Spodoptera exigua

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Geographic Range

Spodoptera exigua, an insect known as the beet armyworm, is thought to have originated in southeast Asia. As an adult moth, S. exigua is extremely mobile. Consequently, it has expanded its geographic range extensively outside of Asia, becoming an invasive species and crop pest throughout the world. It is found in temperate and subtropical regions around the globe, including China, Japan, the United Kingdom, Scandinavia, the United States, South America, Africa, and New Zealand. It has been studied extensively in the United States and China. Beet armyworms were first recorded in the United States in Oregon in 1876 and now inhabit most of the southern states, reaching as far north as Maryland and northern California. Because S. exigua can complete multiple generations in a single warm season, it regularly spreads to higher latitudes during the summer into regions where it cannot survive during the winter; however, the identification of specific overwintering regions is considerably controversial. Experts disagree about whether some adults migrate to low enough latitudes during the fall to survive the winter. (Feng, et al., 2003; Ruberson, et al., 1994; Zheng, et al., 2011a; Zheng, et al., 2011b)

Habitat

Found in temperate zones and tropical areas, Spodoptera exigua occupies a habitat that ranges from crop and grass fields to temperate and tropical forests. Adults lay their eggs on the lower sections of host plants and on the top and bottom of leaves to prevent desiccation and predation of the eggs. Beet armyworms use a wide variety of host plants, including vegetables, grasses, weeds, flowering plants and other crops, which the hatching larvae completely consume, leaving only the roots and leaf veins. The environmental conditions and the host plant species directly influence the development of armyworm larvae, such that a warmer climate and nutritious host plants allow the larvae to develop more quickly.

When an adult moth emerges from the pupal stage, it flies to elevations where winds can carry it long distances for migration. Most moths glide at 200 m above the ground, but some have been known to fly above 500 m. If it is early in the season, adults may stay within the same region and reproduce in the spring or early summer. When the moths do not migrate before reproducing, armyworm population densities can reach infestation levels. Extensive crop damage commonly accompanies infestations by this insect. Alternatively, if it is late in the season, the moths migrate to warmer regions or other countries, where their offspring have a higher probability of survival due to increased food availability and increased temperatures. Adults and larvae often are found together in forests and fields; however, adults are not limited to their immediate surroundings, due to their ability to fly. (Feng, et al., 2003; Zheng, et al., 2011a)

Physical Description

The eggs of Spodoptera exigua appear woolly and green; they are shaped like a cone with a round bottom. The larvae vary in color from green to yellow, and they become darker and develop lateral stripes and dark dots as they mature. The larvae do not possess any hairs or spines on their bodies, and they look smooth. The larvae grow from 1 mm to between 25 and 30 mm in length. Once it reaches an appropriate length, a larva forms a brown pupa that has dark brown markings on each abdominal unit and is 15 to 20 mm long. When the moth emerges from the pupa after metamorphosis, it is brown, gray, and white, with brown and gray forewings and white hindwings. The adult has an average wing span of 25 to 32 mm.

Adult males and females are physically similar. Only the females produce sex pheromones to attract mates. The sex of a pupa can be determined, with the help of a magnifying lens, by identifying the structure of the sternal or ventral plates and the position of the gonopore. (Capinera, 2006; Chen and Chang, 1989)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    25 to 30 mm
    0.98 to 1.18 in
  • Range wingspan
    25 to 32 mm
    0.98 to 1.26 in

Development

After 3 or more days as eggs, Spodoptera exigua larvae hatch and then devour their host plant, feeding on the foliage and fruit. The larvae also feed on nearby plants. The length of time in the larval stage is variable, because the development time depends on the ambient temperature, where higher temperatures allow the larvae to develop to the adult stage more quickly. Larval development time also depends on the species of host plant, which ranges from vegetable crops to common weeds and plants. The larvae grow quickly on pigweed and slowly on cabbage, where growth is measured in terms of weight and survivorship. Beet armyworms feed for the duration of the larval stage and develop through 5 instars, or larval growth stages, during which they grow larger and darker.

During pupation, a larva drops to the ground and digs an underground chamber 1 to 2 cm deep and weaves a silk cocoon. This habit allows the pupa to withstand temperatures as low as 4 ºC in the overwintering (hibernation) stage, as the silk insulates the insect against cold weather. The adult moth emerges after about 6 to 7 days, thereby completing metamorphosis. The sex of this insect can be determined at the pupal stage by using a magnifying lens to identify the structure of the sternal or ventral plates and the position of the gonopore. (Capinera, 2006; Zheng, et al., 2011b)

Reproduction

Upon emerging from their pupae, Spodoptera exigua moths are reproductively mature, and males seek out females via sex pheromones. Adult males can detect female sex pheromones from long distances. They can migrate to breeding areas, sometimes traveling thousands of kilometers. Adult males and females have 4 to 10 days to find mates before they die. The average male mates with 5 females, but the number ranges from 1 to 11. Females also mate multiple times, but the number of times has not yet been determined. Age directly influences the mating rates of individuals, with older individuals mating longer and less frequently than younger individuals. Mating lasts from 30 to 60 minutes the first time, but takes as long as 180 minutes by the end of the reproductive lifespan. As a result, younger individuals are more successful at finding mates than older individuals. (Luo, et al., 2003)

When Spodoptera exigua reaches adulthood, it reproduces in the spring and summer until its death. The female moths usually begin to lay eggs 3 to 7 days after eclosing from the pupa, and they lay a total of 300 to 600 eggs in their lifetime. The eggs are laid in clusters of 50 to 150, and each cluster is laid in a separate place, either on the same plant or on different plants, to maximize survival. After laying their last egg clusters, adult females live for just a few more days. (Capinera, 2006; Takai and Wakamura, 1995; Zheng, et al., 2011a)

  • Breeding season
    spring and summer
  • Range eggs per season
    300 to 600

Host plants directly influence the growth and survival rate of the larvae, due to the quality and quantity of the nutrients the plants provide. Adult females accordingly exhibit a preference of laying eggs on specific host plants. For example, Spodoptera exigua females consistently lay higher proportions of their eggs on pigweed than on cotton, bell pepper, sunflower, or cabbage.

Spodoptera exigua females also provide yolk for their eggs. (Berdegue, et al., 1998; Greenberg, et al., 2001; Greenberg, et al., 2002; Showler, 2001)

  • Parental Investment
  • pre-fertilization
    • provisioning

Lifespan/Longevity

At high temperatures, the egg stage lasts about 3 days, the larval stage lasts 18 to 20 days, the pupal stage lasts 5 days, and adult moths live 4 to 10 days. Thus, the shortest lifespan of Spodoptera exigua is about 27 days. However, most individuals have a lifespan of 30 to 40 days. (Capinera, 2006; University of Arkansas Division of Agriculture, 2006)

  • Typical lifespan
    Status: wild
    27 to 40 days

Behavior

A beet armyworm larva does not tolerate cold very well, but it can overwinter and pupate by digging into the soil to form a chamber that is held together by thin strands of silk. It prefers to form a pupation chamber in the soil directly beneath the canopy of its host plants. In its larval stage, Spodoptera exigua is gregarious, particularly in its feeding habits as a first and second instar; large numbers of individuals have been observed feeding together. As armyworm larvae mature, they exhibit less gregarious behavior. (Bradshaw, 2012; Capinera, 2006; Feng, et al., 2003; North Carolina Cooperative Extension Service, 1997; Zheng, et al., 2011b)

Home Range

Although Spodoptera exigua larvae remain in one location through the pupal stage, adults are highly mobile and can travel distances of up to 179 km.

Communication and Perception

Olfaction is important for oviposition choice and mate attraction in Spodoptera exigua. To attract males, virgin females have been known to secrete sex pheromones, of which the compound (Z)-9-tetradecen-1-ol is an essential component.

Larvae also have sensory hairs that can detect the wing vibrations of potential parasites, such as parasitic wasps that burrow into the larvae. Upon sensing the vibrations, the larvae often cease their current activity and become immobile until the threat passes or even fall from the plant to avoid predation. (Showler, 2001; Wakamura and Takai, 1990; Yong, 2008)

Food Habits

Spodoptera exigua larvae use chewing mouthparts to feed on foliage and fruit. When feeding on fruit, S. exigua larvae make shallow, superficial wounds on the surface. The larvae primarily feed on the undersides of leaves, leaving behind small webs of silk. After S. exigua feeds on a plant, the leaves become skeletonized. Larvae also have been reported to weave the leaves of plants together using their silk to form a makeshift cocoon around themselves for protection when feeding.

Spodoptera exigua is known to feed on more than 50 plant species, which are distributed across 10 plant families around the world. Crops that are susceptible to S. exigua include corn, alfalfa, peas, tomatoes, potatoes, legumes, soybeans, onions, lettuce, cotton, and tobacco, among others. The larvae also have been known to feed on ornamental plants such as roses, sunflowers, geraniums, and carnations. Additionally, larvae have been observed feeding inside the flower buds of some species, which protects them from predators and insecticides. Sometimes they exhibit cannibalism, especially when food resources are low.

Spodoptera exigua adults use sucking mouthparts to drink the nectar of many flowering plants. (Capinera, 2006; Drees and Jackman, 1998; Natwick, et al., 2012; Showler, 2001; Zalom, et al., 2011)

  • Plant Foods
  • leaves
  • fruit
  • nectar
  • flowers

Predation

Spodoptera exigua eggs and small larvae are especially susceptible to predation. They fall prey to a few species in the groups Heteroptera (e.g., damsel bugs, shield bugs), Coleoptera (beetles), Neuroptera, and Hymenoptera, as well as a few unidentified species of Arachnida (spiders). Most of these predator species are generalists that prey on various organisms.

When threatened by ant predators, S. exigua defends itself by regurgitating. Ants that contact the oral secretion are repelled, stop their attack, and begin to groom themselves intensively.

Spodoptera exigua also can detect the wing vibrations of wasps and other parasitoids as the attackers approach. When these predators come near, S. exigua becomes motionless on the plant until the threat passes, or it drops from its host plant to avoid an attack. The most common parasitoids that attack S. exigua are wasps such as Chelonus insularis, Cotesia marginiventris, and Meteorus autographae and the tachinid fly Lespesia archippivora. (Alborn, et al., 1997; Capinera, 2006; Rostás and Blassmann, 2009; Ruberson, et al., 1994)

Ecosystem Roles

Spodoptera exigua serves as a host for numerous wasp and dipteran parasitoids, such as Chelonus insularis, Cotesia marginiventris, Meteorus autographae, Hyposoter exiguae, and Lespesia archippivora. These parasitic wasps and flies are attracted by the volatile compounds emitted by plants when S. exigua saliva contacts damaged leaves.

Fungal diseases can afflict S. exigua populations, and the nuclear polyhedrosis virus is an important cause of mortality for larvae. (Capinera, 2006; Natwick, et al., 2012; Ruberson, et al., 1994)

Economic Importance for Humans: Positive

Spodoptera exigua has no known positive economic importance to humans.

Economic Importance for Humans: Negative

Spodoptera exigua is considered one of the worst agricultural pests worldwide. Due to its larvae consuming a wide range of vegetation, S. exigua can have a disastrous effect on crop production. The armyworm is a well-known pest of vegetables, flowers, and other commercial crops such as cotton and tobacco. Its skeletonization of leaves not only renders plants aesthetically unpleasant but also slows growth in many field crops, leading to decreased yields and substantial economic losses for the agricultural industry. Larvae consume the seedlings, squares, and bolls of cotton plants, thus reducing yield and negatively impacting the textile industry. The holes that larvae create in tomatoes and lettuce can lead to rotting. Larvae also may leave feces in crops, remain in the crop, or cause significant scarring, all of which negatively influence crop yields. (Capinera, 2006; Natwick, et al., 2012; Zalom, et al., 2011)

  • Negative Impacts
  • crop pest

Conservation Status

There are currently no efforts to conserve this species.

Contributors

Wilber Hua (author), The College of New Jersey, Serge Zemerov (author), The College of New Jersey, Elizabeth Wason (author, editor), Animal Diversity Web Staff, Keith Pecor (editor), The College of New Jersey.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

internal fertilization

fertilization takes place within the female's body

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

migratory

makes seasonal movements between breeding and wintering grounds

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

References

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