By Amelia Genis
Cold weather will kill the African armyworms, which were recently found for the first time in the Western Cape, says an entomologist from North-West University.
Prof. Johnnie van den Berg of the North-West University (NWU) says that when it starts to get cold, it is highly unlikely that there will be a next generation that will lead to a re-infestation – even if there is food for the worms.
“When the temperature drops below 10°C, they start to disappear. They literally die out. They don’t move somewhere else.”
The worms thrive in average temperatures above 25°C. “As soon as the temperature drops below 15°C, they start to develop more slowly and some of the larvae will die. It is very unfavourable for them when the evening temperatures drop below 10°C.”
Fewer worms, say farmers
Farmers from Porterville, such as WG Treurnicht and Piet du Plessis, say they are indeed seeing fewer worms. Treurnicht says he hasn’t seen a worm in a long time. There were still worms on the ground against the mountain last week.
Du Plessis says there are parts of his farm where the worms have decreased. “There are still worms on the growing spots and down the wires.”
Short life cycle
The worms have a short life cycle, says Van den Berg. “From the time the eggs are laid until the first little larvae hatch, it’s four to five days. Then they start eating. The worms grow incredibly fast, which is why it’s such a surprise when they’re first seen.”
As the worms get bigger, their food intake increases exponentially. When food starts to run low, they move on and literally march in columns in search of other food, says Van den Berg. “Within 10 to 12 days, those worms are big enough to cause terrible damage.”
When the worms are large, they pupate in the soil for eight to ten days, after which a moth emerges again, but only if the conditions are right.
‘Farthest south’ yet
The African armyworms are a permanent tropical pest in Central and East Africa, which occur very sporadically in the northern parts of South Africa, says Van den Berg.
The African armyworms that are now in the Western Cape are the furthest south he has ever seen them in his 40 years as an entomologist. A month ago, a farmer from Humansdorp did let him know that he had found them there.
He cannot say for sure exactly how they ended up in the Western Cape. The moths can fly very far, but they are also picked up by air currents in Uganda, the eastern Congo and Tanzania, but where they return to earth is almost a random event, says Van den Berg.
“They just fall out somewhere, start laying eggs and then the worms come out.”
He says the pyrethroid pesticides that farmers spray against them have a broad effect, but a short after-effect. The worms only eat grass crops and not broadleaf crops, such as soy, tomatoes or lucerne.
More to read:
