|Traditional Field Crops (Peace Corps, 1981, 283 p.)|
|Pest and disease control|
This list is not complete but deals with the more prevalent reference crop pests. Full or partial (genus only) scientific names are given in parentheses. More specific control measures will be given at the end of the insect section. Stored grain Pests, some of which attack the crops before harvest, will be covered in Chapter 7.
Major Pests Of Maize
White grubs (Phyllophaga, others): Brown headed, plump, six-legged, white larvae up to 25 mm long. Many are larvae of May (June) beetles and attack roots of maize and other grass family crops, sometimes causing serious damage. Especially common where maize is planted on recently cleared pasture land. Occasionally attacks legumes. Larval stage lasts one to three years.
Rootworms (Diabrotica, others): Small, slender, whitish larvae with brown heads, measuring up to almost 20 mm. They attack the roots and sometimes bore into the underground portion of the stalk while adult beetles feed on the silks and attack other crops. They are most prevalent in Latin America. Affected plants often become "goosenecked" because of lodging caused by root damage. Ten or more larvae per plant or a brown discoloration of 50 percent of the root system indicates serious damage.
Wireworms (Elateridae): Shiny, brown, hard larvae up to 1.5 - 3.5 mm long with six legs. The larval stage of click beetles attack germination seeds and below ground plant parts. Larval stage lasts two to six years.
Wireworm Larva (top) and Adult (bottom)
Cutworms (Agrotis, Feltia, Spodoptera): These are caterpillars ranging from bright green to black. Most are rather plump and curl up when disturbed. They attack young plants and cut off stems at or slightly above the soil surface, but some will feed on the leaves. Most remain below ground during the day and emerge at night to feed.
Lesser cornstalk borer (Elasmopalpus: Caterpillars, usually light green with faint stripes and distinct vertical bands of brown. They are most common in Latin America. Young larvae feed first on the leaves and then bore into the stalk about 2-5cm above ground. Each builds a tunnel made of soil particles and silk that runs from the soil to the stalk hole. May also attack the root system. Larval stage lasts about three weeks and pupation takes place in the soil in a silken cocoon.
Seed corn maggots (Hylemya): Yellowish gray fly larvae up to 6-7 mm long with a blunt rear end and a sharply-pointed head. They attack germinating seeds, sometimes eating out the entire kernel.
Maize Foliage Insects and Borers
Fall armyworm (Spodoptera frugiperda: Larvae have a green and brown coloring with a prominent, white, inverted "Y" mark on the head and grow to about 40 mm. One of the most serious and prevalent maize insects in the lowland tropics. The caterpillars are larvae of night-flying moths that lay eggs in clusters of 100 or more on the leaves. Eggs are covered by a coating of body hairs and scales and hatch in two to six days in warm weather. The larvae are cannibalistic and attack each other until only a few are left. They then move to the leaf whorl and feed on the unfolding leaves, but may also damage the growing point in older plants. Larva will sometimes tunnel into older plants. The larval stage lasts about three to four weeks and the pupal stage only 10 days, so maize can be attacked by several generations. Damage is easy to spot by the ragged appearance of the leaves and the large amount of sawdust-like excrement found around the leaf whorl. Diseases and predators may greatly reduce their numbers. Liquid or granular insecticides applied to the leaf whorl are effective and should be applied before the larvae have reached 16-18 mm.
A Mature larva; BAdult; CInjured germinating seed.
Corn earworm (Heliothis zea): A striped yellow, brown or green caterpillar. The moth deposits her eggs individually on the maize silks. Eggs are white, round, and smaller than the period at the end of this sentence, but can be easily seen with a low power magnifying glass. They hatch in three to seven days, and the larvae feed on the young silks and kernels near the ear tip. Earworms seldom interfere with pollination, since most silks become pollinated the first day they emerge from the ear. Eggs are sometimes laid on the leaves of younger plants, followed by leaf feeding in the whorl as with the armyworm. Ear damage is rarely serious enough to justify using insecticides, which would have to be applied to the silks-a time-consuming process. Varieties with long, tight husks have good resistance.
AEgg; BMature larva; and CAdult.
Miscellaneous leaf-feeding caterpillars (yellow striped armyworm, true armyworm, measuring worm, etc.): These may occasionally require foliar insecticide sprays.
Southern cornstalk borer (Diatraea), Southwestern corn borer (Zeadiatraea): Prevalent in lowland areas of Latin America. Moth larvae are about 25-mm when fully grown and are white with dark spots. Eggs are laid in overlapping rows of 10-12 on the leaves near the central veins. Eggs hatch in three to six days, and young larvae spend two to three days feeding on the leaves, making circular holes, before they bore into the stalk. Larval stage lasts several weeks, and pupation takes place inside the stalk. Control is only partly successful and requires spraying the plants during the short period before the larvae bore into the stalks or the use of systemic insecticides, some of which are very toxic.
Stalk borers (Busseola, Sesamia, Eldana, Chilo): Very common in Africa and parts of Asia and can cause serious losses. Busseola and Sesamia prefer young plants and can kill them by damaging the growing point. All four types may attack the ears on older plants in addition to the stalks. Busseola moths mate soon after emergence from the pupal stage and deposit their eggs in groups of 30-100 on the inner leaf sheath near the whorl. The larvae feed on the whorl and then tunnel into the young plant. Systemic insecticides applied to the soil or to the leaf whorl give fair to good control. Eradication of wild grasses that serve as borer hosts helps reduce numbers.
Leafhoppers (Cicadulina Dalbulus: Small, light-green, wedge-shaped insects with piercing-sucking mouthparts. Cicadulina transmits maize streak virus in Africa, and Dalbulus spreads corn stunt virus ("achaparramiento") in Latin America. Both diseases can cause serious losses. Insecticides are effective.
Grasshoppers: Cause serious losses in parts of Africa. Foliar sprays and baits are effective unless the infestation is severe.
AphidsWingless and Winged (USDA)
Maize aphids (Rhopalosiphum): Small, soft-bodied, green or blue, green insects that suck sap from plants and secrete a sweet substance (honeydew) on which a black mold grows. They can stunt and deform the tassels, causing poor pollination. Treatment should be considered if 50 percent of the plants have some aphids and 10-15 percent are heavily infested. Systemic insecticides give longterm control.
Common Storage Insects of Cereal Grains
Maize weevil (Sitophilus zeamais), rice weevil (S. oryzae), and granary weevil (S. granarius): All have long snouts and are about 8.3mm long. Only the maise and rice weevils can fly and infest crops in the field. Females live several months and lay 200-400 eggs by boring holes in the kernels and depositing the eggs inside. The white, legless larvae feed on the inside of the kernels, then pupate, and finally emerge as weevils. All three species are more common in humid than dry regions.
Angoumis grain moth (Sitotroga cerealella): A small cream- or tancolored moth with a wingspan of about 12.7 mm that is often the major stored grain pest in drier regions. Adult moths have a black fringe on the tip of each forewing. They can infest grain both in the field and during storage, but can penetrate only about the top 4-inch layer in stored, threshed grain. Maize stored as ears can be completely infested, however. Each female lays about 40-400 eggs on the outside of the kernels, and the tiny larvae burrow inside to feed. Pupation takes place inside the kernel, and the young moths emerge to begin a new cycle. The moths themselves do no feeding. Unlike most other storage insects, the angoumis grain moth can be controlled by spraying or dusting only the surface layer of stored, threshed grain with an approved insecticide like Malathion or pyrethrin.
Major Sorghum Pests
Sorghum is attacked by many of the same insects that attack maize, but two other insects can also cause serious damage.
Sorghum midge (Contarinia sorghigola: A small orange fly about 2 This is the most important sorghum pest worldwide. The adult lives only about a day and lays eggs on sorghum grain heads during flowering. Larvae hatch in two to four days and spend 9-11 days feeding on the juices of the developing seeds, preventing them from developing. The pupal stage lasts two to six days for a total life cycle of just 15-20 days.
Some local varieties show fair resistance to this pest. Sorghum heads can be sprayed with an insecticide three to five days after they emerge from the boot. Sorghum should not be planted near young sorghum or Johnsongrass, and out of season sorghum heads should be removed from fields. In cooler areas, the larvae pupate in a silken cocoon, but may also do this in very hot, dry weather. Plowing under residues may help control the pest in these cases.
The sorghum midge, Contarinia sorghicola (Coq.). Adult female and larva in its cocoon.
Sorghum shoot fly (Atherigona soccata): A major pest in Africa and Asia. Adults look like small houseflies and lay eggs on the leaves of young plants. Larvae move down into the leaf whori and then bore into the young stem, often killing the growing point. The youngest leaf then turns brown and withers-this condition is called "deadheart". Some sorghum varieties show shoot fly resistance. Insecticides applied to the whorl are not as effective as preplant applications of systemic insecticides to the soil.
Maize Weevil (Sitophilus zeamais). The rice weevil (S. oryzee) looks identical.
Millet is attacked by many of the same inects as sorghum, including the shoot fly, midge, and stem borer, but damage is usually less serious. The millet grain midge (Geromyia pennisetti) is common in the savanna region of Africa. A caterpillar (Masalia spp.) has increased in numbers in the northern savanna and Sahel during the 1970's and can cause serious head damage.
Sorghum Shoot Fly
The flower thrips, Frankliniella tritici
White grubs, wireworms, and rootworms attack peanut roots, and the latter two also attack the pods.
Termites can severely attack the pods, but damage is usually patchy. Treating planting seed with an insecticde, destroying the nests with Chlordane or other insecticides, or applying insecticides broadcast or banded along the crop row are effective on termites.
The lesser cornstalk borer may bore into stems and pods. In Senegal, about a dozen types of millipedes damage pods. Any pod damage increases the likelihood of aflatoxin ( a harmful toxin and carcinogen produced by Aspergillus fungus; see section on diseases).
Thrips: These tiny(1 mm) yellow to black insects have two sets of fragile wings which are fringed with hairs along the rear edge. Immature thrips (nymphs) are light yellow to orange and smaller than the adults. If disturbed, thrips will jump or hop. They can cause serious damage by feeding in the buds or folded leaflets. They have rasping-sucking mouthparts which cause the leaves to be scarred and distorted as they unfold. Thrips can also spread spotted wilt virus.
Leafhoppers: Can be another major pest. Adults are around 3 mm long, pale green, and wedge-shaped. Immature leafhoppers (nymphs) are similar in appearance to adults, but smaller and without wings. Both stages have piercing-sucking mouthparts. The first signs of leafhopper damage are yellow "V" formations at the leaf tips, and severe cases can cause stunting and leaf drop.
Spider mites (Tetranychus and other species: Common in hot, dry conditions. They are sucking insects, and feeding damage may appear as translucent dots on the leaves. Some insecticides will not control mites, while Kelthane is effective only against mites.
Corn earworms (Heliothis spp.), armyworms (Spodopters, Pseudaetia), and other caterpillars feed on the leaves. Blister beetles (Epicauta spp.) are brightly colored with alternate bands of black and red or yellow-they feed on the flowers. Aphids occasionally attack peanuts. One species (Aphis croccivora) spreads rosette virus, a serious problem in Africa.
Peanuts are very susceptible to attack by storage insects. The groundnut bruchid (Caryedon spp.) is a serious pest in West Africa. This weevil lays eggs on the pods after the crop has been lifted from the ground, and the larvae tunnel into the pods and kernels.
A Spider Mite (Univ. of Arizona)
The following information is based on The International Center for Tropical Agriculture (CIAT) studies on the mayor insect pests of common beans (Phaselous vulgaris) in Latin America.
Seedling Stage Insects
Cutworms and white grubs may cut off the stems of young seedlings. White grubs are usually only serious when beans are planted following pasture. The lesser cornstalk borer may bore into the stem just below the soil surface and move upwards and kill the plant. Clean fallowing for long periods or heavy flooding will control these borers as will granular insecticides applied near the seed row at planting.
Leaf Feeding Insects
Many species of beetles, such as the banded cucumber beetle (Diabrotica balteata), bean leaf beetle (Cerotoma), flea beetle (Epitrix), and Mexican bean beetle (Epilachna), attack bean leaves. The most serious damage is caused during seedling stage when the insects can defoliate the plant more readily, or during flowering. Both larvae and adults of the Mexican bean beetle feed on the leaves. The larvae of the other beetles feed mainly on the roots of beans, maize, and certain weeds.
Caterpillars usually do not cause economic damage on bean leaves. The bean leafroller (Urbanus or Eudamus), saltmarsh or wooly bear caterpillar (Estigmene), and Hedylepta caterpillar are the most common.
The leafhopper species Empoasca Kraemeri is the most serious insect pest of beans in Latin America and is also found in other regions. It does not transmit virus (some other leafhoppers do) but causes severe stunting, yellowing, and leaf curling. Work done by CIAT has shown that yields are reduced about six percent for each leafhopper present per leaf. Eggs hatch in eight to nine days and the nymphs feed on the plants for eight to eleven days before becoming adults. The adult stage lasts about 60 days and is more damaging. Beans grown with maize are less affected than pure stands. Mulching reduces leafhopper populations. Leafhopper problems are generally more severe in hot, dry weather.
Several species of aphids attack beans, although their feeding causes little direct damage, they can transmit bean common mosaic virus.
Mexican Bean BeetleAdult and Larva
Several species of mites attack beans. The red spider mite is found on the lower leaf surface, and heavy infestations turn the leaves brown. The tarsonemid mite is too tiny to be seen without a magnifying glass, but causes young leaves to curl up-ward. Mites are seldom serious except during the dry season.
Whiteflies (Bemisia spp.) do not usually cause direct damage but can transmit bean golden mosaic virus and bean chlorotic mottle virus. They are often controlled by natural predators, and most insecticides are effective.
The bean pod weevil (Apion godmani) is a serious problem in Central America. Adults are black and about 3 mm long and they feed on flowers and pods without causing much damage. However, the female chews a small hole in young pods and deposits an egg. The larva feeds on the inner pod and the developing seeds. Pupation takes place in the pods, and the adults emerge near harvest time. Bean types vary in their resistance. A number of insecticides give good control if applied once at a week past flower initiation and again a week later. Carbafuran applied at planting gives excellent control.
Bean bruchids (Acanthoscelides obtectus and Zabrotes subfasciatus ) are snout less weevils about 2.5mm long and are the major storage pests of beans. A. obtectus predominates in cooler areas, while Z. subfasciatus prefers warmer regions. Life cycles for both are very similar with eggs being laid on stored beans or in cracks of growing pods in the field. The larvae tunnel into the seeds to feed.
Adult weevils are short-lived and do little feeding. Both types of weevils may be present initially, but A. obtactus is a better competitor at lower temperatures and will eventually predominate under these conditions. These bruchid weevils are estimated to cause storage losses of up to 35 percent in Mexico and Central America.
Slugs occasionally cause serious leaf damage and are mainly active at night or on wet, cloudy days. Damage is msot likely along field borders but may move inward. Cleaning the field of weeds and plant debris helps control them, but baits are the most effective means of control. Slime trails on the leaves indicate the presence of slugs.
The caterpillar Maruca testulalis is the major cowpea pest in the Savanna region of Africa. It attacks flowers, pods, and leaves, causing yield losses up to 70-80 percent.
Coreid bugs (plant bugs) are larger sucking insects that feed on green pods and cause them to shrivel and dry prematurely.
The leaf feeding beetle Ootheca mutabilis can cause yield reductions when young plants are heavily attacked. It also carries yellow mosaic virus.
The flower thrip (Megalurothrips sjostedti) is a major cowpea pest in tropical Africa. Thrips have suckingrasping mouthparts and are very small (about 1 mm or less).
The snoutless bruchid weevils (Callosobruchusspp) infest cowpeas both in the field and in storage. The adults can fly up to a kilometer and are most likely to infest crops downwind from strong facilities. The 2.5 mm adults lay eggs on the pods or seeds, and the larvae bore into the grain.
IITA, in Nigeria, estimates that one-third of the cowpea crop in Africa is destroyed by bruchids.
Cowpea bruchids (Callosobruchus spp.)