Economic impact of cutworm damage to maize and soybean

October 2025

THE INCREASED PEST STATUS OF THE COMMON CUTWORM, AGROTIS SEGETUM (LEPIDOPTERA: NOCTUIDAE), HAS BEEN A CONCERN FOR GRAIN FARMERS OVER THE PAST FOUR CROPPING SEASONS. ALTHOUGH THE REASONS FOR THE INCREASED IMPORTANCE OF THIS PEST ARE NOT CLEAR, IT IS MOST LIKELY DUE TO A COMBINATION OF FACTORS. THESE INCLUDE CHANGING CLIMATE AND RAINFALL PATTERNS, THE ADOPTION OF HERBICIDE-TOLERANT CROPS, AND THE IMPLEMENTATION OF REDUCED TILLAGE PRACTICES THAT RESULTED IN MORE SUITABLE ENVIRONMENTS FOR THIS PEST TO SURVIVE AND MULTIPLY.

Cutworms are an early-season pest of maize and soybean. The larvae sever seedlings just above or below the soil surface, after which seedlings wilt and die. If infestations are severe, significant stand losses may be experienced soon after seedling emergence, often necessitating costly pesticide applications and even replanting.

A severed and wilted maize seedling and a cutworm larva curled into its distinctive C-shape.

SURVEY ON ECONOMIC IMPACT OF CUTWORMS 
A survey was conducted during 2024 to assess the economic impact of this pest and to determine whether certain agronomic practices may be associated with the increased pest status of cutworms. A total of 65 farmers participated, all of whom were directly affected by cutworm infestations. For the purpose of this survey, the grain production area was subdivided into the following three regions: Eastern, Western and Eastern Cape + KwaZulu-Natal (EC + KZN).

Results on the area in each region that was affected and that had to be replanted due to severe stand losses are presented in Table 1. A total of 12% of the maize surface area under cultivation by the 65 respondents was affected by cutworm and of that 3,9% had to be replanted. In total, 6 062 ha of maize and soybean had to be replanted.

Overall, respondents reported stand losses on 12 183 ha in maize fields and 6 054 ha in soybean fields. Although cutworms are mostly associated with maize, this survey showed for the first time that soybean is also greatly affected by this pest.

The economic impact of the abovementioned cutworm damage was estimated by using available data on input costs to calculate the cost of replanting and chemical control, as indicated by affected farmers (Table 2). The economic impact of replanting of maize and soybean after cutworm-inflicted stand losses on the surface areas indicated in this survey illustrates the importance of this pest.

The overall cost to replant maize and soybean during the 2023/2024 season was estimated at R30,6 million (Table 2). These costs do not include direct expenses in terms of insecticide applications. Assuming that at least a single insecticide application was made with planting to prevent cutworm damage, it is estimated that the overall cost of chemical control on the total maize and soybean affected area of 18 587 ha, amounted to R14,2 million. It is therefore estimated that the overall economic loss experienced on the total surface area of 150 363 ha represented in this study was R44,8 million.

Weeds are the host plants of cutworms during winter months.

AGRONOMIC PRACTICES THAT INFLUENCE CUTWORMS
Conyza, grasses and Amaranthus were the most common weed species reported by farmers. The period between pre-plant weed cultivation and planting ranged widely between farmers, with the mechanical and chemical burndown of these weeds being implemented, on average, 14 days before planting. This period is too short to have a significant influence on the survival of cutworm larvae that use these weeds as their host plants.

Most respondents that experienced severe infestations indicated that the crop residue cover (%) on fields at the time of planting ranged between 26% to 50%. Crop residue does not only provide shelter for cutworm larvae, but also has a negative effect on the efficacy of pesticide applications. This is because the amount of pesticide that reaches the soil surface where larvae move around at nighttime, is not sufficient. Furthermore, pesticides may also be adsorbed by organic matter.

As a result, fields with abundant crop residue cover and an abundance of weeds shortly before planting are likely to provide suitable conditions for cutworms to survive until crop seedlings emerge.

A cutworm larva feeding on a maize seedling.

MANAGEMENT OF CUTWORMS
Correct timing of weed control or chemical burndown of weeds is crucial for effective cutworm management. Previous recommendations that fields must be kept free of weeds for a period of 35 days before planting are still valid. Keeping fields free from weeds for such a long period ensures that any larvae that survived the winter on weeds will starve to death before crop seedlings emerge.

The cultivation of herbicide-tolerant maize and soybean and the move towards reduced tillage practices are factors that may have contributed to the increased pest status of cutworm.

This study was partly funded by Grain SA who also distributed the questionnaire to its members. The assistance of Syngenta, who initiated the survey and managed the online questionnaire, is highly appreciated (specifically Gustav Venter).

Cutworm damage to a maize seedling.

Publication: October 2025

Section: Pula/Imvula

Author: PROF HANNALENE DU PLESSIS, PROF JOHNNIE VAN DEN BERG AND ZUNEL VAN EEDEN, ALL FROM THE NORTH-WEST UNIVERSITY. FIRST PUBLISHED IN SA GRAAN/GRAIN, SEPTEMBER 2025