By Jasper Raats
African armyworms (Spodoptera exempta), which are related to the more common fall armyworm (Spodoptera frugiperda), are devouring pastures in Limpopo and possibly other provinces pastures.
WATCH | Scroll down for videos of the worms
CJ Pieterse, a crop advisor for AECI Plant Health on the Springbok Flats, says this black cousin of the fall armyworm was first observed just over a week ago on farms in Limpopo’s Waterberg region, especially on the Springbok Flats. Meanwhile, there have also been sightings in Groblersdal, Bronkhorstspruit, Limpopo’s western and central Bushveld regions, and parts of the Lowveld.
Pieterse says infestations like those seen this year are typical in exceptional rainfall years when the moths – known as owlet moths – migrate from the tropical regions north of South Africa’s borders to lay their eggs. These moths can travel up to 160 km in a night.
These moths’ ability to use the wind to their advantage and quickly migrate over long distances to lay millions of eggs makes African armyworm infestations so unpredictable. In 2010, they advanced as far south as the Eastern Free State and Lesotho, where that country’s government had to intervene with urgent aerial spraying programmes to save local livestock farmers’ pastures.
South Africa does not offer a viable habitat for the African armyworm and pupae, which do not survive the winter. Therefore, this pest rarely appears in South Africa, but when moths migrate south, the infestations are devastating.
Grass eaters
It is typical for this worm to appear in good and wet years late in February and March. The worms eat almost exclusively grass and grass-related crops, posing the greatest threat to pastures. Although grass in the warmer Bushveld and Lowveld regions can usually still recover before winter, the African armyworm puts livestock farmers in the highveld under tremendous pressure. The grass there rarely recovers before the growing period ends and colder temperatures arrive. Then, the farmers must enter winter with almost no pasture.
The worms’ preference for grass makes it difficult to combat the pest chemically. Pieterse says that although pyrethroids quickly kill the African armyworm, the withholding period of chemical agents registered for pastures varies from two to 35 days, which is a big problem for game farmers.
He says although biological agents such as Broadband (with the fungus Beauveria bassiana as the active ingredient) can kill the worms, it doesn’t work fast enough. Worms only die after seven days of this agent and can still actively feed for at least four days and cause enormous damage.
Warning
AJ Botes, AECI’s technical advisor for the northern region, also warns his clients in a letter that livestock can die under certain circumstances, especially where high numbers of armyworms feed or have recently fed if they graze in the same camps.
There is still no certainty about what causes the poisoning. The suspicion is that the cyanide levels in the remaining leaves of certain grasses, such as couch grass, increase when armyworms attack, leading to livestock deaths.
Another theory is that mycotoxins (extremely toxic natural toxins), which are secreted by fungi growing on the armyworm’s droppings, can be ingested by livestock. Animals can die from very low concentrations of mycotoxins, and Botes suspects that this is probably the main reason for the deaths.
