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Field Notes: Weed and insect management challenges in an extended planting season

Liz Stahl, Extension educator - crops

Photo: Jared Goplen
After a good start, wet conditions have stalled planting across much of the state. Dr Debalin Sarangi, Extension weed scientist, and Bruce Potter, Integrated Pest Management specialist at the Southwest Research and Outreach Center by Lamberton discussed weed and insect management challenges faced by growers this year due to an extended planting season during the May 8 Strategic Farming: Field Notes session. They were joined by moderator Dr. Anthony Hanson, Extension educator – Integrated Pest Management.

Weed management updates

Preemergence herbicides were activated - now what?

Preemergence (PRE) herbicides are the foundation of a strong weed management program, regardless of the postemergence (POST) program used. Their use is strongly encouraged in part due to the prevalence of weeds resistant to one or more of our commonly used POST herbicides. The residual activity they provide is also key in controlling weeds like waterhemp, which emerges over an extended period of time and later into the season.

Typically a quarter to a half inch of rainfall is needed to activate PRE herbicides, so activation should not be an issue this year due to the amount of rain received by most across the state. The bigger question this year from growers who have been able to plant and apply a PRE earlier this season, is how long will their PRE herbicide last? 

Dr. Sarangi reports that residual activity typically lasts three to four weeks after application, so where crops were planted mid-April, these products may be running out of steam soon. It will be some time until crops reach canopy closure. Check fields for weed emergence and prepare to apply POST products early. The addition of a chloroacetamide (Group 15) herbicide like Dual, Warrant, or Outlook, to POST applications will help extend herbicide residual activity.

For crops that have yet to be planted, keep in mind herbicide label restrictions. For example, Fierce herbicide should not be applied once soybeans have begun to crack, and Valor should be applied within three days of planting but before soybean emergence, or severe soybean injury will occur.

Special weed problems

Where giant ragweed is a concern, tillage prior to planting at this time of year will help take out at least 50% of the plants expected to emerge this season. Giant ragweed emergence usually ends by mid-May.

Palmer amaranth continues to be a weed of concern in the state. Eradication efforts in MN have been a success story to date due to efforts of the Minnesota Department of Ag, U of MN Extension, farmers, and ag professionals. With eradication efforts now shifting to farmers and landowners due to a sunset of available funding, scouting and continued efforts to prevent introduction of this weed into the state will be critical. More information on Palmer amaranth identification, reporting, and removal can be found here.

Carryover concerns

Since 2021, drought conditions have contributed to herbicide carryover issues. Although spring rains have erased drought or abnormally dry conditions across much of the state, fields are still at an elevated risk for herbicide carryover this year, particularly if dry conditions return later in the season. Farmers are encouraged to keep an eye out for issues this year.

Herbicide resistance limits POST options

Dr. Sarangi has led efforts to document herbicide-resistant weed issues with POST products across the state by screening populations of waterhemp against commonly-used herbicides. As expected, resistance to glyphosate and the Group 2 herbicides (ALS inhibitors such as First Rate) was found to be widespread in waterhemp across the state. Resistance to Group 5 herbicides (e.g. Atrazine) was found in 47% of the populations tested, while resistance was detected to Group 14 herbicides (e.g. Flexstar) and Group 27 herbicides (e.g. Callisto) in 31 and 22 % of the waterhemp populations tested, respectively. Resistance to Group 4 herbicides (e.g. Enlist, Xtendimax) was also detected at a handful of sites.

Herbicide-resistance to POST products continues to be a complicating factor in weed control. Although resistance has not been documented to Liberty in the state to date, its overuse must be avoided to help prevent or delay resistance. Liberty is more sensitive to conditions at the time of application than other commonly used products, and applications early in the morning or late in the day should be avoided. Care should be taken to achieve good coverage since Liberty is a contact herbicide, and application should be targeted to small weeds (ideally 2-inch weeds).

Insect management updates

Black cutworm

Bruce Potter, is leading the black cutworm (BCW) monitoring network once again this year. This network is made up of a group of volunteers who set out pheromone traps to help monitor BCW migrations into the state. BCW does not overwinter in Minnesota - moths move into the state via strong southerly winds, and thunderstorms are often associated with significant moth flights. The larval stage is the damaging stage of BCW. Once larvae reach the 4th instar stage, they are large enough to cut small corn plants.

The first BCW moth captures this year were on April 8 in Brown County. Significant flights detected around April 15-16 and April 26-27. Most large detections have occurred along the MN River Valley from Nicollet to Swift Counties but scattered high counts have been observed across southern Minnesota. By the third week of May, larvae from the early flights may be big enough to cause damage to corn. Results and predicted cutting dates can be tracked at the Black Cutworm Monitoring Network.

True armyworm

Potter is also leading monitoring efforts for true armyworm. True armyworm is another migratory pest that does not overwinter in Minnesota but is brought up from the south often through storm systems. Armyworm moths are attracted to grassy areas to lay their eggs. The larvae are the damaging stage of this pest as well. Larvae feed on grasses including crops such as wheat and corn, but they have been known to feed on soybeans when a grass host is killed. Care should be taken to monitor corn that is planted into ground where a grass cover crop like cereal rye was growing this spring, especially if the cover crop was terminated less than 10 to 14 days prior to corn planting. For more information, visit the Armyworm webpage and earlier Crop News articles on scouting true armyworm and infestations

Alfalfa weevil

Potter reports alfalfa weevil had an easy winter. He has been picking up weevils in sweep net samples as the weevils come out of hibernation. Pay attention to the first cutting of alfalfa to determine if a cutting should be taken early or if the alfalfa should be cut and then an insecticide applied.

Regarding insecticides for alfalfa weevil, crop tolerances for chlorpyrifos have been restored, but it will take some time for products to be re-registered in the state. If you see control issues with any insecticides, please let us know as to date there have not been confirmations of insecticide resistance in alfalfa weevil, although control issues have been noted in the past.

Field Notes podcasts available

Check out these links if you’d like to listen to the podcast of this session ( ) or review additional resources (

Join us Wednesday mornings!

Join us next week when we welcome Dr. Jeff Couter, Extension corn agronomist and Dr. Seth Naeve, Extension soybean agronomist to discuss "Corn and Soybean Planting Update: Are we on schedule?”.

University of Minnesota’s Strategic Farming: Field Notes webinar series, offered Wednesdays morning from 8 to 8:30 am through August 21, provides useful, timely, and relevant research-based information on cropping issues throughout the growing season. For more information and to register, visit

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

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