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Strategic Farming: Field Notes session discusses early-season pest and weed management challenges

Angie Peltier, UMN Extension crops educator and Phyllis Bongard, UMN Extension educational content development and communications specialist
Early soybean aphid infestation.
Photo: Bob Koch

On June 15, UMN Extension IPM specialist, Bruce Potter, Extension entomologist, Dr. Bob Koch and UMN/NDSU Extension agronomist Dr. Tom Peters, joined Extension educators Dave Nicolai and Dr. Anthony Hanson for a discussion about strategies for managing pests and weeds in 2022. This was the sixth episode of the 2022 Strategic Farming: Field Notes program this year.

To listen to a recording of this episode subscribe to Strategic Farming: Field Notes on your favorite podcasting platform or visit this website:

Managing insect pests in 2022

How will late planting affect soybean aphid and bean leaf beetle populations? 

Pests like soybean aphid and bean leaf beetle tend to prefer to colonize the earliest planted fields. Bean leaf beetle which overwinters in leaf litter, has already been observed on soybeans this spring.

Recent moisture and temperature conditions have spurred on aphid growth and development. In both southwest Minnesota and on the St. Paul campus, aphids have begun to move from buckthorn, where they spent the winter as eggs. On a positive note, the recent hot and windy weather may have a negative effect on colonization and survival of the aphids.

In a typical year, by the time soybean aphid colonizes a field, insecticidal seed treatments might no longer be at effective concentrations in the plant. With late planting this year, there may be more of a chance that seed treatments will be at an effective concentration when aphids begin to move into soybean fields.

What alternatives are there for those that used to rely upon chlorpyrifos for pest management? 

Now that the US Environmental Protection Agency has removed the residue tolerances on crops treated with chlorpyrifos harvested for food and feed, farmers and ag service providers are concerned about the availability of other effective insecticides. To further complicate things, for the past 7 years crop producers have also been battling soybean aphid populations resistant to pyrethroid insecticides (Group 3A).

Over the last several years, newer, more selective insecticides have come on to the market that have proven effective at managing threshold-level soybean aphid infestations. Products such as Transform (Group 4C), Sivanto (Group 4D) and Sefina (Group 9D) kill soybean aphids, while being less harsh toward the aphids’ natural enemies such as Asian lady beetles and parasitic wasps. Miticides are also available for treating TSSM infestations.

What other insect pests have been observed in 2022? 

Grasshopper nymphs

There have been reports of grasshopper nymphs emerging. Producers are encouraged to scout and to do their best to treat vegetation that borders crop fields before nymphs move into fields to cause injury.

Black cutworm  

There have also been reports of significant black cutworm feeding on sugarbeet seedlings in Renville, Meeker and McLeod Counties. The black cutworms are feeding on what are already thin stands from below ground, making management simultaneously very important and difficult.

Alfalfa weevil 

Alfalfa weevil has also begun to worry producers that have harvested their first alfalfa this year. Alfalfa weevils tend to concentrate at the bottom of alfalfa windrows, from which they can easily feed on regrowth at damaging population densities. The areas of the state most at risk of weevil injury might be related to area in which there was snow cover during last winter’s cold snaps as snow provides an insulating layer for overwintering alfalfa weevil adults.

Soybean gall midge  

With funding from the North Central Soybean Research Program, UMN Extension is part of a multi-state project studying the newest economic pest of Minnesota soybean: soybean gall midge (SGM). As a part of this work, researchers such as Potter and Koch are monitoring emergence traps set up along the edges of fields that were known to be infested in 2021. By studying SGM emergence in this way, they will be better able to predict when adults are active and mating and when larvae will begin feeding. SGM females lay their eggs in expansion cracks that tend to form toward the base of the stem. It is thought that late planting of soybean fields may mean that there may be a mismatch between SGM adult emergence and the presence of expansion cracks.

An audience question: Does using a PPO-inhibitor (Group 14) herbicide such as Flexstar have any effect on insect pressure?

While the presenters were unaware of research related to this question, they theorized that leaf injury characteristic of PPO-inhibitor use may make plants less attractive to soybean aphids looking for a meal.

Managing weeds in 2022

Section 18 label for Blazer and Ultra Blazer use on sugarbeets

Dr. Tom Peters has sought and obtained a Section 18 label for applying acifluorofen (ex. Blazer, Ultra Blazer) to Minnesota sugarbeet acres. While this is an older chemistry, glyphosate-resistant waterhemp populations necessitated an additional effective, post-emergence herbicide option to manage plants that escape herbicide treatments applied pre-emergence. Tank mixing acifluorofen with glyphosate and the various adjuvants that come standard in glyphosate can also increase efficacy.

Additional tips to improve herbicide efficacy

Sugarbeet growers use herbicides at planting and on sugarbeet greater than 2-lf stage for control of weeds including waterhemp. Research has found a split application of a chloroacetamide herbicide through the sugarbeet 8-lf stage improves waterhemp control.

In general, herbicides tend to work best to kill weeds outright when emerged weeds are less than 4-inches tall or have fewer growing points (axillary buds). Each growing point needs to be covered to avoid weeds growing back from unaffected growing points so coverage is important, even with systemic herbicides. The number of growing points is lower and the chances of covering them higher when the weeds are smaller. 

Now that warm, humid weather has moved into Minnesota and weeds have begun to emerge after planting, fields should be scouted for emerged weeds at least once per week to increase the potential of being able to target small weeds. An audience member asked how best one should properly time herbicide applications as there always tend to be a range of sizes among both weeds of different species and weeds within the same species. Folks will do their best to spray smaller weeds, with the understanding that in biological systems like farm fields we will seldom be able to perfectly time an application.

Adequate carrier volume (15 gallons minimum) is essential to get good herbicide coverage. Although applicators will need to haul more water and refill spray tanks more often, this is what is required to get the best control.

Using label-recommended adjuvants and nozzles and the full label rate can also help to improve weed management.

What can we do now if there are broadleaf weeds in soybean?

If you planted a traited soybean variety in 2022, be sure and pay close attention to both the calendar and outdoor thermometer. June 12 was the last day when anyone South of I-94 could legally apply an approved over-the-top dicamba formulation to dicamba-tolerant soybeans, while north of I-94, producers have until the end of the month.

For those that planted glufosinate-tolerant soybeans (ex. Liberty Link, Xtendflex), the growing season just entered a period in which conditions will be quite favorable for maximizing glufosinate efficacy. Glufosinate tends to work best when conditions are warm, humid and sunny.

Temperatures in the 90’s don’t make for good application conditions as pesticides are more likely to volatilize before hitting their intended target or herbicides may be more phytotoxic and cause excessive injury to cultivated crops. The latter is especially true with contacts herbicides. Many postemergence herbicides control weeds by stopping various enzymatic pathways in weeds. Weeds protect themselves from stress environments by slowing the speed of operation of some pathways. Thus, some active ingredients will also not work as well at high temperatures as weeds tend to ‘shut down’ and the processes that herbicides disrupt will not be occurring.

Be mindful of length of control from soil residual herbicides following incorporation into soil. For example, chloroacetamide herbicide activity (Outlook, Warrant, S-metolachlor products, group 15) lasts for approximately 3 weeks, activity of Valor (group 14) and metribuzin products (group 5) is longer -  perhaps 4 to 5 weeks - and Authority products (group 14), 8 to 10 weeks. 

Because all herbicides are being applied later than in an average growing season, should we worry about carryover?

Laudis & atrazine are both common corn herbicides, but if one were planning to plant soybeans or sugarbeets in 2023, neither should be applied after June 15 or at the lastest June 21, depending on soil texture and soil organic matter. Most herbicide degradation in our region occurs in June, July and August and is facilitated by soil microbes that require water and warm soils.

Other weed management considerations

Weed ecologists tend to think about each growing season and weed species as a lock and key. It is the combination of moisture and temperature conditions over time that ensures that a particular year is a ‘good’ waterhemp year or a ‘good’ giant ragweed year. One could therefore refer to the 2022 growing season as a ‘good’ thistle year, as Dr. Peters has received more questions this spring about managing thistles than in the previous 10 years combined.

Join us Wednesday mornings for Field Notes!  

To watch the live webinar from 8 to 8:30 am each Wednesday morning and ask your questions directly of presenters, get your season-long passport to Strategic Farming: Field Notes by registering once.

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

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