Weed control key to ending cutworm plague

15 March 2024

Before farmers can effectively combat cutworms, the weeds that feed and protect them must be effectively controlled, says Prof Johnnie van den Berg, head of the Integrated Pest Management Group at North-West University.

Speaking at an eastern Free State farmers’ day hosted by Bayer on Jaco Breytenbach’s farm, Lorenzo, near Fouriesburg, Prof Johnnie van den Berg said chemical control is not the first step when fighting cutworms.

Over the past few years, the pest has caused damage in the eastern summer crop areas, especially the eastern Free State. At the beginning of the current summer season, some farmers had to replant portions of their fields after cutworms devoured the seedlings.

Holistic and extensive answers must be sought. “Some farmers say the insects’ behaviour has changed and the insecticide no longer works, that the cutworms are resistant to it,” said Van den Berg. 

“No, the insects’ behaviour hasn’t changed, and most insecticides are still the same. What has changed are the farming practices in the month or two before planting time. I think that’s where the possible answer to the problem lies.”

Understanding the life cycle of the moth

Cutworm moths are active throughout the year, even mid-winter. A moth can lay up to 2 000 eggs, which then hatch and go through larval stages.

“This is where the difficulty begins,” said Van den Berg. “In summer, when it’s nice and warm, a larva’s stage is 42-56 days. But in winter, the larva can remain in the soil where there are weeds for 100 or even 140 days. Then it becomes a pupa, then another moth emerges.”

The cycle is driven by temperature and available food. “And the food they survive on before planting time is weeds.” 

Van den Berg explained that the moths and larvae are active throughout the year wherever there are weeds. “These are the pest’s natural hosts. The eggs are mostly laid on weeds but also on maize. The larvae that cause damage in summer have long since laid their eggs back in the winter on weeds.”

Temperature is important in determining the length of the life cycle. “If the average temperature is 18°C, as in our winter, it takes 72 days from when the larva hatches until it becomes a pupa. If the average temperature is just one degree higher, the cycle shortens to 60 days. At an average temperature of 28°C, the cycle is 43 days.”

Weed species are host plants for cutworms

The important host plants that harbour and feed cutworms are weeds, but the insects can eat hundreds of plant species in seedling form. “Plants with rosettes lying flat on the ground are where the little worms will stay because they provide good protection,” said Van den Berg.

Examples of weed species that cutworms like include the red poppy (Papaver aculeatum), the Conyza species, wild radish (Raphanus raphanistrum), fragrant evening primrose (Oenothera stricta), Walafrida densiflorus, Nemesia floribunda, prickly poppy (Argemone subfusiformis), Senecio consanguineus and Helichrysum argyrosphaerum.

The number of cutworms that survive from winter to planting time determines how many will be present at planting time. 

“Small larvae, up to about 15 mm long, stay exclusively on top of the ground under the weeds’ rosettes, and where there is a hole in the ground where there is a bit of plant material they can just eat inside the hole, but they don’t go underground,” said Van den Berg. 

The large larvae, on the other hand, stay inside and under the ground, usually coming out at night to feed. “If there is food available above ground, the larvae will move around a lot and feed, even four hours per night.”

Stubble and crop residues are not the problem

There was much talk last season about the possibility of worms eating stubble and crop residues on the ground before planting time. Van den Berg said research has shown that large larvae – the type that cause damage – cannot survive on stubble for longer than 10 days. 

“The stubble mainly provides protection for the larvae. There is no nutritional value for a cutworm in dry stubble.”

The stubble can, however, have an impact on how much sprayed chemicals ends up on the ground.

There is no solution to cutworms that will work on every farm. It varies between districts, farms and even different lands on the same farm. “It’s a dynamic system we’re dealing with here,” said Van den Berg,

“The key factor is the number of worms that will survive the winter, and the winter weeds create the ‘green bridge’ that helps cutworms to persist from one season to the next.”

The traditional recommendation was always to kill all weeds that emerge after harvest and keep fields weed-free for at least five weeks before planting time. “Then there is no food for the worms during that time, they die of hunger, and you can plant. But this practice has changed and it no longer necessarily fits in with contemporary farming methods.”

However, Van den Berg believes the shorter the period between weed control and planting time, the greater the chance of a cutworm problem. 

He recommends that the chemical weed control done before planting time, the so-called burn-down, happens long enough before planting time to ensure the weeds die and the worms starve and die before seedlings emerge.

“If you mechanically control weeds, two or three weeks before planting is actually short because the worms will survive. They can survive for three or four weeks until there are seedlings.”

Chemical pest control must be applied correctly “You’ll notice I haven’t said a word about chemical pest control of the worms themselves,” Van den Berg said, adding that several registered insecticides all work equally well. Research will be done this year to see how long each insecticide’s residue remains active on the ground.

“The most important thing when it comes to chemical control is that you must stick to label recommendations. The soil must be moist when you spray – otherwise, the worms won’t come up to walk around for those two to four hours per night and thus make contact with the insecticide.”

His recommendation is that farmers apply the insecticide in the late afternoon so it can lie on the ground for a while before the sun starts to break it down. “There must be at least some moisture that first night so that the worms come up to make contact with the insecticide.”

Van den Berg says as far as researchers can determine, there is no genetic resistance in the insects against the pesticides. “However, we continue to test. Indications currently are still that the increased occurrence of cutworms has more to do with farming practices and the weeds standing in the fields.”

Inquiries: Email: johnnie.vandenberg@nwu.ac.za

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