Strategic Farming: Field Notes – Crops update and prepping for soybean insect scouting

Eric Yu, Extension Educator – IPM, Matt Pfarr, Extension Educator – Crops, Bob Koch, Extension Soybean Entomologist

Recent rainfall across Minnesota has brought much-needed moisture to many crop acres, though conditions remain variable depending on location. During the June 24 Strategic Farming: Field Notes webinar, Matt Pfarr, Extension Educator – Crops, and Dr. Bob Koch, Extension Soybean Entomologist, provided timely updates on crop development, nutrient management, disease concerns, and soybean insect scouting. The webinar was moderated by Dave Nicolai, Extension Educator – Crops.

Corn update: Rapid growth and management timing

Corn across much of Minnesota is progressing well, with many fields approaching or surpassing the V8 growth stage. As corn enters rapid vegetative growth, management decisions related to herbicides and nitrogen become increasingly important.

Pfarr emphasized that while some post-emergence herbicide labels allow applications at later growth stages, applicators should carefully follow label restrictions and understand the risks of crop injury and reduced efficacy. Once corn exceeds V8 or reaches approximately 30 inches in height, many common herbicides, including glyphosate, glufosinate, dicamba, and Enlist products, require drop nozzles to keep concentrated spray out of the corn whorl. For more information on corn post-emergence herbicide cutoff stages, refer to this Crop News article.

Nitrogen demand is also increasing rapidly. According to University of Minnesota Extension recommendations, approximately 80% of a corn crop’s seasonal nitrogen requirement should be met by the V10–V12 growth stage. Corn is now entering the period of peak nutrient uptake, where nitrogen demand can reach up to seven pounds per acre per day. This season’s dry fall in 2025 and relatively dry spring conditions contributed to high residual nitrate levels in some soils. As a result, early-season nitrogen deficiency symptoms have been limited in many areas, though that could change as crop demand accelerates.

Disease Outlook: Tar spot remains on the radar

Although no southern rust has been reported in Minnesota so far in 2026, tar spot remains a disease to watch. Tar spot activity has increased in neighboring states, including Iowa, Nebraska, Kansas, and Indiana. While the disease has historically caused the greatest concern in southeastern Minnesota, current reports suggest it has not yet become widespread in central or western regions of the state. Tar spot development is favored by cool, humid conditions, particularly temperatures around 70–75°F combined with frequent moisture. Pfarr described tar spot as a “Goldilocks disease,” requiring environmental conditions that are neither too dry nor too wet, and neither too cold nor too warm.

When considering fungicide applications, growers are encouraged to scout fields and use risk assessment tools such as those available through the Crop Protection Network. University of Minnesota Extension does not recommend prophylactic fungicide applications; treatment decisions should be based on disease presence and expected economic return.

Hail damage: Wait before making decisions

Recent storms brought hail and wind damage to some Minnesota fields. Pfarr advised growers not to make immediate management decisions following hail events. For corn, regrowth depends heavily on whether the growing point remains intact. At current growth stages, the growing point is still relatively low in the plant and often protected, improving recovery potential. Soybeans typically have strong regrowth ability because they possess multiple axillary growing points at each node. If stems remain intact below the lowest growing point, soybeans can often recover well. Pfarr recommended waiting one to two weeks before evaluating yield loss or making management changes.

Soybean herbicide timing and IDC

Second-pass post-emergence herbicide applications are currently underway in many soybean fields. Pfarr emphasized the importance of targeting small weeds, particularly waterhemp, for optimal control. He noted that waterhemp should ideally be treated when plants are 2 inches tall or less, especially when using contact herbicides such as glufosinate. Pfarr also reviewed soybean herbicide growth stage cutoffs, noting that glufosinate (Liberty) can be applied up to the R1 growth stage, Enlist (2,4-D choline) can be applied up to R2, and glyphosate (Roundup) can be applied through R2. Additional soybean postemergence herbicide products and their application cutoffs are outlined in the following Crop News article.

Pfarr also discussed iron deficiency chlorosis (IDC), a common issue in soybeans grown on high-calcareous soils. IDC occurs when soybean plants are unable to meet their iron needs, resulting in chlorosis, or yellowing, of the newest trifoliate leaves. While soybeans can often grow out of IDC symptoms over time, the duration and severity of stress ultimately determine the potential yield impact. Management options primarily include selecting IDC-tolerant soybean varieties and, in some cases, applying iron chelate fertilizers to improve iron availability.

Soybean aphids: Start scouting now

Koch reported that soybean aphids are present in many Minnesota fields, but populations remain well below economic thresholds. Growers should begin regular scouting now and continue through at least the R5 growth stage. The established economic threshold remains 250 aphids per plant, with 80% of plants infested and populations increasing. This threshold is designed to allow time for treatment before yield loss occurs and to provide beneficial insects such as lady beetles, lacewings, and parasitic wasps an opportunity to naturally suppress the infestation. Recent rainfall may have temporarily reduced aphid populations by knocking aphids off plants.

Pyrethroid resistance remains a major concern

Koch warned that pyrethroid-resistant soybean aphid populations continue to be widespread in Minnesota. Recent research at the University of Minnesota Rosemount Research and Outreach Center showed that some pyrethroid-containing insecticide applications not only failed to control aphids but actually resulted in higher aphid populations compared to untreated plots. This likely occurs because pyrethroids can kill beneficial insects, such as predators and parasitoids, while resistant aphids survive and continue reproducing. Newer insecticides such as Sefina, Transform, and Sivanto Prime have continued to perform well against resistant aphid populations and tend to be less disruptive to beneficial insects. These products also affect aphids differently than older pyrethroid chemistries. Transform and Sivanto Prime act similarly to neonicotinoids by disrupting chemical signaling between nerves, while Sefina targets stretch receptors in the aphids’ joints, disorienting them and causing them to stop feeding almost immediately. Koch noted that aphids treated with Sefina may still appear on plants after application, but they are no longer feeding and will eventually die from starvation, desiccation, or by falling off the plant.

Soybean gall midge continues expanding

Koch also discussed soybean gall midge, a relatively new insect pest that continues expanding in Minnesota. The pest is now established across much of southern Minnesota, with the heaviest infestations still concentrated in southwestern counties. Last year, soybean gall midge was detected in Dakota, Ramsey, and Chisago counties. Symptoms include wilting or dead soybean plants along field edges, particularly adjacent to fields that were planted to soybean the previous year. For these potentially symptomatic plants in edges, growers should more carefully inspect the base of stems for dark lesions and bright orange larvae inside stem tissue. Symptomatic plants may begin to appear in early July, but become easier to detect later in the summer as injury accumulates.

Join us Wednesday mornings!

Join us on July 1 for the next Strategic Farming: Field Notes webinar featuring Dean Malvick, Extension Field Crops Pathologist, and Jeff Coulter, Extension Corn Agronomist.

University of Minnesota’s Strategic Farming: Field Notes webinar series, offered Wednesdays morning from 8 to 8:30 am through August 12, provides useful, timely, and relevant research-based information on cropping issues throughout the growing season. For more information and to register, visit z.umn.edu/strategic-farming.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their generous support of this program!