Spodoptera exempta (Walker, 1856)
African Armyworm
 (one synonym : Prodenia bipars Walker, 1857)
ACRONICTINAE,   NOCTUIDAE,   NOCTUOIDEA
 
Don Herbison-Evans
(donherbisonevans@yahoo.com)
and
Stella Crossley

Spodoptera exempta
(Namibia, 2010)

These Caterpillars are an international pest, feeding on various grasses ( POACEAE ), including:

  • Para Grass ( Brachiaria mutica ),
  • Rice ( Oryza sativa ),
  • Dallisgrass ( Paspalum dilatatum ),
  • Kikuyu ( Pennisetum clandestinum ),
  • Sugar Cane ( Saccharum officinarum ), and
  • Sorghum ( Sorghum bicolor ).

    Spodoptera exempta
    (Photo: courtesy of J. Dupree, Bulawayo, Zimbabwe)

    The caterpillars are initially green, but later instars can become brown with dark stripes of triangles. The caterpillars vary their colour according to their degree of crowding. If sparse, they are inclined to be greener. If crowded they become darker. This has been found to correlate with an increased resistance to disease if crowded. They often advance in a mass across field, consuming the grass as they go (hence the term "Armyworm"). They do this blatantly in full daylight. This is extensively illustrated in the BBC video "War of the Worlds" in their series: "Alien Empire". The young caterpillars only eat the surface layer of the leaf, but later instars eat large pieces out of the sides of the leaves.

    Spodoptera exempta
    (Photo: courtesy of Buck Richardson, Kuranda, Queensland)

    The adult moth is brown with a pattern of light and dark markings on each forewing. The hindwings are silvery white, with brown edges and veins. The pheromones of this species have been identified. The adult moth has a wingspan of about 3 cms.

    Spodoptera exempta
    (Photo: courtesy of the Photography Group, Centre for Biodiversity Genomics, University of Guelph)

    The species occurs across much of the world, including :

  • China,
  • Hawaii,
  • Japan,
  • Zimbabwe,

    and also in Australia in

  • Western Australia,
  • Northern Territory,
  • Queensland, and
  • New South Wales.

    The moths undertake periodic migrations, which complicate the control of the species. These migrations have been observed by radar.

    The species might be controlled by :

  • digging a ditch in front of the caterpillars, as they then find the extensive section of loose soil difficult to scale,
  • the bacterium Bacillus thuringiensis ( BACILLACEAE ),
  • the nematode Steinernema carpocapsae ( STEINERNEMATIDAE ),
  • a nuclear polyhedrosis virus ( BACULOVIRIDAE ), and
  • trapping using artificial pheromones.

    Further reading :

    J.R. Agnew (ed.),
    Australian Sugarcane Pests,
    Bureau of the Sugar Experiment Stations,
    Indooroopilly, 1997, pp. 54-55.

    Ian F.B. Common,
    Moths of Australia,
    Melbourne University Press, 1990, pp. 42, 65, 462.

    Buck Richardson,
    Tropical Queensland Wildlife from Dusk to Dawn Science and Art,
    LeapFrogOz, Kuranda, 2015, p. 121.

    Francis Walker,
    Noctuidae,
    List of the Specimens of Lepidopterous Insects in the Collection of the British Museum,
    Part 10 (1857), p. 355, No. 114.

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    (updated 1 July 2010, 22 October 2017, 30 August 2021, 7 April 2022)