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ARMYWORM OUTBREAK IN SOUTH AFRICA 2003
Reports of large numbers of armyworm ("Kommandowurm") larvae have were received at the ICOSAMP desk in 2003 from 4 Provinces in South Africa viz. Kwazulu/Natal, Gauteng, North West, and Mpumalanga (See Map). Newspapers in Gauteng and Kwazulu/Natal also printed short articles about this outbreak.

armwyorm in hand armyworm close up
Armyworm collected in the area of the Rietvlei Dam, Pretoria, on Tuesday 15th April 2003. (Photo:M Powell, ARC-PPRI)

General information
Identification
Life Cycle
Control
Warning to farmers
Reporting
 
General Information
The African Armyworm or kommandowurm (Afr.) (scientific name = Spodoptera exempta), is widely distributed in Africa south of the Sahara, and is a serious pest in countries north of South Africa. A characteristic feature of armyworm outbreaks is their unexpected sudden appearance, and this has led to the common name of "mystery worm". Often, large areas of lawn, pasture or crops, sometimes extending over a whole Province, are seen to be covered in dense swarms of worms, virtually overnight!

Small outbreaks occur every year in the high-rainfall areas of Mpumalanga and Kwazulu/Natal Provinces, but a large outbreak such as the one we are currently experiencing, only occurs once in every 6-7 years. The last major outbreak occurred in 1981.

The worst outbreaks in South Africa occur in seasons where there are "late summer rains" after a drought.

Identification
The moth is a nocturnal insect measuring about 30mm and is capable of migrating thousands of kilometres, especially during plague years. Moths may be recognised by the patterned brown colour of the forewings, and whitish hindwings.

The larvae (or worms) are greenish or brownish-green when they occur singly, but as they start crowding together their colour changes to black in the older stages, with thin green/yellow lines running along the length of the worm on either side. The fully grown larva is about 25mm long.

Life Cycle
The female moth lays her eggs at night in clusters, and each cluster can contain from a few to 400 eggs. The eggs hatch in 3-6 days depending on the temperature - the warmer the temperature, the quicker the eggs will hatch.

Larvae pass through a series of "moults" and the stage between each moult is known as an instar. There are 5 or 6 instars, and it is during the last two instars that the armyworm causes the most severe damage to crops and grasses. The duration of the larvae stage is between 11-24 days, after which the larvae burrow into soft damp soil to pupate. The moth emerges after 7-12 days. The total life-cycle is about 45 days under suitable conditions.

The diagram below shows the life-cycle of the armyworm and the duration of each stage.

The main migration flights of the moths start at dusk or in the first part of the night, and moths ascend to a height of about 300m above ground level. They move downwind on prevailing air currents. In most parts of South Africa the infestations are usually due to moths migrating from warmer, more northerly areas, often from Zambia into Zimbabwe, or Mozambique.



Control
It is important to know that for control measures to be effective, the worms must be found in time!

If the caterpillars are discovered when they are fully grown, the use of insecticide control is often not recommended as most of the damage to crops will already have been done, and the emerging adults will probably move off and not produce a second generation in the same place.

Another factor that plays a role in South Africa is temperature. The caterpillar requires temperatures of between 24-32 degrees Centigrade to develop, and therefore anything below this will hinder development and often cause death of the larvae.

Furrows: Where the caterpillars are moving from one land to another, they can be halted by ploughing a furrow with "pits" dug at intervals. The larvae will crawl along the furrows and eventually fall into the pits where they can be covered up or treated with chemical.

Chemicals: Chemical control is most effective if applied as soon as the worms have emerged (1-5mm long), as these instars are more susceptible to poisons than older instars. Two of the insecticides registered in South Africa for use against armyworm are: cypermethrin and decis (synthetic pyrethroids).

Warning to cattle farmers
One aspect of armyworm outbreaks is the poisoning which sometimes follows and infestation. This has been recorded on Kikuyu grass and only affects cattle under field conditions. Symptoms in cattle usually appear about 10 days after the appearance of the worms, and only some kikuyu pastures produce this toxicity.

Symptoms in cattle: The swallowing of affected cattle is paralysed, appearance of large 'strings' of watery saliva drooling from the mouth, and animals exhibit an apparent severe thirst. Slight symptoms of bloat, grinding of teeth, and nervour twitching may occur.

As soon as symptoms are observed, ALL animals should be removed from the affected pastures and a Vet called in. A good prevention of further poisoning is the removal of all animals from the pasture for a period of at least 40 days.

Reporting of outbreaks
The control of armyworm in South African is not part of Government Policy and financial assistance is therefore not available for farmers. However, ICOSAMP is particularly interested in recording as many localities as possible in the migrant pest database.

If you have experienced or observed any outbreak of this pest, please send an email to the ICOSAMP Co-ordinator (Margaret Kieser) at
icosamp@ecoport.org stating clearly:
  • The locality of infestation (district, farm name)
  • Stage of larvae (green or black)
  • Size of infestation (number of hectares)
  • Crop infested
  • Your name and a contact number
Yours assistance in this matter will be greatly appreciated as the data obtained will enable scientists to research this pest more effectively.

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