insects?
2. How would you classify insects based on their economic importance?
3. Mention 2 crops of economic importance and list two insect pests associated with them
4. What is the difference between ‘Noctuids’ and ‘Pyralids’?
5. Discuss with diagram the life cycle of Busseola fusca
6. How can you differentiate between the larvae of B. fusca and S. calamistis based on their     life cycle?
7. Which insect larva feeds with the rear part of their body exposed outside the fruit?
8. Itemize 4 pests of cocoa in Nigeria and discuss the damage caused by any one of them.
9. Discuss one serious pest of cassava and its control.
10. Mention one major pest of banana. Discuss the non-chemical control method of this pest.

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