Spodoptera exempta
(black armyworm)

Pictures

Top of page

Identity

Top of page

Preferred Scientific Name

• Spodoptera exempta Walker

Preferred Common Name

• black armyworm

Other Scientific Names

• Agrotis exempta Walker 1856
• Caradrina exempta
• Laphygma exempta Hampson 1909
• Leucania exempta
• Prodenia bipars Walker 1857
• Prodenia exempta Walker
• Prodenia ingloria Walker 1858

International Common Names

• English: african armyworm; armyworm, african; armyworm, true; hail worm; mystery armyworm; rain worm
• Spanish: gardama africana; gusano soldado africano
• French: chenille defoliante; chenille légionnaire; chenille processionaire

Local Common Names

• : Barnosay; mystery worm; Viwavi jeshi
• Australia: Day-feeding armyworm; Leaf-eating grassworm; variegated armyworm
• Ethiopia: Geiry; Temch
• Germany: Heerwurm
• Kenya: Keenyu; Kungu; Ngonga; Ng'Urrto
• Malawi: Chipakusu; Kapuchi; Nchembere; Zandonda
• Mozambique: nyanja
• Netherlands: Geelgestrepte legerrups; Legerworm
• Somalia: Diirta afrikaana
• South Africa: Kommandoworm
• Sudan: El-Afrigia; El-Dudah; El-Zahfa
• Uganda: N'Kungula; Omor
• USA/Hawaii: Nutgrass armyworm
• Yemen: Gidami
• Zimbabwe: Imhogoyi; Mhundururu

EPPO code

• LAPHEX (Spodoptera exempta)

Taxonomic Tree

Top of page

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Arthropoda
•             Subphylum: Uniramia
•                 Class: Insecta
•                     Order: Lepidoptera
•                         Family: Noctuidae
•                             Genus: Spodoptera
•                                 Species: Spodoptera exempta

Notes on Taxonomy and Nomenclature

Top of page This species was originally named Agrotis exempta by Walker in 1856. It was assigned to the genus Laphygma by Hampson in 1909, and to the genus Spodoptera Guenée by Zimmerman in 1958. The genus Spodoptera currently contains 22 species of which seven are found in Africa, one of them, S. malagasy Viette being recorded only from Madagascar. Three of the species found in Africa are significant pests: S. exigua (Hübner), S. littoralis (Biosduval) and S. cilium Guenée. Three species are of significant pest status elsewhere: S. mauritia (Biosduval) as a severe pest of rice in Asia; S. litura (Fabricius) as a serious pest of cotton and other crops in India, the Far East and Australasia; and S. frugiperda (JE Smith) as an outbreak pest of poaceous crops in North America and the Caribbean. For further information, see Brown and Dewhurst (1975).

Description

Top of page

Eggs

The eggs of Spodoptera exempta are pale-yellowish, darkening through development until, just before hatching, the black head-capsules of the larvae can be seen through the shells. Each egg is about 0.5 mm in diameter, conical with a slightly rounded apex and a densely sculpted surface. Eggs are laid in batches of 10-600 which are covered by black scale-hairs from the tip of the female's abdomen; this covering may be sparse on later batches.

Larvae

Larvae occur in two principle forms: the gregarious (gregaria) form characteristic of high-density populations and the solitarious (solitaria) form found at low larval densities. Intermediate, 'transiens' forms may also be present. Gregarious larvae have a velvety-black upper surface with pale lateral lines, a green or yellow ventral surface, and no hairs on the body. There are three parallel lines on the dorsal surface of the prothoracic (first body) segment and a stripe running longitudinally down the mid-dorsal surface of the body is always paler than the black pigmentation on either side of it. The head is always shiny-black. Solitarious larvae are cryptically coloured in a variety of shades of green-brown or pink, appear fat, and are extremely sluggish. They are difficult to distinguish from other grass-feeding caterpillars (see section on Detection and Inspection Methods). Larvae of the IV and later instars may be sexed by the presence, in females, of small pits on the ventral surface of the eighth and ninth abdominal segments.

Pupa

Mahogany-brown, 10-14 mm long, with a smooth, shiny surface. They are difficult to distinguish from pupae of other Spodoptera species. They may be sexed by examining markings on the ventral surface of the terminal abdominal segments.

Adult

Adult S. exempta are stout-bodied moths of typical noctuid appearance, 14-18 mm long and with a 29-32 mm wing span. The abdomen is covered with pale grey-brown scales except for the tip in the female which has black hair-scales characteristic for this species. Forewings are dark-brown with distinctive grey-black markings. An inner (orbicular) spot is elongated and pale in both sexes but more pronounced in the female. An outer (reniform) spot is arrow/kidney shaped in both sexes but much less apparent in the female. Hindwings are white with dark veins. See Rose et al. (1996) and references therein.

Distribution

Top of page S. exempta is found in Africa, on the Arabian Peninsula, and in South-East Asia, Australasia and Oceania (including Hawaii, USA). Moths have been taken in Japan and New Zealand but there is no evidence that the species is established there. S. exempta is widespread in Africa south of the Sahara, being most prevalent in the east and eastern central regions of the continent. In South-East Asia, it is most frequently recorded from the Indonesian islands, including Kalimantan, Sulawesi and the Philippines. In Australasia, it is confined to Papua New Guinea, the Solomon Islands and New Caledonia, and to the northern and eastern seaboard of Australia.

Distribution Table

Top of page

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Risk of Introduction

Top of page S. exempta is not listed as a quarantine pest by EPPO.

Hosts/Species Affected

Top of page Hosts of S. exempta are primarily plants of the Poaceae and Cyperaceae. Major economically important hosts are: barley, Pennisetum typhoides, Eleusine coracana, maize, oat, rice, sorghum, sugarcane, Eragrostis tef, wheat and pasture grasses, especially Cynodon and Pennisetum species. The larvae exhibit strong host preferences within the Poaceae and there are major differences between varieties of cereal crops in their susceptibility to attack. S. exempta has once been found causing serious damage to coconut seedlings and once to young tea and, during high-density outbreaks, non-host plants including tobacco and cotton may be eaten, though not extensively. See Rose et al. (1996) for a full list of wild and cultivated host plants.

Host Plants and Other Plants Affected

Top of page

Growth Stages

Top of page Flowering stage, Fruiting stage, Seedling stage, Vegetative growing stage

Symptoms

Top of page The symptom of S. exempta attack is gross feeding damage to foliage, growing points and young stems. Severe infestation results in total defoliation or destruction of the plant to ground level.

List of Symptoms/Signs

Top of page

Biology and Ecology

Top of page S. exempta larvae develop in markedly different forms or 'phases' depending on larval densities in the early instars. Although similar 'phase polyphenism' occurs in other Noctuidae, it is most strongly developed in S. exempta, in which the highly conspicuous, heavily pigmented and active 'gregarious' ('gregaria') form, typical of outbreak populations, is the familiar armyworm. The cryptic, sluggish 'solitarious' ('solitaria') form, typical of low-density populations, is difficult to find and often not recognized as an armyworm. A second key feature of the biology of S. exempta is the absence of any pre-imaginal diapause; a single report of an extended pupal period has never been confirmed.

In eastern Africa, there are typically six to eight outbreak generations per year, with four to five in southern Africa. There is a period of 3-5 months when no outbreaks are reported, during which low density populations persist in areas supporting continued growth of host plants, where a total of 13 generations per year is possible.

Oviposition usually begins at 20.00-21.00 h, eggs being laid in batches of 10-600 and covered with black hair scales. A single female lays about 1000 eggs over a period of up to 6 nights but little or no oviposition occurs at temperatures below 20°C. Oviposition often occurs on substrates other than host plants, including dry grasses, leaves of tall plants, twigs of bushes and trees, or on buildings. Eggs hatch in 2-5 days (typically 3 in normal outbreak conditions), with a temperature threshold for development of 10-12°C. Eggs generally hatch in the early morning and the translucent larvae, after feeding on the egg shells, drop from the oviposition site on silken threads to be dispersed on the wind, often over several metres. Mortality can be high, with larvae failing to reach suitable hosts, being drowned by rainfall, or taken by predators. On reaching a host plant, positive phototaxis and negative geotaxis takes them to the upper, often youngest, foliage where they commence feeding by rasping the lower lateral epidermis of the leaves, creating 'windows' and gradually becoming green. They remain green for the first three instars, diverging into the solitarious or gregarious forms at the III-IV instar moult. At this moult, the form of the mandibles also changes, enabling the larvae to feed on the leaf edge. There are normally six larval instars and food consumption increases rapidly in the latter stages, reaching about 0.2 g/day for a fully grown larva on maize. The increase in weight from first to last instar is about 1000-fold.

Gregarious larvae are predominantly black, very active, and feed on the upper parts of host plants, exposed to the sun and avoiding shade. They are adapted behaviourally and physiologically for accelerated development; in particular, their behaviour and pigmentation result in elevated body temperatures by the absorption of solar radiation. The development of gregarious larvae in outbreaks is often highly synchronized, with most larvae pupating over a 3-7 day period. There may also be fewer than six larval instars. Older gregarious larvae may 'march' in large numbers, especially when food plants are locally depleted or when development is complete and they are searching for pupation sites. Solitarious larvae are predominantly greenish and highly cryptic in their coloration and behaviour. They are sluggish, actively avoiding the sun, and sheltering and feeding at the bases of grasses. Although usually sparsely distributed and difficult to find, they can occur at relatively high densities (e.g. 10/m²) where thick vegetation prevents contact between early instars.

The full grown larva burrows into soft, damp soil to construct a silk-lined chamber 2-3 cm below the surface, where it pupates. High mortality may occur if the ground is too dry and hard.

In addition to temperature and larval density, development rates are influenced by food-plant type and quality, sex and the effects of parasitism. The larval period lasts from 11-24 days (21 days in typical outbreak conditions). Pupal periods range from 7-12 days (typically 10 days in outbreaks). The temperature threshold for larval and pupal development is 13-14°C. The larval period of females is longer, and the pupal stage shorter, than for males and, as a result, the peak of female emergence during outbreaks is usually one night earlier than that of males.

Moths emerge in the early part of the night, peak emergence being between 20.00 and 22.00 h. In a single outbreak, emergence may extend over about 12 days with a peak during the first 4 days. Moths are ready for flight in 1.5-2 h when some ascend in migratory flight to be carried downwind. However, the majority fly into nearby trees greater than 1.5 m in height where they may accumulate in numbers of several thousands. Numbers of these moths may embark on migratory flights at any time of the night, forming ascending 'plumes'. Those that remain in the trees at dawn undertake short flights to shelter during the day in grass clumps, under cowpats or in crevices in the bark of trees. The following dusk, they emerge from their day shelters to embark on downwind migratory flights, some moving briefly into trees before ascending. Where nectar is available, moths feed voraciously.

Current evidence suggests that S. exempta is an obligatory migrant, i.e. all individuals emigrate from the site of their emergence. Reports of successive generations at the same site are rare; they appear to occur only when emigration has been prevented by rainfall after moth emergence or by re-invasion by immigrant moths from distant sources. When they occur, they are likely to suffer high mortality from natural enemies, including pathogens and exhaustion of local food plants.

During the main outbreak season in East Africa, the prevailing easterly to south-easterly winds are strong in the first half of the night so migrants embarking at dusk or early in the night can be carried for long distances, of 100 km or more. Radar studies show migratory flight at up to 420 m above ground level in open savanna country and double that altitude in the Rift Valley near the Nairobi escarpment. The winds tend to lighten and become variable in the second half of the night when migrants may orientate across or upwind and displacements are consequently more limited. Radar also shows that the moths do not fly in cohesive swarms but are dispersed as they move downwind. In moderate to heavy rain, migrants descend to the ground.

In females, migration terminates when reproductive development reaches the stage at which the moths call (release sex pheromone) and is, therefore, exclusively pre-reproductive. Thereafter, only local flights occur during mating and oviposition. The oocytes are not developed when the females emerge but develop to approximately half full size within 24 h. Oocyte development may then be arrested for up to 13 days. Whether this arrest occurs, and for how long, is genetically determined. Once it is over, females require water to hydrate the maturing oocytes and complete development. Moths have been seen to feed avidly on nectar and to drink dew before, between, and after migratory flights but the availability of food or water does not terminate the arrest of oocyte development.

Variation between individuals in flight capacity (duration) on individual nights is also genetically determined although there is evidence that it is moderated by larval density (moths from gregarious larvae show enhanced flight performance). A high level of variation in both of the traits which determine migratory potential (capacity for displacement), i.e. flight capacity and pre-reproductive period (the number of nights over which migratory flights occur) has been demonstrated in field populations. This variation is thought to reflect the selection imposed by the pattern of distribution, within and between seasons, of suitable habitats. On average, male moths reach maturity earlier than females and the available evidence suggests that they may continue migration after attaining maturity, mating with females at different locations on different nights.

Marked moths have been recovered up to 147 km from an emergence site but studies of the spread of outbreaks suggest the frequent occurrence of much longer displacements of 200-700 km. In New Zealand, they have been recovered 3200 km from the nearest known source although these moths must have been constrained to fly by day and night across the sea.

Thus, moths emerging from a high-density outbreak become widely dispersed downwind, in both space and time, as a result of: emergence and emigration over a period of about 10-12 days; differences between individuals in migratory potential; and dispersal in flight on winds which vary in speed and direction across both time and space. The usual outcome is widely dispersed oviposition resulting in low-density, solitarious populations.

However, during the rains, flying moths have been shown, on radar, to be concentrated by wind convergence associated with rainstorms and topography. Particularly important is the strongly convergent airflow in rainstorm outflows, at the gust front, where further dispersal is often curtailed by the descent of flying moths as they encounter the rain. The complex patterns of eddies established in the lee of hills and other topographical features may also accumulate moths flying into them. It is the localized oviposition by these concentrations of moths that result in outbreak populations of gregarious caterpillars.

At the end of their migratory flight, mature female S. exempta settle in trees, releasing sex pheromone to attract males. The pheromone has two major (Z-9-tetradecen-1-yl acetate and (Z,E)- 9, 12-tetradecen-1-yl acetate) and four minor components. Mated pairs are found from 21.00-05.00 h with a peak between 24.00 and 02.00 h. Multiple mating is common.

S. exempta has an extremely high reproductive capacity and thus potential for population increase. Laboratory studies indicate Ro (net reproductive rate) from 15 on poor quality grasses to 125 on Cynodon sp. and 142 on maize. Assuming Ro = 100 and 80% mortality, increases of 10,000-fold are possible in two generations. These levels are easily achieved for early season infestations feeding on flushes of new grass growth.

The annual cycle of S. exempta in eastern and southern Africa can be summarized as follows.

In eastern Africa, the rains are associated with the passage of the Inter-Tropical Convergence Zone (ITCZ) which moves northwards and southwards seasonally, moving over northern Sudan and the southern Arabian Peninsula in July to August, southward through Ethiopia and Somalia during October, across Kenya during November, and reaching Tanzania by December and as far south as northern Mozambique and Zimbabwe in January. During February and March, it moves north through Tanzania, crossing Kenya in April and May and southern Sudan, Ethiopia and Somalia in May to June, to reach western Arabia by July. The southward movement of the ITCZ is associated with the 'short rains', and the northward movement with the 'long rains', in southern Ethiopia, Somalia, Kenya and northern Tanzania. Regions at the extremes of its traverse usually have only one rainy season.

During the dry season, grasses dry out over most of eastern Africa, becoming unsuitable for larval development. However, populations persist where host plants continue to grow, for example, where there is intermittent rainfall through this period. In the absence of concentrating weather systems, these populations remain at low densities in the solitarious form. In eastern Africa, dry season populations have been detected in Malawi, western Uganda, on the shores of Lake Victoria and in highland areas, and probably most importantly, in coastal regions of Kenya and Tanzania. Similar areas presumably support dry-season populations elsewhere in Africa. These populations are the source of moths initiating the first outbreaks of the subsequent rainy season.

First outbreaks typically occur in Kenya and Tanzania where there are two rainy seasons. They are associated with the first rainstorms of the short rains which tend to occur on the eastern side of high ground where moths, flying from coastal source areas on the dominant easterly winds at this time, meet storm-induced wind convergence. These 'primary' outbreaks, derived from low-density populations, are distinguished from subsequent 'secondary' outbreaks which are initiated by moths originating from earlier outbreaks. Outbreaks (usually primary) leading to significant sequences of secondary outbreaks are termed 'critical'. The sequence of outbreaks that occurs seasonally follows the pattern of seasonal winds. In eastern Africa, outbreaks move progressively westwards from eastern Tanzania and Kenya, in some years as far as Burundi, before the start of the major northward movement with the passage of the ITCZ through Kenya and Uganda, Ethiopia, Somalia and southern Sudan, in some years reaching the Yemen. In southern Africa, the spread of outbreaks seems to originate in southern Tanzania, northern Mozambique and Malawi, moving into Zimbabwe, Swaziland and South Africa, with less frequent invasions of Zambia and Botswana.

The extent and severity of outbreaks appears to depend largely on the successful development of primary outbreaks at the start of the rains. This has been shown to be associated with preceding drought, frequent early storms to concentrate flying moths and initiate flushes of grass growth, and dry and sunny periods during the larval stages to promote survival and rapid development. Therefore, major upsurges occur in seasons of sporadic rainstorms and long sunny periods throughout the outbreak period. Prolonged cool, cloudy and rainy weather in the short rains is associated with high larval mortality, particularly from viral disease, and suppresses the development of subsequent outbreaks.

With its rapid development, high reproductive capacity, and mobility by migration, S. exempta is well adapted to exploit the highly seasonal and ephemeral habitats provided by the rain-induced growth of host plants in tropical grasslands.

This account is based on extensive, long-term studies in East and southern Africa. Very little is known of the biology and ecology of S. exempta elsewhere in its range.

See Rose et al. (1996) and references therein.

Natural enemies

Top of page

Notes on Natural Enemies

Top of page Natural enemies sometimes have a significant impact on S. exempta populations though their influence is difficult to quantify. Lists for eastern and southern Africa of species attacking all stages are given by Rose et al. (1996). There is no published data on natural enemies elsewhere in the species' range. Of the 21 genera of Diptera and 23 of Hymenoptera represented among armyworm parasitoids, only four Diptera and four Hymenoptera are regularly found in significant numbers. Because of its short life cycle and obligatory migration, there is little opportunity for parasitoid populations to build up to levels capable of constraining population development significantly. Only on rare occasions, when successive generations occur at one location, have high levels of parasitism (exceeding 70%) been recorded. Avian predators (storks [Ciconiidae] and crows [Corvus]) may decimate small outbreaks but have little influence on larger ones. The major cause of mortality in S. exempta larvae is probably the species-specific nuclear polyhedrosis virus (SpexNPV). First outbreaks of the season may be virus-free because of the lack of transmission in low density populations but later outbreaks may be almost eliminated by this virus, mortalities of >98% being frequent. Widespread virus attack is often associated with overcast, cool, wet weather. A Cytoplasmic Virus (CPV) is also an important pathogen, killing pre-pupae and pupae. Fungi and bacteria are thought to be of minor importance. See Rose et al. (1996) and references therein.

Impact

Top of page Losses to agricultural production due to S. exempta are frequently significant and intermittently devastating on a local, national and regional scale. Significant losses are most consistently reported from eastern and southern Africa. However, in recent decades, the frequency of reports from West Africa has increased, possibly due to the extension of suitable grassland habitats following forest and bush clearance for agriculture. Outbreaks are occasionally reported from elsewhere in the species' range, notably from Papua New Guinea, northern Australia and Hawaii, USA.

Damage to cereal crops results principally from direct attack on young plants by larvae hatching or dispersing into the crop as first instars, and by invasion of the crop by older larvae from adjacent wild grasses. Where these invasions are caused by late-instar larvae moving from heavily infested grasslands, even maturing crops can be totally destroyed. If drought conditions follow an outbreak, plants may not recover from defoliation and replanting may fail to produce a crop.

Smallholders are particularly vulnerable to the effects of infestation as they rarely have the resources for effective control or spare seed for replanting. Infestations frequently affect large areas, eliminating the possibility of relief by mutual support and assistance between farmers. Government crop protection and extension services may be able to provide only limited assistance as a result of financial and logistical constraints.

Yield reduction caused by defoliation in maize is almost directly proportional the percentage of leaf area available to the larvae at the time of attack. Reported losses range from 9% in plants attacked at the early whorl (four leaves) stage to 100% in those damaged at the pre-tassel stage. The ability of young maize plants to recover from armyworm damage depends on the position of the apical meristem at the time of attack and the amount of root development when the larvae cease feeding. Damage is always serious if the apical meristem is affected but, as it remains at the base of the plant until near to the pre-tassel stage, it may be below ground during the outbreak and remain undamaged.

Tentative nominal action thresholds for control measures have been determined for maize. To avoid yield losses of >15%, action thresholds for early whorl maize should be taken as 200 second (II), 80 third (III), or 20 fourth (IV) instar larvae per 100 plants. Serious damage develops rapidly once larvae reach the IV instar.

Replanting maize after armyworm have eaten the first-sown plants to the ground is frequently unsatisfactory as the optimum planting dates will have been missed. Yield losses of 6% have been estimated for each day's delay after the optimum planting date in high-rainfall areas in Kenya. Late planting may result in much higher losses in areas with less rainfall; yield losses of up to 92% have been recorded in such areas in Malawi and Kenya.

In sorghum, millet, rice and Eragrostis tef, armyworm damage may stimulate tillering which can, in favourable conditions, increase yield. If subsequent rainfall is adequate for crop growth and development, yield losses may be limited, providing the damage occurs before the critical grain-initiation stage has been reached.

Damage to pasture and rangeland can be extensive and severe. Armyworm damage to grasses and the consequent advantage to dicotyledonous weeds results in changes in the composition of the sward which may be re-inforced by drought and overgrazing. However, good rainfall after infestation is an important factor in pasture recovery. Surveys in Tanzania indicate that effects of infestation may last for more than eight weeks but, in areas with good rainfall, they seldom last more than 5 weeks. In Kenya, vegetation changes in infested pastures have been reported to persist for many years before good grass cover has been restored by management of dicotyledonous weeds. As a general rule, control measures for the protection of pasture are not recommended unless larval densities exceed 10/m².

Deaths among cattle grazing recently infested pasture have been reported by herdsmen in southern Ethiopia, Somalia (where 100 cattle were reported to have died on one occasion), and Kenya, as well as in southern Africa. Speculations as to the causes of death include high cyanide levels induced in Cynodon spp. grasses by armyworm damage, and ingestion of larvae or fungal mycotoxins on armyworm faeces.

See Rose et al. (1995) and references therein.

Detection and Inspection

Top of page

Infestation of the crop by S. exempta is evident from the `windowing' or skeletonizing of younger leaves caused by rasping of the epidermis by young larvae, or gross feeding by older larvae. The horizontal middle leaves of the plant, where photosynthetic activity is greatest, are particularly affected, but the growing points, other leaves, and the stems of younger plants are also affected. Large numbers of black larvae feeding on poaceous plants can be assumed to be S. exempta, whereas larvae feeding in significant numbers on leaves of dicotyledonous plants are unlikely to be (although damage to non-poaceous plants has occasionally been reported). S. exempta egg batches can be distinguished from those of all other Spodoptera spp. by the presence of a covering of black hair-scales. The main distinguishing features of gregaria-phase larvae are:

* velvety black-dorsal surface
* three pale parallel lines on the dorsal surface of the prothoracic segment
* central longitudinal stripe along the top of the body always paler than the black areas on either side
* pale lateral lines
* green or yellow ventral surface
* lack of hairs on the body
* shiny black head

They can be distinguished from the following species, with which they are sometimes confused, by the following features:

* Spodoptera exigua - often very similar but paler dorsally than laterally
* Helicoverpa spp. - the larvae are green or brownish, covered with short hairs, and do not have a black head-capsule
* sawflies (Symphyta) - larvae are typically black and hairless with more than five (usually seven) pairs of prolegs on the abdominal segments. Spodoptera larvae lack prolegs on the first two abdominal segments.

Solitaria-phase larvae may be difficult to distinguish from other grass-feeding caterpillars. Their green, pink or brown coloration is cryptic and the larvae hide in the grass mat during the day, emerging to feed at night. When found, they appear fat, are usually curled up, and behave very sluggishly. The distinguishing characters of older larvae of both phases are the three parallel pale lines on the dorsum of the prothoracic segment, the white spot at the rear of each abdominal segment, and the shape of the mandibles. Pupae of all Spodoptera species are very similar and can only be distinguished with difficulty. However, if very large numbers of pupae are found over large areas by digging, they are likely to be from an outbreak of S. exempta. Adults may be distinguished from other Spodoptera spp. by their forewing pattern and genitalia using keys published by Brown and Dewhurst (1975) and modified by Rose et al. (1996). The black hair-scales at the tip of the abdomen of the female moth and the grey, racket-shaped scales on the outer part of the genitalia of males are characteristic of S. exempta, and useful for identifying moths which have lost their wing scales, e.g. in light traps.

Prevention and Control

Top of page

Forecasting and Control

Effective forecasting and management of pests like S. exempta, whose long-range migrations result in the rapid spread of outbreaks from country to country, requires international cooperation. This is most easily achieved if regional organizations have responsibility for coordinating information and, if possible, the infrastructure required for both forecasting and control. Several national crop protection services, especially in eastern, central and southern Africa and the Yemen, have departments with special responsibility for control of migrant pests, including armyworm. These may assist farmers in countries where armyworm is a notifiable pest, by providing and/or applying insecticide. The Desert Locust Control Organization for Eastern Africa (DLCO-EA) and the International Red Locust Control Organization for Central and Southern Africa (IRLCO-CSA) have regional responsibilities for armyworm. Where infestations are widespread, governments have the option of requesting (or hiring) spray aircraft from them. International cooperation also allows the development of a strategic approach to limiting the development and spread of outbreaks. By focusing attack on early season infestations providing sources of migrants, it should be possible to reduce or eliminate the occurrence of subsequent outbreaks, or sequences of outbreaks, downwind. Such an approach has been developed and implemented by DLCO-EA and its member countries. For further discussion of strategic control of S. exempta, see Cheke and Tucker (1995), Rose et al. (1996), and references therein. Forecasting An effective forecasting system is essential for successful management of Spodoptera exempta, to allow preparations to be made in time to control infestations and reduce crop losses. There may be little time to respond as infestations frequently go unnoticed until larvae become conspicuous at the fourth instar and the amount of damage then increases exponentially with every day that passes. A forecasting system for S. exempta has been in operation in East Africa since 1969. All forecasts and warnings are currently based on: the distribution of armyworm populations currently reported through the monitoring system; the distribution of previously reported populations, with allowances made for their development with time (timing of oviposition following large trap catches of moths and subsequent hatching and development of larvae, timing of emergence of, and emigration by, moths from earlier outbreaks); predominant winds during the periods of moth migration, and synoptic and meso-scale zones of wind convergence in which moths could become concentrated; the distribution of rainstorms with associated wind convergence capable of concentrating moths, particularly early in armyworm seasons; and historical precedents for the development anticipated. The necessary information is derived from monitoring outbreaks and sampling larvae. Accurate monitoring and prompt reporting of armyworm outbreaks is essential for forecasting and control. The procedures developed in member countries of DLCO-EA involve: searches for newly hatched larvae one week after first high catches in pheromone traps, especially catches associated with the first heavy rains after a drought period; larval sampling to determine the age of outbreaks; reporting outbreaks to local district agricultural offices and national plant protection services immediately they are found, preferably with samples of larvae; assessment of larval age by national plant protection service laboratories. Details of these procedures are given by Rose et al. (1996). Trap data from moth trap networks Networks of moth traps in each country are the most effective way of monitoring armyworm populations. Pheromone traps are recommended for widespread use in national networks. However, they catch only sexually receptive males so some light traps, which catch immature, migrating moths (sometimes in large numbers), should be included at selected sites where electricity and expertise for sorting and identification of catches is available. Detailed information on trap design, pheromone lures (chemistry, formulation and availability), and siting and operating traps is provided by Rose et al. (1996). Meteorological data Forecasting services require rainfall, windfield and rainstorm distribution data. These are generally available through national meteorological services with which close cooperation is essential. In recent years, satellite remote-sensing data have become available at an affordable cost. In East Africa, Cold Cloud Duration data from Meteosat are used to identify locations of rainstorms with the potential to concentrate moths, as well as to determine whether there has been little or no, intermittent and scattered, or prolonged and widespread, rainfall on a regional scale. This information is used to guide monitoring to locate outbreaks and to assess the level of infestation that can be expected, given the known influence of weather, especially rainfall, on the population dynamics of S. exempta (see section on Biology and Ecology). Historical data archives Archives of S. exempta trap and outbreak data have been accumulated nationally and regionally in eastern Africa (these archives are stored also at DLCO-EA, Nairobi and, up to 1988, at the Natural Resources Institute, Chatham, UK). The archive data are now routinely accessed by forecasters and provide analogues of outbreak distribution and trap characteristics for the evaluation of current armyworm situations. Computerised databases, using the specific data management system 'WormBase', are established in eastern Africa (Crop Protection Branch in Kenya, Pest Control Services, Tanzania, DLCO-EA), and are being introduced in IRLCO-CSA countries of central and southern Africa. Historical meteorological data are available from national meteorological services. Forecasts and warnings have different levels of urgency: forecasts are prepared weekly, or every 2 weeks, based on information received at the national or regional offices from the monitoring systems and describe expected future armyworm developments. They may be regional or national in scale. An expert system to produce computer-assisted forecasts has been developed at DLCO-EA. A warning is issued as an alert to the immediate potential occurrence or redistribution of infestations, ideally while they are still in the moth or early larval stages. Warnings should be sent by the quickest possible means (telephone, radio, or broadcast media and newspapers) to agricultural offices in the affected areas, to be acted on immediately. Verification of the reliability of forecasts and warnings is essential if their accuracy and value are to be improved. This is now undertaken routinely by the Regional Forecasting Office at DLCO-EA, Nairobi, by plotting locations of reported outbreaks in relation to predictions, using a computer. Reasons for errors are analysed to avoid repetition. Further information on forecasting is given by Rose et al. (1996).

Cultural Control

Weed-free maize crops greater than 50 cm high are unlikely to become infested by newly hatched larvae of S. exempta because the leaves are too tough to allow them to establish. However, if larvae are able to develop on grass weeds, subsequent infestation of the crop may occur. Farmers are advised to keep crops free of grass weeds but, if fields do become infested, to leave the weeds until the larvae have pupated or been controlled. Some maize varieties are more susceptible to attack than others, e.g. Katumani, a dryland variety grown widely in Kenya. These varieties are most at risk where probabilities of armyworm infestation are high.

Biological Control

Predators and parasitoids of S. exempta are never numerous enough to achieve natural control of outbreaks. The nuclear polyhedrosis virus (SpexNPV) has been used to control infestations by spraying with a water suspension of diseased larvae. The feasibility of laboratory production and formulation of the virus has been demonstrated. Recent field trials in Tanzania indicate that SpexNPV could have a potential role as a substitute for chemical insecticides in strategic armyworm management programmes (Grzywacz et al., 2008). A formulation of Bacillus thuringiensis has also been identified as promising. Full laboratory and field evaluation of these products may result in their adoption in the future.

Chemical Control

Only the larval stage is accessible to control by insecticides; eggs are difficult to find, pupae are underground and moths fly at night at extremely low aerial densities. S. exempta larvae are susceptible to a wide range of insecticides and there is no record of resistance; insecticides are never applied consistently enough, in space or time, to impose adequate selection for it to evolve. The major problem is that larvae are not usually detected until they turn black at the III-IV instar moult, when they have already been developing for 7-10 days and serious damage is imminent. Any delay in applying control measures, during the further 8-12 days until pupation, will result is escalating levels of damage. Rapid response is, therefore, essential and can only be achieved by being well prepared and equipped for control operations and acting on forecasts and warnings. In the absence of warnings or trap data, farmers are advised to check crops for newly hatched larvae one week after a rainstorm, particularly the first storms of the rainy season. Recommended insecticides include organophosphorous compounds, carbamates and synthetic pyrethroids, several of which (especially fenitrothion and cypermethrin) are effective when applied at low dose rates by Ultra Low Volume (ULV) spraying. For environmental and safety reasons, organochlorine insecticides are not recommended for armyworm control. Insect Growth Regulators (IGRs) have recently been shown to have considerable promise for use against S. exempta (Fisk et al., 1993) but require further field evaluation. ULV spraying is the most efficient method of armyworm control, allowing rapid treatment of large areas with low volumes of insecticide. Application may be achieved by hand-held, vehicle-mounted, or aircraft-mounted rotary atomisers, according to the scale and accessibility of outbreaks and the resources available. Lever-operated knapsack sprayers, fitted with low-volume hydraulic nozzles, are frequently used for armyworm control. Further details of chemical control (calibration of equipment, conditions for spraying, application techniques, assessment of kill, environmental considerations, safety, logistics and responsibilities) are provided by Rose et al. (1996).

References

Top of page

Brown ES; Dewhurst CF, 1975. The genus Spodoptera (Lepidoptera, Noctuidae) in Africa and the Near East. Bulletin of Entomological Research, 65(2):221-262.

Cheke RA, 1995. Potential rates of increase of solitarious and gregarious phases of the African armyworm Spodoptera exempta (Lepidoptera: Noctuidae). Ecological Entomology, 20(4):319-325.

Cheke RA; Tucker MR, 1995. An evaluation of potential economic returns from the strategic control approach to the management of African armyworm Spodoptera exempta (Lepidoptera: Noctuidae) populations in eastern Africa. Crop Protection, 14(2):91-103

Dewhurst C, 1999. Forecasting armyworms in the tropics. Pesticide Outlook, 10(1):27-30.

Dewhurst CF; Page WW; Rose DJW, 2001. The relationship between outbreaks, rainfall and low density populations of the African armyworm, Spodoptera exempta, in Kenya. Entomologia Experimentalis et Applicata, 98(3):285-294.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.eppo.int/DATABASES/pqr/pqr.htm

Fisk T; Cooper J; Wright DJ, 1993. Control of Spodoptera spp. using ULV formulations of the acylurea insect growth regulator, flufenoxuron: field studies with Spodoptera exempta and effect of toxicant concentration on contact activity. Pesticide Science, 39(1):79-83

Grzywacz D; Mushobozi WL; Parnell M; Jolliffe F; Wilson K, 2008. Evaluation of Spodoptera exempta nucleopolyhedrovirus (SpexNPV) for the field control of African armyworm (Spodoptera exempta) in Tanzania. Crop Protection, 27(1):17-24. http://www.sciencedirect.com/science/journal/02612194

Haggis MJ, 1984. Distribution, frequency of attack and seasonal incidence of the African armyworm Spodoptera exempta (Walk.) (Lep.: Noctuidae), with particular reference to Africa and southwestern Arabia. London, UK; Tropical Development and Research Institute, iii + 116pp.

Haggis MJ, 1996. Forecasting the severity of seasonal outbreaks of African armyworm, Spodoptera exempta (Lepidoptera: Noctuidae) in Kenya from the previous year's rainfall. Bulletin of Entomological Research, 86(2):129-136.

Harvey AW; Mallya GA, 1995. Predicting the severity of Spodoptera exempta (Lepidoptera: Noctuidae) outbreak seasons in Tanzania. Bulletin of Entomological Research, 85(4):479-487.

International Red Locust Control Organisation for Central and Southern Africa, 2010. Migratory pest situation during January 2011 and forecast for February to April 2011. Nambia, Southern Africa: Zambia Emergency Preparedness and Response Information System (ZEPRIS), 7pp

Lomer CJ, 1999. .

Mensah BA; Gatehouse AG, 1998. Effect of larval phase and adult diet on fecundity and related traits in Spodoptera exempta. Entomologia Experimentalis et Applicata, 86(3):331-336.

Mushobozi WL; Grzywacz D; Musebe R; Kimani M; Wilson K, 2005. .

Rose DJW; Dewhurst CF; Page WW, 1995. The bionomics of the African armyworm Spodoptera exempta in relation to its status as a migrant pest. Integrated Pest Management Reviews, 1(1):49-64.

Rose DJW; Dewhurst CF; Page WW, 1996. The African armyworm handbook. Nairobi, Kenya: Desert Locust Control Organisation for Eastern Africa.

Sarpong GA, 2004. The legal regime for the control of invasive alien species (IAS) in Ghana: Some lessons in the implementatioin of treaty norms. FAO legal papers online, 40. http://www.peacepalacelibrary.nl/ebooks/files/FAO_lpo40

UK CAB International, 1972. Spodoptera exempta. [Distribution map]. Distribution Maps of Plant Pests, December. Wallingford, UK: CAB International, Map 53 (Revised).

Walangululu JM; Mushagalusa GN, 2000. The major pests of cabbage (Brassica oleracea var. capitata subs sabouda) in Bukavu and around. (Les principaux ravageurs des choux pommés (Brassica oleracea var capitata subs sabouda) à Bukavu et ses environs.) Tropicultura, 18(2):55-57.

Distribution Maps

Top of page