2026 University of Minnesota Cooperative Black Cutworm Trapping Network
This report was prepared May 13, 2026
Dr. Fei Yang - Extension Corn Entomologist, University of Minnesota Department of Entomology
Bruce Potter
Greetings,
Black Cutworm
Correction from last week’s report: The Martin County 2 report of 3 moths on 4/28 should have been Jackson County.
Very few moths arrived this week with no significant captures. This is not unexpected due to a lack of weather systems to aiding moth movement combined and several nights with temperatures near freezing (Table 1, Figure 1). Some Northerly winds were not for migration.
Cool weather continued to slow both crop and cutworm development. I did see some early-planted corn pushing 3 leaves in Rock County yesterday.
Early planted corn is at most risk from April 12-14 flight egg laying. These larvae should be large enough to show visible leaf feeding on emerged weeds, corn and other crops. They should be large enough to cut 4-leaf and smaller corn by May 24.
Based on projected temperatures, black cutworm larvae from the late-April flights will not be large enough to cut until the 1st week of June (Table 2). Now, t’s a race between corn and cutworm development.
Armyworm
Low trap captures this week. Cottonwood, Freeborn, Houston, McLeod, Olmsted Polk, Stearns and Steele County traps captured armyworm moths, but at low levels.
Managing black cutworm problems
Economic thresholds – When to treat a problem
(adapted from: corn-pest-management/black-cutworm-corn)
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.
Corn
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 mean 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.
Managing Armyworm problems
Economic thresholds – When to treat a problem
(adapted from: corn-pest-management/armyworm )
Corn
Grassy weeds are attractive to egg-laying moths. When scouting, pay close attention to field borders and within-field areas with current or past dense grass weed pressure. If not killed before moths arrive, grass cover crops, winter rye may also be attractive egg laying sites.
Examine plants for feeding damage and larvae. On larger plants, the larvae can often be found in the whorl, where the nighttime feeding often occurs. When there are large amounts of plant residue (cover crop, dead weeds, etc.) larvae may hide on the ground. Small corn (5 leaves or less) can typically recover from armyworm defoliation.
Treat whorl stage corn when 25% of plants have two larvae/plant or 75% of plants have one larva or more. On tassel stage corn, focus on minimizing defoliation at or above the ear leaf.
Wheat, Barley, oats
Some studies indicate a difference in preference among the small grain species, but all are hosts. When trying to detect larval populations, pay close attention to early planted, taller fields and to areas that are lodged, are near lodged grass borders, or have grassy weeds. When an economic armyworm infestation is suspected in a small grain field, populations per square foot should be estimated.
Head clipping is a behavioral change and usually occurs after leaves have been defoliated or senesced.
Shake the plants and look for larvae on the ground in a square foot area. In small grains the treatment threshold is 4-5 larvae/square foot. Check under debris and soil clods. Do this in at least five locations within the field.
Pastures and other crops.
The same scouting methods and economic thresholds described for small grains can be used for pastures, forage grasses, and grass seed production fields.
What about other crops?
Despite their strong preference for grasses, armyworms may be forced to feed on less preferred hosts. Armyworms may clean out the weedy grasses while leaving a less desirable broadleaf crop alone, but on occasion, starving armyworms will switch to the broadleaf crop when the grass food source has been eaten or killed. When terminating a grass cover crop in corn, sugarbeets, newly seeded alfalfa, or other crops, it is worth looking for armyworm larvae based on feeding or sweep net samples. The same applies to areas of dense grassy weeds. Removing their food source will force the armyworms to feed on the crop or move to a nearby field.
Bt Hyrids
Only Bt corn hybrids containing the Vip3a protein (Viptera), alone or in stacks, are labeled for controlling armyworm. In addition to lush small grain and forage grasses, grassy spring weeds and cover crop growth, the Bt protein in the planted hybrid can help prioritize fields for scouting. The Handy Bt corn trait table shows which Bt proteins control armyworm and several other insect species.
At-plant insecticides
High rates of neonicotinoid seed treatments (e.g., Poncho, Cruiser, Gaucho) are effective on many seed and seedling insects and may provide some early seedling protection against armyworms. Seed applied insecticides containing a diamide (e. g. chlorantraniliprole and cyantraniliprole) may show increased control of armyworm.
Post-emerge rescue insecticides
Fortunately, armyworms can be controlled effectively with a timely application of a rescue, foliar insecticide.
Many post-plant insecticide products provide effective control of armyworm with products in the pyrethroid, organophosphate, carbamate and diamide insecticide groups.
Do not base treatment decisions solely on field-edge populations. The presence of live armyworm larvae should be confirmed before an insecticide is applied to a field. Insecticide treatment of populations that are starting to pupate or are heavily parasitized is not likely to provide economic benefit. Long insecticide residuals are generally not needed because of the short time a larval generation is damaging. Refer to the insecticide label for rates.
Spot treatments can be effective when combined with careful scouting. Partial field or border insecticide treatments for armyworm are often sufficient when infestations are well identified by scouting early or when armyworm populations are migrating. Treat several boom widths ahead of the infestation.
Most insecticides have little activity on armyworm eggs. Moth migration occurring over a long period of time and prolonged egg-laying can increase the difficulty of timing the insecticide. Applying too late risks economic crop loss and too early allows late-hatching larvae to escape.
During daylight hours, armyworms spend much of their time hiding low in the canopy and under lodged plants so good insecticide coverage is important. Use appropriate water volumes and pressures matched spray nozzles suited to insecticide application. Do not use reduced rates, particularly when large armyworms are the target. Spraying in the evening may help control but could also expose insecticides to temperature inversions and drift, reducing effectiveness.
There have been recent reports of issues with performance pyrethroid insecticides against armyworm. However, pyrethroid insecticide resistance has not yet been documented in Minnesota true armyworm populations. In addition to resistance, these issues might have been related to coverage, environmental conditions, short insecticide residual, missing late-hatching larvae, and applications made too late to non-feeding prepupal larvae.
For later season outbreaks, it is particularly important to check the pre-harvest interval of any small grain or forage pesticide. In corn, take precautions to protect pollinators, particularly if corn is nearing the tassel stage.
Scouting and rescue insecticide applications are the best defense against yield loss from armyworms. Before deciding to treat, make sure armyworms are still present and actively feeding.
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
This and previous reports can be found at 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