African Armyworm

The African armyworm (Fig. 21) is a larva of a nocturnal moth, Spodoptera exempta (Walker). This species, although commonly referred to as the African armyworm, occurs rather widely in the grasslands of tropical and subtropical Africa and Asia. In Africa, where S. exempta is of major economic importance, its occurrence is confined to countries south of the Sahara: Tanzania, Kenya, Uganda, Ethiopia, Somalia, Malawi, Zimbabwe, Zambia and South Africa. Outside Africa, S. exempta has been reported from southwest Saudi Arabia in the republic of Yemen, southeast Asia, Australia, New Zealand and Hawaii.

During an armyworm outbreak, larvae of S. exempta march together in long columns, akin to army columns, in search of susceptible plant material. This is the basis for the name “armyworm.” When susceptible plant material is found, it is oſten voraciously devoured to ground level. In typical armyworm outbreaks, larval density may exceed 1,000 per m² over areas covering tens or even hundreds of square km.

In Africa, S. exempta is adapted for survival on seasonal grasslands by combining a high intrinsic rate of increase with “migration” to places of rainfall where grasses are suitable for survival of its caterpillars. Because of its capability to move over long distances, hundreds and sometimes thousands of km across national boundaries freely, S. exempta is truly an international pest. Often times it appears sporadically and suddenly in dense outbreaks capable of causing extensive and enormous damage to susceptible rangeland grasses, cereal crops and sugarcane. Because of its ability to appear suddenly and then disappear equally suddenly, the African armyworm has sometimes been referred to by farmers as the “mystery worm.” The scale of devastation to crops and pastures by armyworm is comparable only to that caused by locusts. Thus, the armyworm is greatly feared wherever it occurs.

Biology and Ecology

Adult moths of S. exempta have a wing span in the range of 20–37 mm. The forewings are characterized by an overall dull gray-brown appearance. The hind wings are whitish with dark veins. The two sexes can be distinguished by examining the number of bristles on the frenulum (the mechanism that couples the fore and hind wings during flight), which are single in the males and multiple in the females. A characteristic feature of the African armyworm is the presence of racquet-shaped scales at the tip of the abdomen of the males and black scales on the tip of the body of the females.

Females of S. exempta lay between 100 and 400 eggs per night in a mass covered by black scales from the tip of their abdomen. Eggs are small, 0.5 mm in diameter, whitish in color, but then turn black prior to hatching. Eggs are often laid on the lower side of leaves and hatch in about 2–4 days aſter oviposition. There are six larval instars extending over a larval period of between 14 and 22 days depending on the temperature and the host plant on which the larvae have been reared. Fully grown sixth instar larvae are often 25–35 mm long. Pupation occurs 2–3 cm below the soil surface. This process is often preceded by a sudden and synchronized disappearance of larvae that quickly burrow into the ground, particularly if soil conditions are moist enough.

Adults emerge within 7–12 days aſter pupation and can live up to 14 days if appropriately fed. In their lifetime, females can lay up to 1,000 eggs. Spodoptera exempta is not known to enter into any type of diapause. This probably explains why this species has to migrate soon aſter emergence.

Spodoptera exempta exhibits a phenomenon called polyphenism, or phase polymorphism (i.e., the occurrence in a population of two or more phenotypes due to exposure to different environmental conditions). For example, up until the third molt, all larvae of S. exempta remain green in body color. However, at this stage, depending on whether there are many larvae or just a few, they will turn black or remain in various shades of green or brown. If there are large numbers of larvae present as in a typical outbreak situation, larvae tend to be characteristically velvety black on top with pale lines on each side and greenish-yellow underside; this phenotype is called the gregarious phase. It is during this phase that S. exempta is most devastating to crops. Larvae in the gregarious phase tend to be very active and often march on the soil in one direction only looking for fresh food. They also feed high on the plant during the day.

However, if not crowded, the developing larvae remain one of the many shades of green, pink or brown color until they pupate. In contrast to black gregarious larvae, they are sluggish, living mostly at the bases of plants and are not as destructive to crops. Although their appearance is so different, they are the same insect and one may easily be converted into the other. Nevertheless, because moths derived from gregarious and solitary larvae exhibit the same level of readiness to fly, phase change in S. exempta is construed as being merely a stress phenomenon associated with crowding. It is unknown whether this aspect of phase polymorphism is of any evolutionary significance to S. exempta. In the case of locusts, it is thought that the solitary form is the one that enables the populations to persist at a low level during the dry season when there are no outbreaks occurring. It is worth speculating, in the case of S. exempta, that at such low densities, its populations continue to breed during the dry season in areas where grasses remain green, such as in the cool highland areas and, more especially, the coastal areas where it is hot and there are periodic showers during the dry season. This form may, therefore, be of some critical survival value for this pest.

Seasonal Movements and Armyworm Outbreaks

Upon emergence, adult moths of S. exempta are fully capable of movement from their breeding sites to new areas. Such flights can be very short, or very long, depending on whether they are carried in a downwind direction or not. Nevertheless, because they are one-way journeys, they cannot, therefore, be regarded as migration in its strict ethological sense – where there is invariably a return flight to breeding sites.

The question of how armyworm outbreaks start has baffled scientists and farmers alike for a long time. Because moths emerge over a period of up to 12 days, and can also fly off on “migration” at different times, they become widely dispersed and do not form swarms as occurs with locusts. Moreover, the moths are weak fliers and are often carried in a downwind direction. Thus, moths disperse in space and time downwind. For purposes of this narrative, an armyworm outbreak is simply described as the sudden appearance of larval infestations, often simultaneously on many farms in one region.

Two hypotheses, the continuity and the concentration hypotheses, have been put forward in order to explain how outbreaks begin during an armyworm season. The continuity hypothesis, which is based on biogeographical analyses of past outbreaks, proposes that much of the armyworm population is always at crowded, outbreak densities, and that the seasonal absence of reports represents not a real absence of outbreaks, but a temporary loss of contact with the main population in remote and perhaps uninhabited areas. This continuity implies that the first outbreaks to appear in East Africa are due to the migration of parent moths from the north at the end of the season. Although there is ample evidence of adult dispersal on the wind over distances up to several hundred km in a few successive nights, there is little evidence in support of a southward dispersal at the start of an armyworm outbreak in East Africa.

The concentration hypothesis, on the other hand, postulates that because of occasional capture of moths in traps during the off-season, as well as the finding of rare caterpillars aſter concerted searches, S. exempta persists during such times of year as uncrowded populations (the solitary phase, in which caterpillars remain green and unreported), and that the first outbreaks of the season are due to concentration of moths before the synchronized mating and egg laying.

Thus, there seem to be two types of armyworm outbreaks: primary and secondary outbreaks. During primary outbreaks, sources of outbreaks are the low density populations that survive and breed during dry seasons in green areas of the coast and the highlands. Secondary outbreaks occur downwind from the coast near the first highlands. During years of serious armyworm outbreaks, the first outbreaks often start in Tanzania, Zimbabwe and Malawi at about the beginning of the wet season in December, and are followed by a progression of outbreaks at about one generation time-intervals from Tanzania through Kenya, Uganda, Ethiopia, Somalia to the Yemen and from Zimbabwe to South Africa. Wind convergence plus localized weather and moth behavior provide the mechanism for transporting and concentrating moths emerging from primary outbreaks.

Flight mechanisms of S. exempta prior to outbreaks have been the subject of extensive studies. Aſter drying and hardening their wings, moths first move up into the trees. Then, when they are ready, they fly up several hundreds of meters into the air, where if caught up by prevailing wind, are carried away downwind. When dawn arrives, the moths descend and hide on the ground in the grass, but at dusk they take off again. They will continue to do this for several days, either until they die, or until they come to an area where rain is falling. Rain causes the moth to descend to the ground. The winds coming out of the rainstorm have the effect of concentrating the moths, rather as though they were being swept together with a brush. This is the reason why armyworms occurring during outbreaks are not evenly distributed. Upon descent to the ground, moths tend to drink water if this is available, mate and then lay their eggs. At this stage dispersal will have come to an end.

In Africa, seasonal rains are brought by the meeting of large scale winds from the northeast and the southwest at the Inter-Tropical Convergence Zone (ITCZ). The position of the ITCZ moves with the sun across the tropical zone twice each year, from north to south between July and December and from south to north between January and June. The first outbreaks, designated as primary outbreaks in East Africa, usually occur in central Tanzania in November or December. Occasionally they occur further south, in Mbeya, Mtwara and Lindi regions of Tanzania. More rarely, they occur in southeastern Kenya.

Moths produced by these primary outbreaks are then carried by the wind towards the ITCZ, which has meanwhile moved north. The moths are thus concentrated in areas where the rains are just beginning, and so they breed and multiply yet again. If conditions are suitable, they will increase at an enormous rate. Although the first outbreaks are only a few hectares in extent, by February and March there may be outbreaks of hundreds of square km.

Thus, as the ITCZ takes the rains northwards, so the moths, carried by the prevailing wind, move with it, bringing new armyworm outbreaks, designated secondary outbreaks, to northern Tanzania, Kenya, Somalia, Ethiopia, Sudan and eventually Yemen. Armyworm outbreak seasons vary greatly in severity, extent and timing in each of these countries.

From September to November, there are seldom armyworm outbreaks in any part of eastern Africa. The armyworm seems to disappear during the dry season. But since they have no diapause, they must be living and breeding somewhere. It has now been established that they survive in quite low numbers along the coastal area, where some rain falls in every month of the year. When the rains begin again in central Tanzania, small numbers of moths migrate inland from the coast and it is these that cause the first outbreaks.

Economic Importance of Outbreaks of S. exempta

During armyworm outbreaks, feeding damage by S. exempta to cultivated and wild host plants is almost entirely restricted to the leaves, although when food is scarce, the young stems or flowers, particularly of wild grasses, may also be eaten up. The young larvae at first eat the upper and lower surface tissue of the leaves, which results in the skeletonization, or windowing, of the leaves. As a rule, armyworm larvae tend to prefer young plants and recently germinated crops, often defoliating them to ground level. It is estimated that two larvae can completely destroy a 10-day-old maize plant with 6–7 leaves and a single larva can consume 200 mg of dry mass of maize leaves in the course of the sixth instar.

Destruction of cereal crops such as maize, rice, wheat or sorghum often necessitates replanting of the entire affected crop. S. exempta larvae are most damaging to cultivated and wild host plants during outbreaks when gregarious bands of larvae travel together on the ground. Such armyworm outbreaks are often capable of causing total crop loss within hours at the local level. Thus, crop losses as a result of armyworm outbreaks can be potentially devastating to local and national economies as they occur in some of Africa’s most impoverished economies. Losses are sometimes difficult to assess quantitatively because of hidden costs such as effects of forage destruction, damage to subsistence farms, the expense of additional seed and, most importantly, food aid from international donors.

Survey and Control

Circumstantial evidence from light and pheromone traps shows that long distance migrations occur between moth emergence and the next breeding areas which are in the vicinity of seasonal passages of low-level wind convergence such as Inter-tropical Convergence Zone or African Riſt Convergence Zones. Furthermore, increasing levels of moth catches in light and pheromone traps have been found to be followed by increased probability of infestations of larvae occurring 2–4 weeks later at up to 200 km from the trap.

Because primary outbreaks of S. exempta originate from moths taken downwind from sources nearest the coasts of eastern Africa, and concentrate where heavy rains are falling, control of this pest involves accurately detecting primary outbreaks and eliminating them before the subsequent moths emerge and move downwind causing secondary outbreaks in another region or country. The major objective is to kill as many armyworms as possible on pasture or crops before subsequent moths emerge and move downwind. Normally, the largest and densest outbreaks are attacked first. This approach is referred to as “strategic control.” However, because primary outbreaks often remain unnoticed until aſter secondary outbreaks have occurred, direct elimination of the latter becomes the main objective in multiple outbreak situations.

Monitoring possible sources of the moths that cause primary outbreaks has involved deployment of a network of light and pheromone traps, as well as ground searches for low-density larvae in the off-season in areas historically known to be sites of primary outbreaks. In addition, for all primary outbreak areas, rainfall stations must report on a weekly or daily basis the amount of rainfall during the first month of the rainy season. Recently, there have been attempts to introduce predictive models that integrate African Real Time Environmental Monitoring and Information System (ARTEMIS) and satellite imagery and synoptic weather data that has been applicable for detection and elimination of gregarious locusts. The major challenge associated with armyworm monitoring, forecasting and control in countries most ravaged by this pest is the inadequacy of funding for these operations.

Because elimination of all primary outbreaks is almost impossible to achieve, management strategies for S. exempta tend to focus more on suppression of secondary outbreaks. This approach has invariably been dependent on the application of pesticide sprays. To date, a series of low toxicity pesticides are often used (e.g., synthetic pyrethroids, carbamates and organophosphates). Previously, highly toxic, persistent and broad spectrum pesticides such as DDT, BHC, dieldrin and a series of others were used due to lack of alternative, safer products. However, due to the need for rapid intervention and coverage of fairly extensive areas during outbreaks, newer oil-based pesticides are being applied as ultra-low volume (ulv) formulations using hand held, battery driven applicators. Nevertheless, suppression tactics are only costeffective when applied in a timely manner on farmland. Outbreaks occurring on uncultivated land often remain unchecked.

Unfortunately, control of armyworms by less toxic means has received far less attention than in the case of other international migratory pests such as locusts, which have been the targets for tests involving fungi, protozoa and even viruses.

Extensive studies in eastern Africa have confirmed that all life stages of the armyworm are subject to attack by a diversity of natural enemies. For example, up to 90% of armyworm caterpillars in the last instar can be killed by a nuclear polyhedrosis virus (NPV). Similarly, pupal and prepupal stages can be killed by a cytoplasmic virus. The only fungus known to attack armyworms during conditions of high humidity and temperature is Normuraea rileyi. Armyworms infected by this fungi typically climb to the top of grass blades where they die amidst masses of mycelia.

Armyworm larvae are also subject to parasitism by some 28 species of tachnid flies (Diptera: Tachinidae). In cases of parasitism from wasps (Hymenoptera), some 25 parasitoids have been isolated from eggs, larvae and pupae of S. exempta. Apart from attack by parasitoids, there are several arthropod predators that prey on armyworms, including ants (Hymenoptera: Formicidae) and beetles (Coleoptera), which oſten prey on eggs and early larval stages of S. exempta. To date, there has been no effort to study the potential of some of these natural enemies for commercialization. Armyworm outbreaks also attract flocks of avian predators, notably storks such as Marabou storks, white (European) storks and Abdim’s storks. Occasionally, such assemblages of predators help to eliminate small primary or secondary outbreaks of S. exempta.

For the more foreseeable future, it is evident that preventive control of armyworms will continue to rely on diligent surveillance during recessions and, as outbreaks occur, intensified scouting will be necessary to locate and then eliminate pockets of solitary morphs before outbreaks.

Figure 21 African armyworm: top left, adult female, top right, adult male; middle left, solitary form of larva; middle right, gregarious form of larva; lower left, pupae in soil; lower right, eggs on foliage.