In the agricultural environment, insect pests are of key interest due to the fact that their development and damage potential are influenced by the changing climate and agricultural practices that have been adopted along the way.

Among the various pests that threaten crop yields, cutworms have re-emerged as significant challengers to maize cultivation over the past three planting seasons.

These polyphagous pests are highly adaptable and capable of feeding on a wide range of plant species, including economically vital crops like maize, soya bean, and sunflower, which are commonly grown in the Highveld region of South Africa.

Understanding the ecological interplay between cutworms and weeds is crucial for effective pest management, particularly in the Highveld production region, where cutworm infestations can lead to significant reductions in crop density and yield.

A few factors that predispose maize fields to cutworm outbreaks include, but are not limited to, previous cutworm infestations, the timing of the removal of weed coverage, and environmental conditions like temperature and soil moisture influencing larval survival.

Cutworm larvae undergo six larval stages before pupation and moth emergence. During these stages, they rely heavily on available food sources. Large instar larvae, which are more damaging, are often observed to feed on weeds stems before transitioning to newly emerged maize seedlings.

However, the presence of small larvae can be identified by their characteristic leaf damage that have a shot-hole appearance.

Producers often overlook the dual role that weeds play in the life cycle of cutworms. After the harvest of maize, many fields are left uncultivated, allowing for the growth of winter weeds.

These weeds serve not only as a refuge for cutworm larvae but also as a primary food source in the early planting season, before the new season’s maize crop is established. Consequently, fields with an abundance of weeds can inadvertently become breeding grounds for cutworms.

Cutworm larvae prefer to feed on various weed species, often developing more robustly on them compared with maize. This preference underscores the need for proactive weed management, as uncontrolled weed growth directly correlates with increased cutworm populations.

The timing of weed management practices is an essential component of effective cutworm control. However, many producers tend to initiate their weed control measures too close to the planting date, which is often too late to mitigate the impact of established cutworm populations.

A simple realisation will influence the way producers think about their winter weeds and the time they intend to cultivate.

Cutworms are already present in the fields at planting time, having developed from eggs laid by cutworm moths on the soil, winter weeds and maize residues left on the fields during the preceding spring.

Understanding the oviposition behaviour of cutworm moths further illuminates the relationship between weeds and cutworm populations. Moths prefer to lay their eggs on dense vegetation, particularly in areas with low-growing plants and ample plant debris.

This preference means that fields with unmanaged weeds accompanied by maize residues are more likely to experience bigger cutworm infestations. As a result, significant damage to maize seedlings can occur soon after seedling emergence, with cutworms capable of drastically reducing plant density within days after planting.

Weeds to look out for

While cutworms will feed on any available weed species, they have frequently been observed to feed and hide under Senecio consanguineus (John Deere bossie), Conyza bonariensis (skraalhans/horseweed), Picris asplenioides (oxtongue), and Portulaca oleracea (porselein/varkkos/hogweed).

To effectively combat this problem, producers should conduct impromptu field scouting at least 40 days prior to their intended planting date. This early assessment allows them to identify any cutworm populations that exist underneath winter weeds that are growing on their fields and adjust their management practices accordingly.

Scouting

Identify areas that have been previously impacted by cutworm. Assess the types of weeds present on your fields. If typical weeds associated with cutworm are abundant, the occurrence of cutworm populations may be likely. Pull large weeds from the root, look underneath the leaves of low-growing weeds, and check the soil.

Investigate the topsoil (5cm to 10cm) beneath weeds, and locate injured maize seedlings within rows or under cover crops. Separate the soil by hand and collect cutworm larvae.

If any evidence of cutworm damage to weed leaves, stems, maize seedlings or cover crops is evident, the surrounding soil must be carefully searched for larvae.

Producers and their selected field representatives of various agrochemical companies can then determine what their plan of action will be, based on the number of cutworms collected during the scouting process.

Protecting maize seedlings

Following the scouting, weed management and cultivation practices should begin at least 35 days before planting.

Eliminating weeds during this critical period means the larvae will either starve or move away from their previous food sources before maize seedlings emerge, thus reducing the likelihood of damage.

The consequences of delaying weed management can be severe.

If weeds are removed after cutworm populations have already established themselves too close to planting, larvae have no other option than to move to maize seedlings, resulting in a loss of seedling stand, which, in many instances, has been so severe that producers were forced to replant, impacting them financially.

Effective management of cutworm populations relies on an integrated pest management approach. This strategy includes early scouting for cutworms, timely weed removal, the application of pre-emergence herbicides, and the responsible use of insecticides registered against cutworm.

By beginning weed management efforts early, producers can disrupt the life cycle of cutworms and reduce their populations significantly before the maize crop is planted. Continuous monitoring of cutworm populations and moth flights is another method for predicting and managing potential infestations.

Producers should consider tracking moth activity, which can help in timing their management practices more effectively. A simple way of doing so is to put up a white sheet in the previously infested fields with spotlights to attract flying insects to the sheet.

Cutworm moths can be easily identified based on the characteristic kidney bean-shaped markings observed on each moth’s wings.

If cutworm moths are observed, producers can anticipate the timing and presence of damaging cutworms based on their life cycle and better align their weed control measures to reduce the impact of this pest.

By adding weed identification and cutworm scouting to the list of management practices used in preparation of the 2024/25 planting season, producers will enhance their chances to gain the upper hand and be in control of the cutworm, not the other way around.

Sources: Drinkwater, TW. 1980. ‘Cutworms in maize’. Farming in South Africa. Maize Series D5/1980: 1–4; Drinkwater, TW. 2017. ‘Insekteplae van mielies in Suid-Afrika’. LNR-Instituut vir Graangewasse-Potchefstroom. 78–85; Drinkwater, TW & Janse Van Rensburg, JB. 1992. ‘Association of the common cutworm, Agrotis segetum (Lepidoptera: Noctuidae), with winter weeds and volunteer maize’. Phytophylactica, 24: 25–28. bit.ly/40wZ5gv; Du Plessis, H. 2000. ‘Common cutworm, a pest of grain crops’. ARC Crop Protection Series No. 19. ARC-Grain Crops Institute. Potchefstroom, South Africa.