An information piece on the website of the Agricultural Research Council (ARC), supplied by Information Core for Southern African Migrant Pests (ICOSAMP), shares important information on the African armyworm.
“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 location.
Another factor that plays a role in South Africa is temperature. The caterpillar requires temperatures of between 24 and 32oC 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 field 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 chemicals.
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 of livestock, which sometimes follows an infestation. This has been recorded on kikuyu grass and only affects cattle under veld conditions. Symptoms in cattle usually appear about ten days after the appearance of the worms, and only some kikuyu pastures produce this toxicity.
Symptoms in cattle: After swallowing the worms, affected cattle are paralysed, they have large ‘strings’ of watery saliva drooling from the mouth, and animals exhibit an apparent severe thirst. Slight symptoms of bloating, grinding of teeth, and nerve twitching may occur.
As soon as symptoms are observed, all animals should be removed from the affected pastures and a vet called in. 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 to 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 [email protected] 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. “