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Armyworm Biology
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ARMYWORM BIOLOGY
The African Armyworm is the larva of a night-flying moth which, in large numbers, causes considerable damage to grasses and agricultural cereal crops (pastures, wheat, maize, rice, millet, sorghum, tef, and sugarcane).
Scientific and common names
Class: Insecta
Order: Lepidoptera
Family: Noctuidae
Genus & sp: Spodoptera exempta (Walker)
Common names:
English: African armyworm; mystery worm; hail worm; rain worm
Malawi: nchembere, zandonda (= old ladies follow each other), chipakusu, kapuchi
South Africa: kommandowurm
Tanzania: viwavi jeshi, ng'urrto
Zimbabwe: nhundururu, imhogoyi
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Economic Status and Geographic Distribution
The African armyworm is a particularly serious pest of pastures and cereal crops (maize, wheat, sorghum, millet, teff and rice) in eastern, central, and Southern Africa.
Normally only small numbers of this pest occur - usually on pastures, but periodically the populations increase dramatically and mass migration of moths occur, covering many thousands of square kilometres and traversing international boundaries.
Outbreaks follow the onset of wet seasons when dry grasslands produce new growth and cereal crops are planted. Major outbreaks of armyworm are commonly preceded by extended drought.
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Life Cycle and identification
Fig.1. Life cycle showing duration of each stage under typical outbreak conditions1
Moths
Adult moths have a wing span in the range of 20-37mm. Forewings are an overall dull grey/brown colouration, and have distinctive markings (Fig.2). Both sexes have white hindwings with dark veins running through them.
Fig.2. Adult moth [Photo: ARC]
Eggs
A female moth is capable of laying more than 1000 eggs during a period of up to 6 nights, usually between 100 and 400 eggs per batch, and are covered by black hair-scales (Fig.3). The covering of these black hair-scales distinguishes the egg masses of this species from all other species of Spodoptera.
Fig.3. Egg mass laid in laboratory, showing black hair-scales. [Photo: ARC]
Larvae (caterpillar/worm)
Larvae occur in two colour forms viz. black (gregarious phase) or green (solitary phase). The gregarious larvae (Fig.4) have:
- A 'velvety black' upper (dorsal) surface
- Pale parallel lateral lines (along the sides)
- Green or pale yellow ventral (under side) surface
- NO body hairs.
- Shiny black head
- Inverted "V" on the forehead.
Fig.4. Gregarious phase of S.exempta larvae showing distinctive markings. [Photo: ARC]
Pupae
Once the larvae have reached full size they burrow into the ground, forming chambers 2-3 cms below the surface, and then moult into pupae (Fig.5). Moths will emerge from pupae under favourable conditions within 7-15 days.
Fig.5. Pupa of S.exempta [Photo: ARC]
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