Cattle, sheep, and goats feeding on grass in an area infested with the green or black worms could become ill or die, provincial agriculture spokesperson Bonolo Mohlakoana said in a statement.
The worms recently emerged in Ngaka Modiri Molema and Dr Ruth Segomotsi Mompati districts in the North West.
Affected livestock could display symptoms, such as difficulty swallowing, excessive salivation, and severe thirst, as well as slight bloating, teeth grinding, and twitching.
Crop scientist William Weeks said livestock should be removed from infected fields for at least 40 days.
The fields should be sprayed with a low dose of insecticide in the morning, when the worms were visible and active.
Animals should still be kept off the affected area after spraying.
Farmers who encountered armyworm infestations needed to alert the local animal health technician or state veterinarian.
Armyworms are not government-regulated pests, as their population outbreaks occur only once or twice in a decade, usually following outbreaks in Tanzania or Zambia.
Weeks said the weather was generally too hot for armyworm to survive in South Africa from April through to winter.
As such, new outbreaks were not expected next summer.