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African armyworm
African armyworm
African armyworm
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African armyworm
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Superfamily: Noctuoidea
Family: Noctuidae
Genus: Spodoptera
Species:
S. exempta
Binomial name
Spodoptera exempta
(Walker, 1856)
Synonyms
  • Agrotis exempta Walker, 1856
  • Prodenia bipars Walker, 1857
  • Prodenia ingloria Walker, 1858
  • Laphygma exempta

The African armyworm (Spodoptera exempta), also called okalombo, kommandowurm, or nutgrass armyworm, is a species of moth of the family Noctuidae. The larvae often exhibit marching behavior when traveling to feeding sites, leading to the common name "armyworm".[1] The caterpillars exhibit density-dependent polyphenism where larvae raised in isolation are green, while those raised in groups are black. These phases are termed solitaria and gregaria, respectively.[2] Gregaria caterpillars are considered very deleterious pests, capable of destroying entire crops in a matter of weeks. The larvae feed on all types of grasses, early stages of cereal crops (e.g., corn, rice, wheat, millet, sorghum), sugarcane, and occasionally on coconut.[3][4] The solitaria caterpillars are less active and undergo much slower development.[1] The species is commonly found in Africa, but can also be seen in Yemen, some Pacific islands, and parts of Australia.[5] African armyworm outbreaks tend to be devastating for farmland and pasture in these areas, with the highest-density outbreaks occurring during the rainy season after periods of prolonged drought.[6][7] During the long dry seasons ("off-season"), the population densities are very low and no outbreaks are seen.[3]

Taxonomy and phylogeny

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Spodoptera exempta was first described by Francis Walker in 1856. The species of Spodoptera are distributed around the world, mostly inhabiting tropical and subtropical areas. There are 30 known species in the genus, and roughly half are considered agricultural pests. There are two final larval stages based on the mandible structure of the species: serrate-like mandibles and chisel-like mandibles. Along with S. exempta, the following species and subspecies have chisel-like mandibles that were evolved to consume silica-rich leaves: S. triturata, S. m. mauritia, S. m. acronyctoides, S. umbraculata, S. cilium, Spodoptera depravata [sv], and S. pecten. Like S. exempta, a few species are migratory as adults and travel downwind for hundreds of kilometers, namely S. exigua, S. frugiperda, and S. litura. Due to the broad distribution of the species, the origin of the genus is unknown. However, dating of phylogenic trees reveal that the common ancestor for the genus lived between 22 and 30.7 million years ago and began diversification during the Miocene Epoch.[8]

Geographic range

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The African armyworm is commonly found in the grasslands of Africa and Asia. Within Africa, it is mostly seen near the Sahara in the following countries: Tanzania, Kenya, Uganda, Ethiopia, Somalia, Malawi, Zimbabwe, Zambia, and South Africa. Outside of Africa, the species also inhabits southwest Saudi Arabia, Southeast Asia, Australia, and New Zealand.[9]

Habitat

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Akin to other species of the genus Spodoptera, S. exempta lives in tropical and subtropical environments near cereal crops and dense vegetation where the larvae of the species can easily feed.[9] Because these larvae thrive in high-density populations, African armyworm outbreaks occur in which fields become overrun by caterpillars. The surrounding vegetation is then consumed in large amounts, often with devastating effects.[9] While seasonal winds and rains allow adult S. exempta to migrate across countries and continents, they are often confined to coastal areas and marshes during dry seasons, where the occasional rainfall supports enough vegetation growth for the moth's survival.[10]

Food resources

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Oplismenus undulatifolius, a type of Poaceae

S. exempta larvae feed almost exclusively on Poaceae, also called Gramineae. Poaceae is a family of flowering grasses which includes cereal grasses and the grasses of grasslands and pastures.[1] The species targets most cereal crops, including maize, sorghum, rice, millet, and other grasses.[11] Young caterpillars are also known to feed on wheat and oat seedlings.[12] The larvae eat the upper and lower surfaces of the plant tissue first, preferring to feed on younger and recently germinated crops. Studies have shown that two larvae can completely consume a 10-day-old maize plant with 6–7 leaves, indicating the devastating potential of a high-density outbreak.[9]

Life cycle

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Eggs

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The female can lay a maximum of about 1,000 eggs in her lifetime. She can lay 100–400 eggs per night, with an overall average of 150.[9][11] The number of eggs females can lay is positively correlated with their pupal weight.[1] African armyworms primarily lay their eggs in clusters on the lower side of leaves. Their eggs are relatively small at 0.5 mm in diameter. They are white when first laid, but will turn black prior to hatching.[9] The eggs hatch into larvae within two to five days.[11]

Larvae

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S. exempta undergo six larval instars that altogether can last between 14 and 22 days depending on the temperature and vegetation in the environment. Fully grown sixth-instar larvae are 25–33 mm long.[9]

The larvae display density-dependent polyphenism where the appearance of the individual depends on the population density in which it was reared. The terms gregaria and solitaria were given to the caterpillars raised in groups and in solitude, respectively. These two phases can also be referred to as gregarious and solitary or crowded and isolated. Gregaria larvae have black bodies and thin yellow, horizontal stripes along their bodies. Solitaria larvae have green coloration with a brown stripe down their backs. This green coloration is due to both pigmentation and ingested plant material visible in the gut. The differences in appearance between the solitaria and gregaria phases become apparent in the third instar and remain until the last instar.[13] The species is most damaging to crops during its gregaria phase because the black-bodied caterpillars are more active and densely concentrated than solitaria caterpillars.[1] Solitaria larvae are less active and tend to stay curled up by the base of the crops, leading to less exposure to the sun.[14] Because gregaria caterpillars have darker coloration and maintain positions higher up on the crops, they have increased exposure to the sun, resulting in more rapid development than solitaria caterpillars.[1] Generally, African armyworms are not noticed by farmers until the caterpillars are ten days old and start exhibiting the gregaria phase.[15]

Larvae that are transferred from isolated to dense conditions or vice versa display the gregaria appearance.[13] The darkness of the gregaria phase is positively correlated with the population density of the environment; two larvae raised together will have a dark brown appearance, while larvae raised in larger groups will be completely black.[13] The caterpillar's phase is determined by non-species specific larval interaction but can be influenced by temperature. Isolated larvae that are raised in low temperatures can gain a darker appearance, while larvae raised in crowded groups at high temperatures will not be as dark as typical gregaria.[13] Despite how sensitive the developing larvae are to larval contact, high densities of solitaria caterpillars have been recorded due to a vegetation-dense environment limiting interaction.[13] Because the developmental and pupal stages for gregaria caterpillars are shorter than the solitaria caterpillars, gregaria-phase larvae tend to be smaller but have more efficient feeding behavior.[1]

Pupae

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Pupation occurs beneath the surface of host plants and away from bare ground, roughly 2–3 cm underground.[9][12] This pupation event leads to a sudden synchronized disappearance of the larvae, especially if the soil is moist.[9]

Adults

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Adults emerge in 7 to 10 days and can live up to 14 days.[9][16] The moths migrate over hundreds of kilometers from their emergence sites to their oviposition sites.[17] This migration often causes outbreaks to occur suddenly in areas that were previously free of the pests.[18] While polyphenism is observed in the larvae, the two phases lead to indistinguishable S. exempta adults. However, the two larval phases go on to display different migratory behavior. The gregaria larvae tend to produce adults that travel longer and further during the migration period.[1]

The adult moth wingspan is between 20 and 37 mm, with dull gray-brown forewings and off-white hindwings with visible veins.[9] Females and males can be distinguished by the number of bristles on their frenulum, where males have a single bristle while females have multiple. Females are also identifiable due to their racquet-shaped abdomen tip and black scales.[9] Males have been observed to mature earlier than females.[10]

Migrations

[edit]

When African armyworms first emerge after pupation, they allow their bodies to dry off and their wings to harden before climbing trees.[9] The moths then fly hundreds of meters above the ground and rely on winds to carry them to their next location.[19] Therefore, migration is based on winds brought by the Intertropical Convergence Zone, which move northwards or southwards depending on the season. The moths only travel during the day and descend during dusk to hide in the grass until dawn. This process is repeated for several days until an acceptable destination is reached or the moths encounter rain.[9] Because rain causes the moths to descend, S. exempta are less likely to migrate during frequent rainfall.[11] After the moths land back on the ground, they drink water, mate, and lay their eggs.[9] Migration is beneficial because it allows the species to travel to a new location with fewer predators and less possibility of parasitism and infection.[10]

Outbreak behavior

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S. exempta are a serious pest for crops, often thriving during the wet seasons and leading to outbreaks. There are two types of African armyworm outbreaks: primary and secondary. Primary outbreaks occur with relatively inconspicuous, low-density populations that can survive without frequent rainfall. Secondary outbreaks occur when the moths from the primary outbreak are carried downwind and the adults mate and lay eggs in same location due to wind patterns.[9] During these secondary outbreaks, gregaria larvae emerge from the ground synchronously, and their density may exceed 1,000 larvae per m2.[5][9] These larvae display highly active feeding behavior, leading to excessive consumption of the nearby vegetation, especially the leaves of cereal crops and grasses. If extremely hungry, the gregaria larvae may also consume the stem and flowers of the plants.[9] The high-density gregaria larvae may then pupate and emerge as adults, leading to a large migration event of moths that travel downwind together. These moths will then develop their eggs together, resulting in more outbreaks.[5] The intensity of outbreaks also increases during the start of the wet season after a drought, which allows moths to lay eggs in areas of dense vegetation which supports the larval stage growth.[11]

Enemies

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Overview

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Natural enemies have limited effects on African armyworms due to the species' unpredictable migration paths and variable population densities. The large number of moths traveling during migration can also lead to predator swamping, overwhelming the predator by exceeding its capacity to eat the moths.[19] Despite these challenges, S. exempta still faces predators, parasites, parasitoids, diseases, and viruses. During the pupal and pre-pupal stages, the species is susceptible to a cytoplasmic virus. High humidity and temperature lead to attacks from the fungi Nomuraea rileyi.[9] Although parasitoids do not accumulate rapidly enough to kill a S. exempta population before migration occurs, if the moths happen to return to the same spot, high levels of parasitism are experienced.[19] Most significantly, the species is infected by the species-specific baculovirus, S. exempta nucleopolyhedrovirus (SpexNPV).[citation needed]

SpexNPV

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SpexNPV, abbreviated from S. exempta nucleopolyhedrovirus, is a baculovirus that can be transmitted vertically, from adults to offspring.[20] SpexNPV can kill 90% of an African armyworm larvae population in their last instar; the high mortality rate is due to the vertically transmitted diseases' capability of traveling with the diseased individual during migration.[9][19] Because vertically transmitted diseases benefit from the host species' survival and are less pathogenic than horizontally transmitted diseases, SpexNPV can persist in an individual with no symptoms.[20] S. exempta attempt to reduce the risk of contracting SpexNPV via density-dependent prophylaxis, which allows the species to invest more in resisting disease and stimulate migration to less dense locations.[19]

Mating

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Female/male interactions

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Pheromones

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Virgin female S. exempta adults are observed to release pheromones that attract males. In particular, this includes the following six compounds: Z9-14:Ac (Z-9-tetradecenyl acetate), Z,E9,12-14:Ac ((Z,E)-9,12-tetradecadienyl acetate), Z11-14:Ac (Z-11-tetradecenyl acetate), Z9-14:OH (Z-9-tetradecen-1-ol), Z9-14:Ald (Z-9-tetradecenal), and Z11-16:Ac (Z-11-hexadecenyl acetate). While the compounds Z9-14:Ac, Z,E9,12-14:Ac, Z11-14:Ac and Z9-14:OH are commonly found sex pheromones in Lepidoptera species, the latter two compounds have only been seen in other Spodoptera species.[21]

Physiology

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Thermoregulation

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While dark coloration in species usually indicates a thermoregulatory function, this does not appear to be true under normal conditions in S. exempta. The black gregaria larvae do not exhibit basking behavior which is usually present in dark-colored species that want to increase their body temperature via exposure to the sun. Although the gregaria larvae are shown to heat up more rapidly than the solitaria larvae, gregaria caterpillars have less body mass, which is the main contributing factor to the increased effect of temperature. However, darker coloration was favored in high temperatures when the species is more likely to overheat, suggesting that the coloration between the two phases could be more significant at extreme temperatures.[13]

Interactions with humans

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Pest control

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Armyworms marching

The gregaria phase of the S. exempta species are considered agricultural pests due to their high densities and feeding behavior. Because it is hard to identify and eliminate all primary outbreaks, the main focus for pest control has been to target secondary outbreaks. In the past, cheap, broad-use pesticides such as DDT, BHC, and dieldrin were commonly used to target the caterpillars.[9] Now, newer insecticides such as azadirachtin and aqueous neem (Azadirachta indica) seed extracts are often applied, but these methods are dose dependent and have adverse side effects on both human health and crops.[11] More focused, rapid, and environmentally friendly intervention techniques now exist to limit the spreading of S. exempta. For example, the S. exempta nucleopolyhedrovirus virus (SpexNPV), a naturally present disease that preys on the species, is now being investigated as a method of biological pest control.[11]

Infestations

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In mid-April 1999, an African armyworm infestation started in southern Ethiopia, spreading into the north the following month and into the Jubba Valley of Somalia in early May. Similar outbreaks affected the Rift Valley Province of Kenya and parts of Uganda at the same time. While Ethiopian officials had stocks of pesticides to treat 350,000 hectares of affected land, neither Kenyan nor Ugandan officials had sufficient supplies to combat the insect and no central government was present to respond to the emergency in Somalia.[22]

STAR radio in Liberia reported in January 2009 that Zota District in Bong County had been invaded by African armyworms, which had consumed vegetation, polluted creeks and running water,[23] and were moving toward Guinea and Sierra Leone.[24] On January 28, 2009, the president of Liberia declared a state of emergency to deal with the infestation of army worms in the country.[25]

African armyworms marching along a road in Tanzania

December 2009 had an infestation of ten regions of Tanzania. The infested regions include three of the five main grain-producing regions. The other two major grain-producing regions were at risk of infestation. Tanzania has trained farmers in fighting armyworms since 2007, and responded to forecasts of the late 2009 infestation by sending out hundreds of liters of pesticides to rural farmers. The first infestation was reported on December 22 and quickly spread to surrounding regions. The previous growing season saw Tanzania produce 10.872 million tons of grain; after 10.337 million tons of domestic consumption, the remaining 0.534 million tons were exported. By December 31 almost 1,400 acres (5.7 km2) of grain had been destroyed by armyworms in just the Lindi Region of Tanzania.[26]

In January 2015, armyworm outbreaks were reported in Zimbabwe, and by November there were reports of similar outbreaks in Botswana.[27][28]

In 2016, the fall armyworm (Spodoptera frugiperda, a species of American origin that behaves similarly) invaded maize crops in Zambia.[29] By January 3, 2017, about 90,000 hectares were affected according to reports released by the Zambian government's Disaster Management and Mitigation Unit.[30] The Zambia National Farmers Union indicated that some farms were completely wiped out. The Zambian president directed the air force to help airlift pesticides to fight the outbreak.[30]

In 2017, fall armyworm infestations were detected in more African countries such as Zimbabwe and South Africa.[31]

See also

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  • Common armyworm or true armyworm (Mythimna unipuncta) (North and South America)
  • Fall armyworm (Spodoptera frugiperda) (North and South America)
  • Northern armyworm, Oriental armyworm, or rice ear-cutting caterpillar (Mythimna separata) (Asia)

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

African armyworm

View on Grokipedia
from Grokipedia
The African armyworm, Spodoptera exempta (Walker, 1857), is a species of noctuid endemic to and southwestern Arabia, whose gregarious larvae form dense, marching bands that voraciously defoliate crops and pastures during irregular outbreaks. Its life cycle spans 30–40 days under typical outbreak conditions at temperatures around 26°C, comprising eggs laid in clusters of up to 600 covered with scales, larvae undergoing 5–7 instars over 14–21 days with biphasic behavior—cryptic green solitary phases at low densities and black, cannibalistic gregarious phases during plagues—pupal development in soil for 7–12 days, and adults living 7–14 days while migrating nocturnally hundreds of kilometers downwind. Outbreaks are triggered by seasonal rainfall following dry periods, starting in semi-arid coastal regions like and before spreading via adult flights influenced by easterly winds and the , often causing 50–100% yield losses in , , millet, and grasslands, thereby threatening and regional . The pest's ecology features low-density "concealed" populations persisting in dry seasons on alternative hosts like grasses, from which post-rain oviposition initiates explosive larval increases monitored via pheromone traps and larval scouting to enable timely chemical or biological controls.

Taxonomy and classification

Scientific nomenclature and synonyms

The African armyworm is classified under the binomial name Spodoptera exempta Walker, 1856, within the family Noctuidae of the order Lepidoptera. This species was originally described by Francis Walker as Agrotis exempta in 1856 based on specimens from Africa, and subsequently reclassified into the genus Spodoptera due to morphological alignments with other noctuid moths exhibiting similar larval gregariousness and migratory patterns. The primary synonym is Agrotis exempta Walker, 1856, reflecting its initial generic placement before taxonomic revisions in the early , such as Hampson's 1909 transfer to Laphygma. No other widely recognized synonyms exist in current entomological literature, though historical misclassifications occasionally appear in older regional pest reports predating standardized nomenclature under the . The accepted name exempta is upheld by authoritative databases due to consistent genitalic and wing venation characteristics distinguishing it from congeners like S. frugiperda.

Phylogenetic relationships

The African armyworm, Spodoptera exempta, belongs to the Spodoptera (Lepidoptera: : Noctuinae), which encompasses approximately 31 exhibiting varied host plant ranges from monophagous to highly polyphagous forms. Recent phylogenetic analyses of the genus, based on a including 28 , resolve two major, well-supported reflecting ecological : Clade I, comprising specialized on grasses, and Clade II, including broader-host-range pests. S. exempta is positioned within Clade I, a monophyletic group of seven Eastern Hemisphere (S. cilium, S. depravata, S. exempta, S. mauritia, S. pecten, S. triturata, and S. umbraculata) characterized by chisel-like mandibles adapted for consuming silica-rich C4 grasses. Clade I exhibits moderate support (topology-based bootstrap 57%, posterior probability 0.92) and is estimated to have diversified 11–12 million years ago during the Middle to Late Miocene, coinciding with the expansion of C4 grasslands. The broader origin of Spodoptera is dated to 17–18 million years ago in the Early Miocene, inferred through Bayesian relaxed molecular clock methods applied to concatenated mitogenomic sequences (13 protein-coding genes and rRNAs) supplemented by nuclear loci (28S rRNA, EF1α, and dopa decarboxylase). These findings were robust across concatenation and multi-species coalescent models, with calibrations incorporating fossil and geological priors. Mitochondrial genome-based phylogenies corroborate S. exempta's placement within with high nodal support but indicate it as a relatively basal or isolated branch relative to species like S. exigua, potentially reflecting ancient divergence or incomplete lineage sorting at the mtDNA level. Earlier multilocus studies similarly highlight dispersal-driven evolution in the genus, with S. exempta's African-centric distribution aligning with radiations in I.

Physical description

Adult morphology

The adult Spodoptera exempta is a stout-bodied noctuid moth typically measuring 14–18 mm in body length with a wingspan of 29–32 mm. The forewings exhibit a pale grey-brown coloration, characterized by a diffuse dark area occupying the basal half, transitioning to a paler distal half; a prominent dark spot is present within the discal cell, accompanied by a smaller dark spot near the wing apex. The hindwings are predominantly white or off-white, marked by visible veins and a diffuse grey terminal band along the outer margin. The abdomen is covered in scales, contributing to its overall drab appearance that aids in camouflage during rest. The head features black antennae and crimson labial palpi tipped in black; additional black patches occur on the vertex, , edges of the tegulae, shoulder stripes (patagia), and fore coxae. These traits align with the ' nocturnal habits, where subtle patterning on the forewings—often described as irregular light and dark markings—provides against bark or soil backgrounds. Males and females show minimal external morphological differences beyond potential subtle variations in antennal structure, with females generally possessing filiform antennae and males slightly more serrate forms typical of the , though size ranges overlap significantly. The and legs are robust, supporting the moth's migratory flights, but lack distinctive ornamentation beyond the scaled integument common to .

Larval stages and color phases

The larvae of Spodoptera exempta undergo six instars, with the entire larval period typically lasting 14–21 days under optimal conditions for the gregarious phase, though duration varies with temperature and density. Early instars are small and initially translucent or pale, feeding gregariously on tissues, while later instars grow to 40–45 in , exhibiting increased mobility and voracious appetite. Development progresses rapidly in warm, humid environments, with larvae dispersing in marching bands during outbreaks, consuming foliage . Larval coloration displays density-dependent , resulting in distinct solitarious and gregarious phases. Solitarious larvae, occurring at low densities, are cryptically colored in shades of green-brown or , with a robust, fatty appearance and sluggish behavior; these forms are often cannibalistic and blend into host vegetation. In contrast, gregarious larvae at high densities adopt a velvety black dorsum with broad yellow-white lateral stripes and narrower pale dorsal lines, appearing slender, active, and aggressive; this phase facilitates synchronized movement in large bands that defoliate crops. The phase shift is triggered by crowding, enhancing survival through reduced predation visibility and coordinated foraging, as observed in field outbreaks across African savannas.

Distribution and ecology

Geographic range

The African armyworm (Spodoptera exempta) is endemic to , where it occurs widely south of the Desert, with larval outbreaks documented across diverse and ecosystems. The ' distribution is influenced by migratory adult moths, which facilitate seasonal range expansions during favorable wind and rainfall conditions, enabling outbreaks to span thousands of kilometers. Outbreaks are most frequent and intense in eastern Africa, including countries such as , , , , and , where populations build up in source areas like coastal and northern before dispersing westward and southward. In southern Africa, occurrences are less regular and typically confined to high-rainfall regions like , , and South Africa's and provinces, with major infestations happening only every five to ten years. Western and central African nations, such as and the of Congo, experience sporadic invasions from eastern fronts, though endemic populations persist at lower densities. Beyond , S. exempta has been recorded intermittently in south-western Arabia (including ) and on islands like and the , likely via long-distance migration rather than established breeding populations. Rare detections in and have been noted, but these lack evidence of sustained reproduction and are attributed to vagrant individuals. Modeling studies indicate potential for range expansion under scenarios, but current verified distributions remain centered on as of 2022 assessments.

Habitat preferences and environmental factors

The African armyworm, exempta, primarily inhabits seasonal grasslands and ephemeral grassy vegetation in tropical savannas across , with a strong preference for areas supporting young, nitrogen-rich graminaceous plants such as cereals (, millet, ) and wild grasses including spp. and Pennisetum spp.. It is well adapted to exploit rain-induced flushes of these host plants, favoring hot and humid conditions in regions like coastal , the highlands of and , , western , southwestern , and shorelines of . Larvae rarely feed on dicotyledonous plants or trees, limiting suitable habitats to gramineae-dominated ecosystems where dry-season grasses become unsuitable due to low nutritional quality. Rainfall is the dominant environmental driver of habitat suitability and , with outbreaks frequently initiated by the first seasonal rains—such as those in –December in eastern —that exceed 50 mm over short periods (e.g., 10 days) and stimulate rapid grass growth. Preceding droughts paradoxically heighten outbreak risk by enhancing post-rain host plant quality through nitrogen accumulation in regrowing vegetation, while wind convergence during storms facilitates migration into these newly favorable areas. Temperature influences developmental thresholds and activity, with minimums required for egg (12°C), larval (14°C), pupal (13°C), and adult (20°C) stages; optimal larval growth and adult flight occur between 20–30°C. In cooler highland areas, development prolongs (e.g., July–October in Kenyan highlands), but moths remain capable of flight in warm air layers, enabling persistence in varied elevations. These factors interact synergistically: seasonal rainfall expands habitable zones eastward of topographic barriers, where combined with suitable temperatures, it supports explosive gregarious larval phases and migratory adults.

Host plants and diet

Primary food sources

The larvae of Spodoptera exempta, known as African armyworm, primarily consume foliage from in the (grasses) and (sedges) families, with a strong preference for graminaceous species that serve as staple crops across . Major host include (Zea mays), (Sorghum bicolor), (Pennisetum glaucum), (), (Triticum aestivum), ( vulgare), and (), as well as various and wild grasses such as spp. and . These preferences align with the pest's outbreaks during rainy seasons when young seedlings and tillering grasses are most vulnerable, often leading to defoliation of entire fields. While S. exempta is highly specialized on monocots, occasional records exist of minor feeding on non-graminaceous plants like certain or under high-density conditions, though these do not constitute primary sources and lack economic significance compared to cereal damage. Early larvae exhibit broader acceptability among grasses, including , before shifting to more mature stages in later development, reflecting adaptations to ephemeral outbreaks in ecosystems. This host specificity underscores the pest's role as a key threat to smallholder cereal production, with documented losses exceeding 50% in and yields during severe infestations in regions like .

Feeding behavior and damage patterns

The larvae of exempta exhibit polyphenic feeding behaviors that vary between solitary and gregarious phases. In the solitary phase, larvae are typically green or brown, feeding individually at night on the bases of grass plants while remaining cryptic during the day, resulting in minimal observable damage to crops. In contrast, gregarious-phase larvae, which are black and form dense aggregations, display heightened voracity, feeding collectively on foliage exposed to . Young gregarious larvae (instars I-III) rasp the undersides of leaves, causing windowing or where only veins remain intact, primarily targeting tender grasses high in . Older instars (IV-VI) employ cutting mandibles to devour leaf edges and entire surfaces, consuming up to 0.7 g of foliage per per day, leading to rapid defoliation of stems and growing points. These behaviors predominantly affect Gramineae such as , , millet, , , and pasture grasses, with occasional impacts on and sedges. Damage patterns during outbreaks appear as irregular patches of eaten foliage progressing to bare stems and ground, with fields stripped within days under high larval densities. Depleted food sources prompt marching bands of older larvae to relocate en masse, covering several kilometers and amplifying destruction, potentially resulting in 9-100% yield losses in depending on growth stage. Trails of and synchronized pupation follow, but the marching facilitates widespread crop devastation in sub-Saharan regions.

Life cycle

Egg stage

Eggs of the African armyworm (Spodoptera exempta) are small and spherical, measuring approximately 0.5 mm in diameter, with a conical shape featuring a slightly rounded apex and a densely sculptured surface. They appear pale yellow or greenish-cream when freshly laid but darken progressively, often turning grayish or brown, with the black head capsules of developing larvae becoming visible shortly before hatching. Oviposition occurs primarily at night, with females depositing eggs in clusters or masses on a variety of substrates, including the undersides of leaves, stems, or twigs of grasses and crops, as well as dry grass stems, bushes, or even non-vegetative structures like buildings. These sites are not always restricted to host plants, reflecting the moth's opportunistic dispersal behavior. Each egg mass typically contains 100–300 eggs, though batches can range from 10 to 600, and the eggs are coated with a protective layer of fine, hair-like scales or black hairs derived from the female's , which may thin in later batches. A single female can produce 400–1,300 eggs total, laid across up to six nights, with varying based on larval phase (gregarious forms yielding higher numbers) and adult nutrition. The incubation period lasts 2–5 days, averaging about 3 days under favorable conditions, but is strongly temperature-dependent. Hatching accelerates at 25–30°C, while development slows below 20°C and fails below 12–14°C, with no at the lower threshold. Upon , first-instar larvae consume the before dispersing to feed nearby.

Larval development

The larval stage of exempta typically spans 5 to 6 instars, though 7 instars occur rarely depending on host plant quality and larval phase. Development proceeds faster on preferred graminaceous hosts such as (Zea mays) and star grass (), where larvae complete the stage in fewer instars with reduced mortality compared to suboptimal plants like guinea grass (Panicum maximum). At 25°C and 70% relative humidity, larvae on star grass and pass through 5 instars, exhibiting a U-shaped mortality pattern with peaks in the first and last instars. The duration of the larval period ranges from 11 to 24 days, averaging 21 days under outbreak conditions at optimal temperatures of 25–30°C. Development ceases below a minimum threshold of 14°C, while higher temperatures within the viable range accelerate growth, shortening durations—typically 2–3 days for early instars and 4–5 days for the final . Larvae grow from approximately 2 mm in length upon to 35–40 mm in the mature stage, attaining weights of 0.5–1 g, with feeding rates peaking at up to 0.7 g per day in the final on . In the gregarious phase, induced by high densities, larvae display enhanced metabolic rates and faster development relative to the solitarious form, contributing to rapid population buildup during outbreaks. Survival is highest on nutrient-rich young grasses, with overall larval mortality often exceeding 80% due to predation, pathogens, and food scarcity, particularly in early instars. Host plant nitrogen content positively correlates with growth rates and subsequent .

Pupal stage

Upon reaching maturity, typically in the fifth or sixth , larvae of Spodoptera exempta cease feeding, leave host plants, and into the to pupate. This process occurs in soft, damp, loose, or moist near plant bases or sandy banks, where larvae construct a silk-lined chamber, often 2–3 cm deep, sometimes forming small surface mounds of earth. The pre-pupal stage lasts 1–2 days, during which the sheds its within the chamber to form the ; pupation depth generally ranges from 2–5 cm below the surface. Dry or hard conditions hinder burrowing and elevate mortality rates, underscoring the dependence on suitable post-rainfall for successful pupation. The is exarate, with free appendages, initially pale green but hardening to a deep red-brown color; it measures 15–18 mm in length and features a smooth exterior typical of . are subterranean and vulnerable to predation by and , as well as infection by and fungi, which can significantly reduce survival during outbreaks. Pupal development duration varies from 7–14 days under typical outbreak conditions, with ranges of 7–12 days or 10–14 days reported in optimal environments, extending to 22–24 days in cooler highland regions. Temperature influences this stage, with a minimum threshold of 13°C for ; warmer conditions accelerate development, while suboptimal moisture or hardness further prolongs it or increases mortality. Adult moths emerge nocturnally, primarily between 20:00 and 22:00, over approximately 12 days, facilitating dispersal and potential outbreak perpetuation. This stage lacks , aligning with the species' rapid, multi-generational life cycle in tropical African climates.

Adult stage

The adult stage of the African armyworm, Spodoptera exempta, consists of stout-bodied moths with a of 29–40 mm. Forewings are dark brown, patterned with lighter markings including a diagonally elongate orbicular spot and a - or arrow-shaped reniform spot, while hindwings are white with dark veins that are often darkened distally. includes feathery antennae and a single bristle in males, contrasted with simpler antennae and 2–3 frenulum bristles in females; body length measures 14–18 mm. Moths emerge nocturnally from pupae between 20:00 and 22:00, resting during daylight under cover such as stones or before becoming active at , midnight, and dawn. Adults are primarily non-feeding, relying on larval-stage energy reserves, though or honeydew consumption, when available, enhances . Field longevity spans 7–16 days, with females outliving males, though conditions can extend this to 36 days; typical duration is 7–10 days. Migratory flight occurs downwind at altitudes of 420–870 m, covering 100–700 km per in dispersed formations rather than swarms, facilitating outbreak dispersal across eastern via convergence and rainstorm influences. Settlement in trees precedes mating and oviposition, with pheromone traps capturing receptive males for monitoring.

Behavioral ecology

Migration patterns

Adult moths of Spodoptera exempta engage in long-distance, wind-assisted migrations that facilitate the pest's dispersal across and beyond, typically occurring at or early night when moths take off from congregation sites in trees. Flights reach altitudes of 420–870 meters above ground level, with airspeeds of 3–5 km/h, enabling ground-covered distances of 100–700 km per night under favorable conditions, though exceptional records exist up to 3,200 km. Migration is downwind, often converging towards areas of rainfall and the (ITCZ), where low-density moth populations accumulate to initiate outbreaks upon descent triggered by moderate to heavy rain or physiological maturation in females. Seasonal patterns align with rainfall onset and wind regimes, starting in primary outbreak areas like coastal and during September–October with initial rainstorms. In , moths migrate inland from coasts during the short rains (October–November), spreading northwards and westwards via easterly winds to regions including , , , , , and ; secondary outbreaks then propagate downwind from November–March to areas like and . Northern movements occur from to northern (February–June), while southern flows from reach (July–September), tracking ITCZ shifts. In , outbreaks advance south from , , and to , , , , and , following regional wind patterns.
RegionSeason/TimingPrimary DirectionKey Areas InvolvedDriving Factors
East AfricaShort rains (Oct–Nov)North/west from coasts → , , , , Easterly winds, initial rains
East/SouthernNov–MarWest/south → , Downwind secondary spread
NorthernFeb–JunNorth → N. ITCZ tracking
NorthernJul–SepSouthN. → ITCZ tracking
Southern AfricaVariable (post-rains)South// → , , S. AfricaRegional winds
Mark-and-recapture studies confirm dispersal distances up to 147 km from emergence sites, with radar observations in Kenya revealing flight in warm air layers below temperature inversions to avoid convective turbulence. Outbreak potential amplifies through generational increases, with wind convergence enabling a 10,000-fold population rise over two generations despite high mortality (around 80%) and female fecundity of approximately 1,000 eggs. Monitoring via pheromone and light traps (>30 male moths per night indicating risk), combined with satellite detection of storm zones and weather data, supports forecasting of these migratory influxes.

Outbreak formation and gregariousness

Outbreaks of Spodoptera exempta originate from low-density, solitarious populations persisting in source areas like coastal Kenya and Tanzania during dry seasons. With the onset of rains, adult moths migrate long distances—up to 700 km—via easterly winds and convergence zones such as the Inter-Tropical Convergence Zone or storm fronts, concentrating in areas with emerging grass flushes. Favorable post-rain conditions, including sunny periods and high-nitrogen young grasses, promote egg hatching and larval survival, leading to rapid density increases exceeding 1000 larvae per square meter within 2-3 weeks. Pheromone traps detecting over 30 moths per night provide early warnings, with larval outbreaks manifesting 5-7 days later. High larval densities trigger a density-dependent phase polyphenism, shifting individuals from the green, solitary (solitaria) form—characterized by sluggish and hiding at bases—to the gregarious (gregaria) phase. Gregarious larvae develop black coloration, heightened voracity, and cohesive marching bands, particularly in instars III-VI, which move nocturnally to exploit new food sources as local vegetation depletes. This phase involves physiological adaptations like sun-basking to elevate body temperature and accelerate development, alongside increased reserves and synchronized , amplifying outbreak impacts. The gregarious phase supports higher reproductive rates and aggregation tendencies compared to the solitarious phase, facilitating sustained population surges under outbreak conditions. Crowding acts as the primary trigger for , mediated by tactile and possibly pheromonal cues, though exact mechanisms remain under study. Rainfall and food availability further modulate this shift, with gregarious forms dominating during severe infestations that can devastate cereal crops across .

Foraging and swarming dynamics

The African armyworm Spodoptera exempta exhibits phase polyphenism in its larval stage, manifesting as solitarious or gregarious forms depending on early-instar . Solitarious larvae, arising from low-density conditions, display solitary feeding with dispersed distribution and slower developmental rates. In contrast, gregarious larvae emerge under high-density conditions, such as exceeding 1000 larvae per square meter, leading to darker coloration, heightened activity, and synchronized group behaviors. Gregarization is triggered primarily by tactile stimulation from crowding and limited food resources, prompting larvae to form dense bands that facilitate collective and movement. In the gregarious phase, larvae feed voraciously in groups on preferred hosts like grasses and cereals, consuming up to 0.7 grams of per day in the final , with younger instars skeletonizing leaves through windowing and older ones defoliating from edges. Group feeding enhances efficiency by concentrating solar radiation absorption, accelerating development and enabling rapid depletion of vegetation in outbreak areas. Swarming dynamics involve the formation of marching bands when sources are exhausted, with larvae moving directionally—often downwind—to seek new feeding sites or pupation grounds. These bands cover distances up to 100 meters daily, advancing at speeds of 5-20 meters per hour depending on environmental conditions and age. The clumped, synchronized progression of these bands, driven by density-dependent aggregation, results in extensive crop damage across , particularly following rains that promote egg hatching in concentrated patches.

Reproduction

Mating behaviors

Adult Spodoptera exempta moths mate nocturnally, with courtship and copulation occurring primarily during the scotophase. Males exhibit a pre-mating maturation period, requiring at least 48 hours post-emergence in laboratory conditions to respond to females, whereas wild-caught males mature within 24 hours. Courtship begins with male wing fanning directed toward the female, followed by antennal contact and extrusion of the female's . Copulation duration ranges from 30 to 90 minutes, after which the male performs post-copulatory behaviors. Virgin females display calling behavior throughout the night but preferentially mate in the middle of the scotophase, ceasing calling until the following night after . Multiple s are common in both sexes, with males capable of copulating several times per night, though females typically only once per night. A single suffices to fertilize the female's entire production, supporting high reproductive potential despite opportunities for remating.

Pheromone roles in attraction

Female Spodoptera exempta moths produce a multicomponent sex pheromone blend from a glandular structure in the ovipositor, which serves as the primary chemical signal for long-range attraction of conspecific males during the mating period. The major constituents are (Z)-9-tetradecenyl acetate (Z9-14:Ac) and (Z,E)-9,12-tetradecadienyl acetate (ZE9,12-14:Ac), typically in a ratio approximating 100:7.5, with minor components including (Z)-9-tetradecenal (Z9-14:Ald), (Z)-11-tetradecenyl acetate (Z11-14:Ac), (Z)-9-tetradecen-1-ol (Z9-14:OH), and (Z)-11-hexadecenyl acetate (Z11-16:Ac). This blend elicits oriented upwind flight in males, characterized by zigzag anemotactic behavior toward the pheromone plume, facilitating mate location over distances relevant to adult dispersal. Pheromone release by virgin females commences around 48-72 hours post-emergence, coinciding with reproductive maturity, and peaks during scotophase, often between midnight and 02:00, under favorable conditions of and low wind speed. Attractiveness escalates with age until mating, after which mated females cease emission and elicit no male response within 72-96 hours. Field trapping experiments in demonstrate that incorporating Z11-16:Ac into the binary major blend significantly enhances male captures, while Z9-14:OH inhibits attraction, underscoring the blend's specificity for species-specific mate signaling. Laboratory bioassays using crude gland extracts equivalent to 0.1 female confirm dose-dependent male responses, including wing fanning and orientation, which validate the in short-range attraction preceding copulation. The consistency of composition across gregarious and solitary phases, as well as geographic populations in eastern Africa, supports its reliability in and population monitoring via traps that detect males up to 100 km from outbreak sources. Cross-attraction to synthetics by related species like triturata occurs but remains limited, indicating partial but not complete overlap in pheromone receptors.

Fecundity and oviposition

Females of Spodoptera exempta typically lay 500 to 1000 eggs over their lifetime, though estimates range from 400 to 2000 depending on nutritional status and environmental conditions. A single female deposits these in up to eight batches over six nights, with each cluster containing 10 to 600 eggs, peaking on the second night of oviposition. Oviposition is nocturnal, commencing between 20:00 and 21:00 and lasting about 30 minutes per batch, often with a secondary period before dawn; eggs are laid in single-layer masses covered by grayish hair-scales from the female's , imparting a woolly appearance. Preferred sites include the undersides of leaves on grasses, cereals such as and , bushes, trees, or even non-host structures like buildings, favoring elevated positions to aid larval dispersal by wind rather than strict host suitability. The pre-oviposition period spans 2 to 13 nights post-emergence, with laying initiating 1 to 3 days after , synchronized to cues like initial wet-season rains that concentrate moths via wind convergence. Fecundity is modulated by larval phase, with from gregarious (outbreak-phase) larvae showing elevated egg production compared to solitary-phase counterparts due to higher metabolic reserves. feeding enhances egg numbers, while host plant quality—particularly young grasses with elevated post-rain—boosts output; conversely, high population densities or poor water availability can suppress it. One suffices for full oviposition, though multiple matings occur in captivity without further reproductive gain.

Physiology and adaptations

Thermoregulation mechanisms

The gregarious phase larvae of Spodoptera exempta, known as African armyworms, exhibit dark coloration with prominent black bands, a phase polyphenism triggered primarily by high population density during early instars. This coloration, as determined experimentally by rearing larvae at varying densities, contrasts with the paler green or brown hues of solitarious phase individuals and has direct implications for thermal balance. Darker pigmentation increases solar absorptance, facilitating greater radiative heat gain and potentially elevating body temperature above ambient levels to support foraging and marching activity in cooler microhabitats or early morning conditions common in outbreak-prone African savannas. However, under typical field conditions, this does not confer active thermoregulatory control but rather passive enhancement of heat acquisition, with darker forms showing higher equilibrium temperatures in solar exposure compared to simulated paler variants. Field observations of gregarious larvae reveal them as thermoconformers, with body (Tb) linearly related to ambient air (Ta) via the regression Tb = 0.218 + 1.14 Ta (P < 0.001, n = 270), indicating a slight amplification and offset likely due to metabolic heat and enhanced radiation absorption from dark cuticles. No evidence of behavioral thermoregulation—such as postural adjustments, basking, or selective shade-seeking—was detected; larval positioning above 10 cm on host plants is driven by food depletion rather than thermal optimization, though clustering in bands may incidentally reduce convective heat loss. Critical thermal limits include a minimum of 11.8 ± 0.13°C and maximum of 55.9 ± 0.45°C, defining the viable thermal envelope for survival and activity. Physiological maintenance of haemolymph osmolality at approximately 330 mOsm kg⁻¹ supports thermal tolerance by mitigating desiccation risks during diurnal fluctuations, indirectly bolstering endurance in hot, dry outbreak environments. Adult moths, being nocturnal, rely on crepuscular activity and diurnal sheltering in vegetation to evade peak solar heat, with limited phase-specific adaptations beyond general ectothermy. Rearing temperature can modulate coloration intensity within phases, with lower temperatures (e.g., below 25°C) yielding darker individuals, suggesting a plastic response that fine-tunes heat balance to developmental needs.

Sensory and metabolic traits

The antennae of Spodoptera exempta adults and larvae function as primary olfactory organs, equipped with sensilla that facilitate detection of sex pheromones, host plant volatiles, and environmental cues through odorant-binding proteins (OBPs) and chemosensory proteins (CSPs), whose expression exhibits sex- and stage-specific patterns peaking in antennae during reproductive phases. Larval chemoreceptors, located on maxillary galeae and preoral cavity structures, mediate contact chemoreception for sugar perception and host discrimination, with responses to stimulants like sucrose suppressed by plant-derived inhibitors such as warburganal, enabling selective feeding amid diverse vegetation. These sensory adaptations underpin behavioral shifts from solitary to gregarious phases, where heightened chemosensory sensitivity supports swarm formation and resource location during outbreaks. Metabolically, S. exempta larvae display elevated respiratory rates during active feeding, correlating with increased energy expenditure for digestion and growth, as measured in controlled assays showing up to twofold rises in oxygen consumption. In preparation for adult flight, pupal thoraces develop key enzymes of flight muscle metabolism, including those of the tricarboxylic acid cycle and lipid oxidation pathways, which mature rapidly post-eclosion to fuel migratory endurance using stored lipids as the primary energy substrate. Gregarious larvae exhibit distinct nutrient regulatory responses compared to solitarious forms, prioritizing protein and carbohydrate assimilation to sustain high-density outbreaks, with adult feeding further modulating lipid and protein reserves critical for ovarian development and sustained locomotion. These traits reflect adaptations for opportunistic energy mobilization, enabling rapid population irruptions in response to favorable conditions.

Natural enemies

Predators and parasitoids

Predators of the African armyworm (Spodoptera exempta) include ants, which destroy eggs and kill young larvae; thrips, which prey on eggs; and spiders, which feed on larvae. Birds such as storks (Ciconia abdimii), crows (Corvus albus), and the yellow-necked spurfowl (Francolinus leucoscepus) attack larvae across stages, capable of decimating smaller outbreaks but exerting limited impact on larger ones due to the rapid larval gregariousness and migration. Other predators encompass beetles (e.g., Carabidae such as Calosoma spp.), amphibians like frogs and toads, scorpions, and the larvae of Heliothis armigera in regions like Kenya, though quantitative data on their predation rates remain sparse. Parasitoids target eggs and larvae, with egg parasitoids including Trichogramma spp. and Telenomus spp. (Scelionidae), the latter recorded in Zimbabwe and southern Tanzania. Larval parasitoids comprise hymenopterans such as Apanteles spp. (Braconidae), Campeletis pedunculata (Ichneumonidae), Euplectrus laphygmae (Eulophidae), and Chelonus curvimaculatus (Braconidae, southern Africa), which reduce host feeding and prolong instar durations; dipterans like Palexorista quadrizonula (Tachinidae, across Africa), Sturmiopsis parasitica, Blepharella analis, and Exorista xanthaspis also parasitize larvae. Parasitism levels can reach up to 70% in later generations, and early deployment of egg and larval parasitoids has demonstrated efficacy in preventing outbreaks. Despite these natural enemies, their regulatory effect is often insufficient to suppress large-scale outbreaks, constrained by the pest's short generation time, high fecundity, and migratory behavior, which outpace enemy responses in peak infestation phases. In eastern Africa, surveys have identified 19 parasitoid species and five predator species, underscoring regional diversity but highlighting the need for integrated management beyond reliance on endemic enemies alone.

Pathogens and viral controls like SpexNPV

The African armyworm, Spodoptera exempta, is affected by a range of microbial pathogens, including viruses, entomopathogenic fungi, and protozoa, which contribute to natural mortality during outbreaks. Among these, baculoviruses—particularly nucleopolyhedroviruses (NPVs)—are prominent for their specificity and potential in biological control, as they infect larvae via oral ingestion of contaminated plant material, leading to viral replication, tissue liquefaction, and host death within days. Fungal pathogens, such as species of Metarhizium and Beauveria, can also cause epizootics under humid conditions, though their efficacy varies with environmental factors like temperature and humidity. Protozoan pathogens, including microsporidia, impose sublethal effects that reduce fecundity and longevity but are less commonly deployed for control. Spodoptera exempta nucleopolyhedrovirus (SpexNPV), an alphabaculovirus, is the most studied and utilized viral pathogen against S. exempta, exhibiting strict host specificity that limits infection to this species and spares beneficial insects, predators, and humans. Infection occurs when third- or fourth-instar larvae consume virus-laden occlusion bodies on foliage, with viral loads amplifying exponentially; mortality rates can exceed 90% in lab assays and 70-98% in field applications when doses of 1-5 × 10¹¹ occlusion bodies per hectare are applied during early outbreak stages. Field trials in Tanzania demonstrated SpexNPV's equivalence to synthetic pyrethroids in suppressing larval populations, with no resurgence observed due to its self-propagating nature post-application. Vertical transmission via infected eggs sustains SpexNPV prevalence in migratory populations, with field surveys detecting up to 20-30% covert infections in adults, enhancing pathogen persistence despite host dispersal. SpexNPV-based controls have been scaled through mass-production facilities, such as the one established in Tanzania in 2011, enabling local formulation from field-collected infected larvae via homogenization, filtration, and UV stabilization for aerial or ground application. Efficacy is optimized under moderate temperatures (20-30°C) and high humidity (>70%), conditions prevalent during S. exempta outbreaks in ; however, UV degradation limits persistence to 2-5 days post-spray, necessitating timely application during gregarious larval phases. Modeling studies indicate that integrating SpexNPV with early warning systems can suppress 98% of outbreaks when coverage exceeds 80% of infested areas, reducing reliance on broad-spectrum insecticides that disrupt non-target ecosystems. Challenges include variable across SpexNPV isolates and potential resistance from high-protein host diets, underscoring the need for isolate screening and mixed-pathogen strategies.

Interactions with agriculture

Economic impacts and yield losses

The African armyworm (Spodoptera exempta) inflicts substantial economic damage primarily on cereal crops and pastures across , threatening and livelihoods of smallholder farmers who lack resources for effective control. Outbreaks lead to defoliation and reduced harvests, with impacts exacerbated during short rainy seasons when young crops are vulnerable. Recent events, such as the February 2025 outbreak in , highlight ongoing risks to agricultural production in maize-dependent regions. Yield losses vary by crop stage, larval density, and control timing, ranging from 9% in early whorl stages to 100% near pre-tassel if unmanaged. In Kenyan trials, mean losses reached 23.7% in Enkoiperiae and 60% in Keturo, while experimental data from and showed up to 92% reduction in yields. and other cereals like millet, , and face similar defoliation, though quantified losses are less documented; simulated studies indicate density-dependent reductions without specific percentages. Action thresholds of 200 second-instar, 80 third-instar, or 20 fourth-instar larvae per 100 plants aim to prevent losses exceeding 15%. grasses and rangelands suffer heavy damage in the final 8-12 days of larval feeding, indirectly affecting and contributing to broader economic strain. Strategic control measures, targeting outbreak sources, yield economic returns up to 10:1 in eastern by averting widespread damage. Replanting costs often exceed chemical interventions, with unmanaged infestations potentially causing 30% yield reductions in cereals. Data on total monetary losses remain sparse compared to invasive pests, reflecting the pest's endemic nature and periodic outbreaks rather than chronic . Nonetheless, uncontrolled epidemics in key production areas amplify vulnerability for resource-poor farmers reliant on rain-fed systems.

Historical and recent outbreaks

Outbreaks of the African armyworm ( exempta) have been recorded in since at least 1919, occurring almost annually in where they pose a serious in nine out of every ten years. Major historical upsurges often follow periods of , which promote larval survival through increased post-rain host availability, with notable events in 1961/62, 1970/71, and 1984/85 across , , and surrounding regions. Severe outbreaks struck in the early 1990s, affecting , , and , while a significant infestation began in southern in mid-April 1999 and spread northward into the Jubba Valley. Outbreak patterns in typically initiate in semi-arid inland areas east of high ground during the short rains (November–December) or long rains (March–May), progressing westward with the and winds from eastern and toward and beyond in peak years. Historical records document specific instances, such as outbreaks in Kenya's District in March 1940 and Meru in October 1984, and in Tanzania's Longido area in late November–December 1971. In , invasions occurred in in January–February 1985 and extensive outbreaks in and during 1992/93. Recent outbreaks have intensified in eastern and , driven by favorable warm temperatures, precipitation, and winds exacerbated by . Severe infestations affected , , , , , and in 2022 and 2023. In early 2025, outbreaks emerged across , including , , , , and , where over 70 cases were reported in the Free State province's Xhariep and Lejweleputswa districts, alongside incidents in , , , , North West, and . These events threaten crop yields and in affected biomes.

Control strategies and efficacy

Integrated pest management (IPM) approaches for Spodoptera exempta emphasize monitoring, timely intervention, and a combination of chemical, biological, and cultural methods to minimize outbreaks while reducing reliance on broad-spectrum pesticides. Early warning systems, including pheromone traps and community-based forecasting networks, enable detection of adult moths and larval masses, allowing interventions before crop damage exceeds 20-50% in cereals like and . Strategic scouting in outbreak-prone regions has demonstrated efficacy in limiting larval densities to below economic thresholds, with models predicting up to 70% reduction in infestation spread through . Chemical control remains the primary rapid-response tactic, utilizing synthetic pyrethroids (e.g., cypermethrin) and carbamates applied via aerial or ground spraying during larval migration phases. These insecticides achieve 80-95% mortality in early instars when applied within 24-48 hours of detection, though efficacy declines against later-stage larvae due to behavioral adaptations like gregarious marching. Repeated applications in high-density outbreaks (e.g., >10 larvae per square meter) have protected yields in Tanzanian maize fields, recovering 60-80% of potential losses, but costs of imported formulations often exceed $50 per hectare, limiting accessibility for smallholder farmers. Overuse has raised concerns about non-target effects on beneficial insects, prompting shifts toward targeted applications based on larval stage-specific thresholds. Biological controls, particularly Spodoptera exempta nucleopolyhedrovirus (SpexNPV), offer a host-specific alternative, infecting larvae via foliar and causing 90-100% mortality within 5-7 days under optimal humidity (>70%). Field trials in reported 75-85% reduction in larval populations when SpexNPV was applied at 10^12 occlusion bodies per during early outbreaks, with no observed resistance development due to its obligate viral nature. Local mass-production protocols using infected cadavers have enabled scalable deployment in , yielding economic benefits through preserved cereal harvests estimated at $10-20 per treated. Parasitoids such as Charops sp. and egg predators deployed preventively have suppressed outbreaks by 50-70% in modeling studies, though field efficacy varies with environmental factors like rainfall disrupting host-parasite synchrony. Entomopathogenic nematodes (Heterorhabditis bacteriophora) show 60-80% mortality against soil-dwelling pupae, complementing NPV for below-ground stages. Cultural practices enhance overall efficacy by disrupting S. exempta life cycles, including early planting to avoid peak larval emergence (October-December in ), crop rotation with non-host , and intercropping cereals with trap crops like Napier grass to divert marching larvae. These methods reduced damage by 30-50% in Kenyan trials when combined with , though standalone efficacy is lower (10-20% ) against migratory swarms. Destroying volunteer weeds harboring eggs and larvae before crop establishment prevents initial infestations, with farmer-led initiatives in reporting sustained reductions in outbreak frequency over multiple seasons. Integrated deployment of these strategies, as modeled for sub-Saharan contexts, yields benefit-cost ratios of 3:1 to 5:1, outperforming sole reliance on chemicals by mitigating resistance risks and environmental impacts.

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  53. https://www.researchgate.net/publication/392862215_Managing_African_Armyworm_Outbreaks_in_Sub-Saharan_Africa_Current_Strategies_and_Future_Directions
  54. https://www.africanfarming.com/2025/03/26/armyworms-outbreak-in-the-free-state/
  55. http://www.armyworm.org/wp-content/uploads/2018/08/PN84pp18-19.pdf
  56. https://www.fao.org/family-farming/detail/en/c/1620521/
  57. https://researcherslinks.com/current-issues/Biocidal-Efficacy-Heterorhabditis-bacteriophora-against-African-Armyworm/30/1/3933
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