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Strategic Farming: Field Notes resistance management episode - When pests bite back

 Angie Peltier, Extension educator - crops

Waterhemp. Photo: Liz Stahl
On the August 10, 2022 Strategic Farming: Field Notes episode, Bruce Potter, UMN Extension Integrated Pest Management Specialist and Dr. Tom Peters, UMN/NDSU Extension Sugarbeet Agronomist joined moderators and Extension educators Liz Stahl and Claire LaCanne for the “Pests bite back” edition to discuss how best to manage pesticide resistant weeds and insects and keep currently effective pesticides effective.

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

Weed management

Audience asked about their weeds’ herbicide resistance

Audience members were polled before the discussion began about whether they think that they have weed resistance to any of the six most commonly used herbicide groups (audience members that responded could select all that applied): 
  • 27% didn’t think that they have resistant weeds on their farm. 
  • 68% believe they have weeds resistant to glyphosate (Group 9, ex: Roundup PowerMax3), 
  • 27% to ALS inhibitors (Group 2, ex: Classic), 
  • 18% to PPO-inhibitors (Group 14, ex: Cobra),
  • 9% to glufosinate (Group 10, ex: Liberty) ,
  • 5% to atrazine (Group 5, ex: AAtrex). 
It is likely some weeds maybe resultant to multiple herbicides, for example, group 9 and group 2 herbicides.

Calling for suspected herbicide-resistant weed seed submissions.

To the best of our knowledge, glufosinate-resistant weed populations have not been found in Minnesota. Dr. Peters asked members of the audience to submit suspected herbicide-resistant waterhemp or common ragweed seed samples to UMN Extension weed scientist Dr. Debalin Sarangi, for additional study and potential lab confirmation. Follow these instructions for collecting and submitting a weed seed sample to Dr. Sarangi’s lab.

Preserving currently effective herbicides.

Thoughtful stewardship of our current herbicides is essential to maintain the efficacy of herbicides. For example, glufosinate is quite effective at managing waterhemp, one of Minnesota’s most difficult to manage weeds. But for this active ingredient to be effective long-term, care must be taken to follow IPM best management practices. For example, the full label rate of glufosinate should be applied to weeds shorter than 4 inches. Reserving glufosinate application only for the crop in the rotation in which waterhemp management is most difficult and combining it with tactics such as planting into a ‘clean’ field and layering soil-applied residual herbicides will help to extend the viability of this active ingredient for effective weed management.

Do late herbicide applications limit weed seed set?

In his travels throughout the sugarbeet production region of Minnesota and North Dakota, Dr. Peters has time to look at how well (or not so well) weeds are being managed. In one soybean field that looked to have a pretty good level of weed control early on, waterhemp plants began poking through the soybean canopy in mid to late July. These two to three feet tall waterhemp plants were recently sprayed, presumably with Liberty. Applying herbicides in an attempt to limit additions to the weed seed bank is generally not a recommended practice as not only is it unlikely that the weeds will be killed, one could actually be helping to select from the weed population those individuals able to survive a sub-lethal herbicide application.

How to manage weeds in mid-August

At this point in the growing season, we need to put our sprayers away and turn to practices that would be more effective for managing weeds. Some folks have had success using a “WeedZapper”. These implements hit weeds with electricity, “frying” the weed and reducing the viability of its seeds. Before the advent of glyphosate-tolerant soybean varieties, “walking” fields to hand rogue weeds was a common practice. While still a not-so-fun task, hand weeding very weedy patches is still an effective practice. However, folks need to understand the weed species that are being pulled from the soil to determine whether rogued plants need to be hauled out of the field or can just stay put. For example, common ragweed and lambsquarters produce seed much later in the season than waterhemp. This means that one can leave rogued common ragweed and lambsquarters plants in the field without risking them adding their seeds to the field’s weed seed bank. Conversely, waterhemp plants can produce viable seeds in as little as 2 weeks after plants flower and so must be removed from the field to avoid having to deal with their seeds in future growing seasons.

Weed management at harvest.

If the area of the field in which weeds need to be rogued is prohibitively large, make a map delineating the weedy area so that weeds can be better targeted in subsequent growing years. Combine around very weedy patches during this year’s harvest, and if weed pressure didn’t completely decimate yields, combine weedy areas and fields last to avoid spreading weed seeds within the field. In many instances, when yields are so low due to weed pressure, harvesting these areas may not be worth the risks associated with the combine spreading weed seeds or ending up with your crop getting docked at the elevator for having too much foreign material. Be sure to thoroughly clean your combine of all debris before storing it away.

Avoid tank mix antagonism when controlling volunteer corn.

When the Field Notes audience was asked what they thought was Minnesota’s most common glyphosate-resistant weed, 77% answered waterhemp, 27% volunteer corn, and 9% thought either common ragweed or kochia. Glyphosate-tolerant (GT) corn makes up the bulk of the volunteer corn on Minnesota farms and so one must seek options other than glyphosate for managing it. The Field Notes audience was cautioned about the antagonism that can occur when mixing a group 1 herbicide (ex. Select Max) with a Group 4 herbicide (ex. XtendiMax, Enlist), making volunteer corn and control of other grasses less effective.

Insect management

The state of the state.

So far this year, spider mite injury has been spotty, relegated to droughty areas of the state, but well-timed rain showers, dew and cooler, cloudy weather can reduce pressure. Soybean aphid infestations have been spotty in 2022, first appearing in river bottoms and along woodlots where their winter host resides. While aphid populations remain low in many areas, over the past week economic threshold-level populations have been observed in an increasing number of southern and western Minnesota fields. As soybean vegetative growth slows and ceases, large numbers of soybean aphids move within and between fields. At this point in the growing season it is very important for folks to be scouting whole plants, particularly for smaller and harder to spot aphids on the lower leaves of the plant or risk underestimating population densities.

What’s the audience’s view on soybean aphid thresholds?

Audience members were polled about how they make the decision to spray for soybean aphid:
  • 59% indicated that they follow the research-derived economic threshold (of building populations of at least 250 aphids per plant on 80% of plants),
  • 14% believe that this economic threshold is too high, 
  • 10% tank mix an insecticide when they apply a fungicide, 
  • 3% spray when custom applicators are spray, 
  • 3% when neighbors are spraying, 
  • 7% spray when their crop consultant tells them to 
  • 7% don’t spray.
Bruce Potter assured the audience that even though crop prices are higher than in the recent past and folks are worried about losing yield, soybean aphids have no idea how high crop prices are and won’t eat any more when crop prices are high. The 250 aphid per plant threshold is based on many years of trials across the Midwestern states (including Minnesota) and is relatively conservative. When population densities reach 250 aphids per plant, they haven’t yet caused feeding injury that will result in yield losses, rather the threshold was designed with a built in 7 day window to get an insecticide on before such yield loss can occur. According to Potter, "the 250 aphid/plant economic threshold should be used through the R5 stage. Aphids have less time to reduce yield as the growing season starts to wind down and yield loss from economic threshold populations becomes less certain after the R4 stage.”

Caution urged when tank mixing insecticide and fungicide

While it is understandable that folks would like to save a trip across fields by tank mixing an insecticide with their fungicide. Unfortunately, while a fungicide application may be warranted at the beginning flowering or beginning pod growth stages, an insecticide application is likely to kill many of the soybean aphid’s natural enemies that suppress soybean aphid populations. This may result in threshold-level aphid populations developing where they might not have. Unwarranted fungicide applications can also negatively affect entomopathogenic fungi that help to suppress aphid and mite populations.

Keeping effective tools in the toolbox

We also do not have an unlimited supply of insecticide active ingredients at our disposal. Several years ago when pyrethroid-tolerant soybean aphid populations were first observed, many Minnesotans learned the “hard way” that soybean aphid is capable of evolving to overcome our management strategies. Reserving our insecticides for spraying threshold-level soybean aphid population densities is one way to slow the evolution of soybean aphid populations tolerant to other insecticide groups. Newer, more selective insecticides (ex. Sivanto, Transform, Sefina) are on the market for treating them, but if folks still want to spray a pyrethroid insecticide, a tank mix with a neonicotinoid or one of the newer products is recommended as pyrethroid-tolerance is likely to be a fixed trait still present in Minnesota soybean aphids.

Potter stresses the importance of season-long scouting for crop pests

Scouting for Bt-resistant corn rootworm populations is an excellent reason to scout corn this year. One can both see how well their Bt trait is/isn’t holding up and the population densities that corn crops in 2023 may encounter.

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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