Report #6 May 3-9, 2025
This issue was prepared by:
Dr. Fei Yang – Extension Corn Entomologist, University of Minnesota Department of Entomology
Bruce Potter – Potter Bros., LLC
May 13, 2025
Greetings,
Black cutworms
A much quieter week. Nevertheless, captures of significant black cutworm moth migration occurred May 6-9 into Rock and May 7-8 into Renville County (Table 1, 2).
Warm temperatures should push faster development of black cutworm larvae. Scouting for leaf feeding and possible cutting should be underway in high risk fields.
Larvae from the earliest mid-April moth arrivals could reach the 4th instar stage and able to cut small corn as soon as May 20. These larvae should cease feeding and being to pupate in mid-June (Table 2).
Figure 1 shows the areas where we know that large numbers of moths arrived last week
Armyworms
Several trap sites across Southern Minnesota reported captures, but these captures continue low
Managing black cutworm problems
Economic thresholds – When to treat a problem
Remember, yield limiting populations of black cutworms are relatively rare in Minnesota and black cutworms can be well managed by scouting and post-plant rescue insecticide treatments.
Cutworms reduce crop yield by decreasing the plant populations. Corn is one of the more sensitive crops with respect to plant population. A generic economic threshold for black cutworm in corn is 2-3% of the plants cut or wilted when the larvae are less than ¾ inch long. The threshold increases to 5% cut plants when larvae are larger. However, with high corn prices, these thresholds could be lowered to 1% wilted or cut for small larvae and 2-3% wilted or cut for large larvae.
Remember to take into consideration corn plant populations in individual fields and adjust threshold numbers accordingly. For example, if the current plant population is at or near yield limiting levels, you can afford to lose fewer plants than in a field with a higher emerged population. Planting date factors into replant decisions as the season progresses and finding a stand loss later in the season compounds the replant decision. The UMN Extension publication “Corn grower's guide for evaluating crop damage and replant options” includes information on the effect of corn stand, planting date, and hybrid relative maturity on yield
Based on the amount of larval feeding remaining, the black cutworm economic threshold for corn varies by larval size. Cutworms must shed their skins (molt) to grow. The stage between molts is called a larval instar. Cutworms will begin to cut corn at the 4th instar (~½ inch long). The smaller larvae tend to cut corn at or near the soil surface while larger larvae tend to feed below ground. The larvae are fully grown and cease feeding between 1½ and 2 inches long. While larger larvae can cut or tunnel into larger plants, they have less time left to feed and as a result, kill fewer plants.
There are more detailed dynamic black cutworm thresholds available that use stand, crop stage, projected damage and crop price. However, caution is advised when dynamic thresholds generate lower thresholds below those described above. Yield loss, actual or measurable, does not begin with the first missing corn plant. High grain prices and a good planted and emerged stand means you could easily be treating cutworm populations that would not reduce stand enough to hurt yields.
The rescue insecticide calculator (Table 3) was adapted from a University of Illinois publication and is an example of a dynamic threshold that is used in several management guides. Modern corn yields and prices could indicate treatment at a very low percentage cut plants using this worksheet, perhaps leading to over-reactive treatment decisions. However, the yield loss factors are still useful when combined with yield loss by stand reduction charts. It might be useful to revisit these dynamic thresholds with higher modern seeding rates.
Other crops
Plant population and planting date effects on yield will vary for crops other than field corn. The growing points of broadleaf crops are above ground. Plants will be killed if cut below the cotyledons so even climbing cutworm species can be a threat. Since yield loss from cutworms is related to stand loss, crops that are less able to compensate for stand loss are at greater risk.
While black cutworm larvae will cut soybeans, they are less likely to create a yield limiting problem in this crop. Soybeans are seeded at a much higher plant density and can compensate (up to a point) for a reduced stands much better than corn.
Sugarbeets are at risk because of yield and quality sensitivity to beet plant populations. In addition, they are planted early and often with an oat cover which may encourage black cutworm egg laying. Cutworms will move to beet seedlings as oats and weeds are killed by herbicides.
Post-plant rescue insecticides
Fortunately, cutworms are controlled well with rescue insecticide applications and many post-plant insecticide products provide effective control of black cutworms. Several compounds within the pyrethroid, organophosphate, carbamate, and diamide groups are labeled for post plant/post-emerge cutworm control. Spot treatments can be effective when combined with careful scouting. Make sure cutworms are still present and actively feeding if you decide to treat them. Check just before spraying to ensure stand loss is still progressing.
Black cutworms tend to remain lower in the soil when the top few inches of the soil profile are dry, meaning that insecticide applications can be less effective. A rotary hoe or row cultivation before application (or after application if below-ground feeding continues) can help improve the efficacy of some insecticides by incorporating insecticides and encouraging cutworm movement. Good coverage of row area and plants is important. Do not skimp on water and match spray volume and pressure to nozzles designed for insecticide application. Although spraying in the late afternoon or evening will place the insecticides in the field closer to larval activity, it can also reduce control if a temperature inversion prevents the spray from settling on the field.
It is important to read pesticide labels. Be cautious of potential interactions between some organophosphate insecticides (e.g., Counter, Chlorpyrifos) and some ALS herbicides. These interactions can cause severe or temporary crop injury. Some PPO, and HPPD herbicides may also interact with insecticides.
In the next issue we’ll discuss other cutworm species you may encounter.
For more information on migration, and other management facts of these two crop pests, see
Corn-pest-management/Black-cutworm-corn and
Corn-pest-management/armyworm.
For more information: bcw-reporting
This network is supported, in part, by the farm families of Minnesota and their corn check-off investment.
Contact me with any questions and Happy trails,
Bruce Potter
[email protected]
(507) 276-1184