This post is the second of a two-part series on black cutworm (Agrotis ipsilon) and its impact on corn producers in Illinois. Part one discussed the biology and lifecycle of black cutworm, identifying the adult moths and larvae, and the common types of plant injury caused by larvae. This post will explore treatment options, the economic threshold, and scouting methods that farmers can use to detect this pest in their fields. With planting season in full swing and some corn fields beginning to emerge, it is important to scout for and treat pests like black cutworm that can reduce yields and profits for corn producers across Illinois and the Midwest.
A 2018 article published in farmdoc by Dr. Nick Seiter of the University of Illinois outlines how economic thresholds are calculated and used in farming operations. The economic threshold is the method for evaluating whether the cost of treatment for a pest is equal to or less than the damage costs caused by that pest. The article notes that the purpose of the economic threshold is to prevent significant monetary losses from a pest, thus the cost of control should match the damage costs inflicted by the pest. This relationship is termed the economic injury level, which can be calculated using an equation that includes the value of the commodity, the economic damage already incurred, and the cost to control the pest. Dr. Seiter highlights that, while most people may never need to use the equation, it is still crucial to understand how the various factors in the equation can affect the economic injury level. Generally, the economic threshold is set lower than the economic injury level to allow for action before significant losses occur. What is the economic threshold for black cutworm in corn? Another 2018 farmdoc article by Dr. Nick Seiter stated the economic threshold as three percent of plants cut with larvae still present in the field. This economic threshold level remains current as of the writing of this blog post.
There are two primary ways to scout for black cutworms: trapping moths and digging for larvae. Trapping adult moths has been ongoing since planting began, using sticky traps that contain pheromones to attract males. If a significant number of moths are trapped within a specific timeframe, the area where the trap is located may be at risk of cutting damage in the coming days and weeks. However, there is no guarantee that trapped moths will indicate cutting will occur. The most important method to scout for cutworms is to dig for the larvae.
The scouting method for larvae involves examining corn plants that have been cut down at the root surface. Refer to part one of this blog series for specific information on damage caused by larvae. Scouting should occur every seven to ten days until the plant reaches the V5 stage. If a plant has sustained damage, dig around its base to locate the larvae. In the same area of the field, select 20 additional plants to check for larvae presence, even if no damage is visible. This process should be repeated in at least five different areas of the field to fully assess the extent of potential feeding.
There are several methods for treating black cutworms and protecting corn plants during this critical early stage. One way to prevent larval populations from establishing is through pre-planting weed control. Recalling part one of this blog series, larvae can thrive in fields with excessive weed cover or in fields with cover crops that have not been terminated on time. Eliminating weeds and any other plants in the field before planting can remove food sources for young larvae and reduce the likelihood of them feeding on corn plants. Simple tillage has not been found to significantly reduce egg populations, so it cannot be the only method used to prevent eggs from hatching into larvae.
Producers can also consider chemical treatments to protect their crops. Some seed treatments with a Bt trait can effectively prevent larvae from feeding on plants, but it is important to determine which traits are more effective than others. A table from Michigan State University and Texas A&M University outlines the Bt treatments that work against black cutworm and other insects. Foliar insecticide treatments are also effective in preventing black cutworm damage. Products containing pyrethroids, organophosphates, carbamates, and diamides are the most effective for treating black cutworm. If feeding is confined to one part of a field, a spot treatment in that area may be sufficient. This could save on input costs and the time needed to protect plants.
This two-part blog series has explored the critical aspects of managing black cutworm in corn, from understanding the pest to implementing effective control strategies. Part one provided a comprehensive overview of black cutworm, including its biology, identification, and the impact it can have on corn plants. Part two focused on practical approaches to scouting for black cutworm and the various control methods available. Effective scouting is essential for early detection, allowing for timely intervention before significant damage occurs. By combining knowledge with action, farmers can protect their corn crops from the damaging effects of black cutworm, ensuring healthy yields and economic viability. As planting season progresses, staying vigilant and proactive in pest management will be key to a successful harvest.
For additional reading, visit the following links:
- Black cutworm: Management decisions in corn (farmdoc, 2018)
- Handy Bt trait table for U.S. corn production (Texas A&M AgriLife Extension, 2025)
- Integrated pest management: What are economic thresholds, and how are they developed? (farmdoc, 2018)
- Reported black cutworm moths (Corn IPM, 2025)