Agronomy Update: Weather, Weeds, Worms

The upper Midwest has experienced a cool, wet spring thus far. Most of us were left 40-80 heat units short of April’s 30-year average while receiving 100 to 200 percent of the month’s average rainfall. Some areas exceeded 200 percent of average rainfall! Watching the rain out the window can lure anyone into a flurry of stressful “what-if” scenarios, but the Dairyland Seed Agronomy Team wants everyone to focus on a few pest issues that we are anticipating with the compromised field preparation across the countryside.

WEEDS

Many common field weeds are getting a head start on the crop. Annual and biennial broadleaf weeds are coming on first and could be nearing the size limits of herbicide labels by the time applications can be made. We are all familiar with the “start clean, stay clean” mantra. To achieve this:

• Scout fields and prioritize weed control where it is needed first.
• Be sure that your herbicides will control any issues that were not anticipated a month ago. Many soil residual herbicides (i.e. Group 15) will not control emerged weeds unless mixed with another active ingredient.
• Changes to your tillage programs might require a more robust herbicide program.
• Pre-plant intervals for burndown applications will affect planting timing. For example, intervals for products with 2,4-D or dicamba vary from 7 to 30 days, so read labels carefully.

CORN INSECTS

Increased weed pressure can compound insect pressure as well. Pay special attention to any alfalfa or small grain fields being taken out this spring; they can be insect hot beds. Black cutworm moths have been arriving in the upper Midwest for the better part of a month.

• Cutworm corn cutting begins at the 4th instar, 312 degree days (base 50) after the first significant flight and ends ~1000 degree days after flight (Ostlie and Potter, 2017). Cutting will likely begin during the last two weeks of May for many of us. A simple internet search will find the university trapping location nearest to you and help localize your projection.
• Many insecticides are labeled for control of cutworms, making rescue treatments generally successful. The economic threshold is rarely reached these days, but local conditions could align.
• The University of Minnesota recommends treatment if cutworms of less than ¾” length have wilted or cut 2-3% of plants. If worms are greater than ¾”, the threshold increases to 5%.
• Soil applied granular insecticides and Bt hybrids are sometimes not enough to stop strong black cutworm infestations. Not all above-ground traits are labeled for control of black cutworm, so prioritize those fields for scouting and alternative controls.

Armyworm flights have also been observed during the past month in the Midwest. Treatments are warranted if larvae are less than 1 inch long and two or more are observed on at least 25 percent of plants OR 1 or more are observed on 75 percent of plants. (Jensen, et al., 2017)

Ostlie, K. and Potter, B. (2017). Black Cutworm. University of Minnesota Extension-Integrated Pest Management.

Jenson, B., Liesch, P., Nice, G., Renz, M., Smith, D. (2017). A3646 Pest Management in Wisconsin Field Crops. University of Wisconsin Extension, p65.