The African armyworm (''Spodoptera exempta''), also called ''okalombo'', ''kommandowurm'', or nutgrass armyworm, is a
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of
moth
Moths are a group of insects that includes all members of the order Lepidoptera that are not Butterfly, butterflies. They were previously classified as suborder Heterocera, but the group is Paraphyly, paraphyletic with respect to butterflies (s ...
of the family
Noctuidae
The Noctuidae, commonly known as owlet moths, cutworms or armyworms, are a family (biology), family of moths. Taxonomically, they are considered the most controversial family in the superfamily Noctuoidea because many of the clades are constantly ...
. The
larva
A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
e often exhibit marching behavior when traveling to feeding sites, leading to the common name "armyworm".
The
caterpillar
Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths).
As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder ...
s exhibit density-dependent
polyphenism
A polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions. It is therefore a special case of phenotypic plasticity.
There are several types of polyphen ...
where larvae raised in isolation are green, while those raised in groups are black. These phases are termed ''solitaria'' and ''gregaria'', respectively. ''Gregaria'' caterpillars are considered very deleterious
pests
PESTS was an anonymous American activist group formed in 1986 to critique racism, tokenism, and exclusion in the art world. PESTS produced newsletters, posters, and other print material highlighting examples of discrimination in gallery represent ...
, capable of destroying entire crops in a matter of weeks. The larvae feed on all types of
grass
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and spe ...
es, early stages of cereal crops (e.g.,
corn
Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...
,
rice
Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...
,
wheat
Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
,
millet
Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most millets belong to the tribe Paniceae.
Millets are important crops in the Semi-arid climate, ...
,
sorghum
''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...
),
sugarcane
Sugarcane or sugar cane is a species of tall, Perennial plant, perennial grass (in the genus ''Saccharum'', tribe Andropogoneae) that is used for sugar Sugar industry, production. The plants are 2–6 m (6–20 ft) tall with stout, jointed, fib ...
, and occasionally on
coconut
The coconut tree (''Cocos nucifera'') is a member of the palm tree family (biology), family (Arecaceae) and the only living species of the genus ''Cocos''. The term "coconut" (or the archaic "cocoanut") can refer to the whole coconut palm, ...
.
The ''solitaria'' caterpillars are less active and undergo much slower development.
The species is commonly found in
Africa
Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...
, but can also be seen in
Yemen
Yemen, officially the Republic of Yemen, is a country in West Asia. Located in South Arabia, southern Arabia, it borders Saudi Arabia to Saudi Arabia–Yemen border, the north, Oman to Oman–Yemen border, the northeast, the south-eastern part ...
, some
Pacific islands
The Pacific islands are a group of islands in the Pacific Ocean. They are further categorized into three major island groups: Melanesia, Micronesia, and Polynesia. Depending on the context, the term ''Pacific Islands'' may refer to one of several ...
, and parts of
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
.
[Rose, D. J. W., Dewhurst, C.F. and Page, W. W. 2000. ''The African Armyworm Handbook'' (2nd Edn). NRI, Chatham, UK, 304 pp ] African armyworm outbreaks tend to be devastating for farmland and
pasture
Pasture (from the Latin ''pastus'', past participle of ''pascere'', "to feed") is land used for grazing.
Types of pasture
Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, c ...
in these areas, with the highest-density outbreaks occurring during the
rainy season
The rainy season is the time of year when most of a region's average annual rainfall occurs.
Rainy Season may also refer to:
* ''Rainy Season'' (short story), a 1989 short horror story by Stephen King
* "Rainy Season", a 2018 song by Monni
* '' ...
after periods of prolonged
drought
A drought is a period of drier-than-normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, ...
.
[Haggis, M. J. 1984. Distribution, frequency of attack and seasonal incidence of the African armyworm, ''Spodoptera exempta'' (Walk.) (Lepidoptera: Noctuidae), with particular reference to Africa and south-western Arabia. Tropical Development Research Institute, London, Report No. L69, pp. 116.] During the long dry seasons ("off-season"), the population densities are very low and no outbreaks are seen.
Taxonomy and phylogeny
''Spodoptera exempta'' was
first described by
Francis Walker in 1856. The species of ''
Spodoptera
''Spodoptera'' is a genus of moths of the family Noctuidae Species description, erected by Achille Guenée in 1852. Many are known as Pest (organism), pest insects. The larvae are sometimes called armyworms. The roughly thirty species are distrib ...
'' are distributed around the world, mostly inhabiting
tropical
The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
and
subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...
areas. There are 30 known species in the
genus
Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
, and roughly half are considered
agricultural pests
A pest is any organism harmful to humans or human concerns. The term is particularly used for creatures that damage crops, livestock, and forestry or cause a nuisance to people, especially in their homes. Humans have modified the environment fo ...
. There are two final larval stages based on the
mandible
In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla).
The jawbone i ...
structure of the species:
serrate-like mandibles and
chisel
A chisel is a hand tool with a characteristic Wedge, wedge-shaped cutting edge on the end of its blade. A chisel is useful for carving or cutting a hard material such as woodworking, wood, lapidary, stone, or metalworking, metal.
Using a chi ...
-like mandibles. Along with ''S. exempta'', the following species and
subspecies
In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...
have chisel-like mandibles that were evolved to consume silica-rich leaves: ''
S. triturata'', ''
S. m. mauritia'', ''
S. m. acronyctoides'', ''
S. umbraculata'', ''
S. cilium'', ', 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
An ancestor, also known as a forefather, fore-elder, or a forebear, is a parent or ( recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from ...
for the genus lived between 22 and 30.7 million years ago and began diversification during the
Miocene Epoch
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...
.
Geographic range
The African armyworm is commonly found in the
grassland
A grassland is an area where the vegetation is dominance (ecology), dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes such as clover, and other Herbaceo ...
s of Africa and
Asia
Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
. Within Africa, it is mostly seen near the
Sahara
The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...
in the following countries:
Tanzania
Tanzania, officially the United Republic of Tanzania, is a country in East Africa within the African Great Lakes region. It is bordered by Uganda to the northwest; Kenya to the northeast; the Indian Ocean to the east; Mozambique and Malawi to t ...
,
Kenya
Kenya, officially the Republic of Kenya, is a country located in East Africa. With an estimated population of more than 52.4 million as of mid-2024, Kenya is the 27th-most-populous country in the world and the 7th most populous in Africa. ...
,
Uganda
Uganda, officially the Republic of Uganda, is a landlocked country in East Africa. It is bordered to the east by Kenya, to the north by South Sudan, to the west by the Democratic Republic of the Congo, to the south-west by Rwanda, and to the ...
,
Ethiopia
Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...
,
Somalia
Somalia, officially the Federal Republic of Somalia, is the easternmost country in continental Africa. The country is located in the Horn of Africa and is bordered by Ethiopia to the west, Djibouti to the northwest, Kenya to the southwest, th ...
,
Malawi
Malawi, officially the Republic of Malawi, is a landlocked country in Southeastern Africa. It is bordered by Zambia to the west, Tanzania to the north and northeast, and Mozambique to the east, south, and southwest. Malawi spans over and ...
,
Zimbabwe
file:Zimbabwe, relief map.jpg, upright=1.22, Zimbabwe, relief map
Zimbabwe, officially the Republic of Zimbabwe, is a landlocked country in Southeast Africa, between the Zambezi and Limpopo Rivers, bordered by South Africa to the south, Bots ...
,
Zambia
Zambia, officially the Republic of Zambia, is a landlocked country at the crossroads of Central Africa, Central, Southern Africa, Southern and East Africa. It is typically referred to being in South-Central Africa or Southern Africa. It is bor ...
, and
South Africa
South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
. Outside of Africa, the species also inhabits southwest
Saudi Arabia
Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in West Asia. Located in the centre of the Middle East, it covers the bulk of the Arabian Peninsula and has a land area of about , making it the List of Asian countries ...
,
Southeast Asia
Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...
, Australia, and
New Zealand
New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
.
Habitat
Akin to other species of the genus ''Spodoptera'', ''S. exempta'' lives in
tropical
The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
and
subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...
environments near
cereal crops and dense vegetation where the larvae of the species can easily feed.
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.
While seasonal winds and rains allow adult ''S. exempta'' to migrate across countries and continents, they are often confined to coastal areas and
marsh
In ecology, a marsh is a wetland that is dominated by herbaceous plants rather than by woody plants.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p More in genera ...
es during dry seasons, where the occasional rainfall supports enough vegetation growth for the moth's survival.
Food resources

''S. exempta'' larvae feed almost exclusively on
Poaceae
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivate ...
, also called
Gramineae
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated i ...
. Poaceae is a family of flowering
grasses
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in ...
which includes cereal grasses and the grasses of
grassland
A grassland is an area where the vegetation is dominance (ecology), dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes such as clover, and other Herbaceo ...
s and pastures.
The species targets most
cereal crops, including
maize
Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout grass that produces cereal grain. It was domesticated by indigenous peoples in southern Mexico about 9,000 years ago from wild teosinte. Native American ...
,
sorghum
''Sorghum bicolor'', commonly called sorghum () and also known as great millet, broomcorn, guinea corn, durra, imphee, jowar, or milo, is a species in the Poaceae, grass genus ''Sorghum (genus), Sorghum'' cultivated for its grain. The grain i ...
,
rice
Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...
,
millet
Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most millets belong to the tribe Paniceae.
Millets are important crops in the Semi-arid climate, ...
, and other
grasses
Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos, the grasses of natural grassland and species cultivated in ...
.
Young caterpillars are also known to feed on
wheat
Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
and
oat seedlings.
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.
Life cycle
Eggs
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.
The number of eggs females can lay is positively correlated with their
pupa
A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
l weight.
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.
The eggs hatch into larvae within two to five days.
Larvae
''S. exempta'' undergo six larval
instar
An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...
s 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.
The larvae display density-dependent
polyphenism
A polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions. It is therefore a special case of phenotypic plasticity.
There are several types of polyphen ...
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.
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.
''Solitaria'' larvae are less active and tend to stay curled up by the base of the crops, leading to less exposure to the sun. 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.
Generally, African armyworms are not noticed by farmers until the caterpillars are ten days old and start exhibiting the ''gregaria'' phase.
[Brown, E. S. 1972. Armyworm control. ''Pest Articles and News Summaries''.]
Larvae that are transferred from isolated to dense conditions or vice versa display the ''gregaria'' appearance.
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.
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''.
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.
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.
Pupae
Pupation
A pupa (; : pupae) is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages th ...
occurs beneath the surface of host plants and away from bare ground, roughly 2–3 cm underground.
This pupation event leads to a sudden synchronized disappearance of the larvae, especially if the soil is moist.
Adults
Adults emerge in 7 to 10 days and can live up to 14 days.
The moths
migrate over hundreds of kilometers from their emergence sites to their
oviposition
The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...
sites.
This migration often causes outbreaks to occur suddenly in areas that were previously free of the pests.
[Jahn, G.C. 1995]
Environmental assessment for armyworm control in Ethiopia
. USAID, Washington, DC. While
polyphenism
A polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions. It is therefore a special case of phenotypic plasticity.
There are several types of polyphen ...
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.
The adult moth
wingspan
The wingspan (or just span) of a bird or an airplane is the distance from one wingtip to the opposite wingtip. For example, the Boeing 777–200 has a wingspan of , and a wandering albatross (''Diomedea exulans'') caught in 1965 had a wingsp ...
is between 20 and 37 mm, with dull gray-brown forewings and off-white hindwings with visible veins.
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
The abdomen (colloquially called the gut, belly, tummy, midriff, tucky, or stomach) is the front part of the torso between the thorax (chest) and pelvis in humans and in other vertebrates. The area occupied by the abdomen is called the abdominal ...
tip and black scales.
Males have been observed to mature earlier than females.
Migrations
When African armyworms first emerge after pupation, they allow their bodies to dry off and their wings to harden before climbing trees.
The moths then fly hundreds of meters above the ground and rely on winds to carry them to their next location.
Therefore, migration is based on winds brought by the
Intertropical Convergence Zone
The Intertropical Convergence Zone (ITCZ , or ICZ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the t ...
, 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.
Because rain causes the moths to descend, ''S. exempta'' are less likely to migrate during frequent rainfall.
After the moths land back on the ground, they drink water, mate, and lay their eggs.
Migration is beneficial because it allows the species to travel to a new location with fewer
predators
Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
and less possibility of
parasitism
Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...
and
infection
An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...
.
Outbreak behavior
''S. exempta'' are a serious
pest for
crop
A crop is a plant that can be grown and harvested extensively for profit or subsistence. In other words, a crop is a plant or plant product that is grown for a specific purpose such as food, Fiber, fibre, or fuel.
When plants of the same spe ...
s, 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.
During these secondary outbreaks, ''gregaria'' larvae emerge from the ground synchronously, and their density may exceed 1,000 larvae per m
2.
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.
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.
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.
Enemies
Overview
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.
Despite these challenges, ''S. exempta'' still faces predators,
parasites
Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...
,
parasitoid
In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
s,
disease
A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
s, and
virus
A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...
es. 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
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
''
Nomuraea rileyi''.
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.
Most significantly, the species is infected by the species-specific
baculovirus
''Baculoviridae'' is a family of viruses. Arthropods, among the most studied being Lepidoptera, Hymenoptera and Diptera, serve as natural hosts. Currently, 85 species are placed in this family, assigned to four genera.
Baculoviruses are known ...
, ''S. exempta''
nucleopolyhedrovirus (SpexNPV).
SpexNPV
SpexNPV, abbreviated from ''S. exempta''
nucleopolyhedrovirus, is a
baculovirus
''Baculoviridae'' is a family of viruses. Arthropods, among the most studied being Lepidoptera, Hymenoptera and Diptera, serve as natural hosts. Currently, 85 species are placed in this family, assigned to four genera.
Baculoviruses are known ...
that can be
transmitted vertically, from adults to offspring.
SpexNPV can kill 90% of an African armyworm larvae population in their last instar; the high
mortality rate
Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular Statistical population, population, scaled to the size of that population, per unit of time. Mortality rate is typically ...
is due to the vertically transmitted diseases' capability of traveling with the diseased individual during migration.
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
symptom
Signs and symptoms are diagnostic indications of an illness, injury, or condition.
Signs are objective and externally observable; symptoms are a person's reported subjective experiences.
A sign for example may be a higher or lower temperature ...
s.
''S. exempta'' attempt to reduce the risk of contracting SpexNPV via density-dependent
prophylaxis
Preventive healthcare, or prophylaxis, is the application of healthcare measures to prevent diseases.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, prolonging life, and promoting physical and mental health a ...
, which allows the species to invest more in resisting disease and stimulate migration to less dense locations.
Mating
Female/male interactions
Pheromones
Virgin female ''S. exempta'' adults are observed to release
pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
s 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
Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...
species, the latter two compounds have only been seen in other ''Spodoptera'' species.
Physiology
Thermoregulation
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.
Interactions with humans
Pest control

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
pesticide
Pesticides are substances that are used to control pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for approximately 50% of all p ...
s such as
DDT,
BHC, and
dieldrin
Dieldrin is an organochlorine compound originally produced in 1948 by J. Hyman & Co, Denver, as an insecticide. Dieldrin is closely related to aldrin, which reacts further to form dieldrin. Aldrin is not toxic to insects; it is oxidized in the i ...
were commonly used to target the caterpillars.
Now, newer insecticides such as
azadirachtin and aqueous neem (''
Azadirachta indica
''Azadirachta indica'', commonly known as neem, margosa, nimtree or Indian lilac, is a tree in the mahogany family Meliaceae. It is one of the two species in the genus ''Azadirachta''. It is native to the Indian subcontinent and to parts of Sout ...
'') seed extracts are often applied, but these methods are dose dependent and have adverse side effects on both human health and crops.
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
Biological control or biocontrol is a method of controlling pests, whether pest animals such as insects and mites, weeds, or pathogens affecting animals or plants by using other organisms. It relies on predation, parasitism, herbivory, or ot ...
.
Infestations
In mid-April 1999, an African armyworm
infestation
Infestation is the state of being invaded or overrun by pests or parasites. It can also refer to the actual organisms living on or within a host.
Terminology
In general, the term "infestation" refers to parasitic diseases caused by animals su ...
started in southern
Ethiopia
Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...
, spreading into the north the following month and into the
Jubba Valley of
Somalia
Somalia, officially the Federal Republic of Somalia, is the easternmost country in continental Africa. The country is located in the Horn of Africa and is bordered by Ethiopia to the west, Djibouti to the northwest, Kenya to the southwest, th ...
in early May. Similar outbreaks affected the
Rift Valley Province
Rift Valley Province () of Kenya, bordering Uganda, was one of Kenya's eight provinces, before the 2013 Kenyan general election.
Rift Valley Province was the largest and one of the most economically important provinces in Kenya. It was dominated ...
of
Kenya
Kenya, officially the Republic of Kenya, is a country located in East Africa. With an estimated population of more than 52.4 million as of mid-2024, Kenya is the 27th-most-populous country in the world and the 7th most populous in Africa. ...
and parts of
Uganda
Uganda, officially the Republic of Uganda, is a landlocked country in East Africa. It is bordered to the east by Kenya, to the north by South Sudan, to the west by the Democratic Republic of the Congo, to the south-west by Rwanda, and to the ...
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.
STAR radio in
Liberia
Liberia, officially the Republic of Liberia, is a country on the West African coast. It is bordered by Sierra Leone to Liberia–Sierra Leone border, its northwest, Guinea to Guinea–Liberia border, its north, Ivory Coast to Ivory Coast–Lib ...
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, and were moving toward Guinea and Sierra Leone.
On January 28, 2009, the president of Liberia declared a state of emergency to deal with the infestation of army worms in the country.
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 of grain had been destroyed by armyworms in just the Lindi Region of Tanzania.
In January 2015, armyworm outbreaks were reported in Zimbabwe, and by November there were reports of similar outbreaks in Botswana.
In 2016, the
fall armyworm (''Spodoptera frugiperda'', a species of American origin that behaves similarly) invaded maize crops in
Zambia
Zambia, officially the Republic of Zambia, is a landlocked country at the crossroads of Central Africa, Central, Southern Africa, Southern and East Africa. It is typically referred to being in South-Central Africa or Southern Africa. It is bor ...
. By January 3, 2017, about 90,000 hectares were affected according to reports released by the Zambian government's Disaster Management and Mitigation Unit.
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.
In 2017, fall armyworm infestations were detected in more African countries such as Zimbabwe and South Africa.
See also
* Common armyworm or true armyworm (''
Mythimna unipuncta
''Mythimna unipuncta'', the true armyworm moth, white-speck moth, common armyworm, or rice armyworm, is a species of moth in the family Noctuidae. The species was Species description, first described by Adrian Hardy Haworth in 1809. ''Mythimna un ...
'') (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
External links
{{Authority control
Spodoptera
Agricultural pest insects
Moths of Cape Verde
Owlet moths of Africa
Moths of Japan
Moths of Madagascar
Moths of New Zealand
Moths of the Middle East
Moths described in 1856
Insect pests of millets