What’s the deal with pest resistance?
Pests – insects, disease and weeds -- are a regular part of life. For large areas, our pest removal practices have included spraying safe pesticides or relying on plant genetics to manage them. But many pests can reproduce quickly and often, allowing them to evolve and develop tolerance to some of our easiest, cheapest and most effective management tools. Some pests are completely tolerant to some pesticides. When this occurs, it is called pest resistance, because they can now resist and survive a specific management tactic that previously controlled them.
Pest resistance is a natural process. However, when it comes to something as valuable as our food and when there’s no easy way to fight a pest, it can lead to yield reductions and increasing costs. Farmers, landowners, scientists, private businesses, and groups of individuals are finding ways to preserve our management tools, prevent pest resistance, and develop management strategies for already resistant pests.
Identifying Biotypes of Glyphosate-Resistant Horseweed
Since being brought to market in 1974, glyphosate has become the most widely used herbicide in the U.S. From less than 44 million pounds used annually in agriculture in 1995 in the U.S., to over 253 million pounds in 2015, the herbicide has become a heavily relied on weed management tool. This can be explained through the advent of glyphosate-resistant row crops, which allowed growers to commit to using the herbicide on weeds without causing any damage to plants. The conventional usage of glyphosate has resulted in numerous incidences of weed resistance to the herbicide over the last decade.
In order to understand the evolutionary outcomes of weeds subjected to strong, continued selection as a result of prolonged glyphosate use in row crops, researchers from both Iowa and Ohio examined the variations of resistance in self-pollinating Conyza canadensis (horseweed). Their research was published this past summer in the Nature Scientific Reports journal. Researchers chose to focus on horseweed due to the fact that this weed has become prevalent among low-til to no-till crops. Horseweed has become a problem in fields due to the fact that in tillage cropping systems, seeds could not germinate in the low tillage depths. With the availability of glyphosate-tolerant crop breeds, low-till to no-till cropping systems could be implemented, which increased horseweed germination rates. Glyphosate was a widely adopted herbicide used to kill horseweed, which has resulted in high resistance rates. Glyphosate resistant horseweed was first reported in Ohio and Iowa in 2002 and 2010, respectively.
Researchers collected seeds from a single maternal plant from 74 biotypes from six north-central Ohio counties, and 74 biotypes from 32 southern Iowa counties, from agricultural and non-agricultural land (parks and ditches). Researchers targeted counties with large areas of soybean production and known horseweed populations, due to the the weed’s problematic rise in no-till soybean fields.
This procedure was used because maternal seed families are expected to be full-siblings due the fact that horseweed self-pollinates, virtually making copies of itself, to very high representing evolutionary units on which selection for herbicide resistance can act. These seeds were then germinated and grown, and then sprayed with various rates of glyphosate, and the rates of tolerance to the herbicide were measured.
Results from the study show that nearly all Ohio agricultural biotypes of horseweed (approximately 76 percent) were classified as R4 or extremely resistant (the study ranked tolerant varieties on a scale of 1-4, with R1 being least resistant to glyphosate exposure and R4 being extremely resistant to glyphosate exposure), as were 62 percent of biotypes from the non-agricultural sites. In Iowa, glyphosate resistance levels were more diverse among the seeds collected. R4 biotypes were clustered in south central/southeast parts of the state, where no-till agriculture is more common (R4 biotypes made up only 26 percent of plants). Approximately 45 percent of non-agricultural biotypes were between R1–R4 resistance, with the remainder being susceptible to the herbicide. These results illustrate that horseweed resistance levels to glyphosate can be very high in both states, and that even non-agricultural sites likely serve as a refuge for glyphosate-resistant biotypes.
The study concludes that there are multiple mechanisms through which weeds can be resistant to glyphosate, with those in the R1 biotype having less mechanisms of resistance than those classified as R4, which allows for horseweed to be more tolerant of the herbicide. These mechanisms for horseweed include reduced translocation and vacuolar sequestration, which would be the common injuries that lead to plant death in glyphosate. There have been other studies that show that some horseweed varieties can actually metabolize glyphosate, which is another mechanism of resistance, as well as target site resistance.
Iowa Pest Resistance Management Plan Unveiled
DES MOINES – Iowa Secretary of Agriculture Bill Northey, Iowa State University College of Agriculture and Life Sciences Associate Dean John Lawrence and Iowa farmer Larry Buss of Logan today announced the release of the Iowa Pest Resistance Management Plan. A video of the announcement is available here.
Version 1 of the IPRMP is an Iowa-specific plan that seeks to engage farmers on the issue of pest resistance management with the goal of keeping technology and tools such as pesticides, seed treatments and biotechnology products and native traits available and effective.
Research proves herbicide selection non-issue with SDS
The world’s most widely used weed killer is not responsible for perpetuating Sudden Death Syndrome (SDS) in soybeans, research shows. A collaborative effort among soybean researchers in the United States and Canada and found that glyphosate does not increase SDS severity or adversely affect yields in soybean fields. Scientists from five Midwest universities and the Ontario Ministry of Agriculture Food and Rural Affairs, led by Daren Mueller of Iowa State University (ISU) in Ames, participated in the three-year study.
2020 Virtual Field Trials Introduction: Harrison County Pest Resistance Management Project
The Harrison County Pest Resistance Management Project team created 2020 field trials to help local farmers see examples of local weed pressure and strategies for management. This video introduces the 2020 soybean trait trials, which include both Dicamba + Glyphosate trials and 2,4-d + Glufosinate + Glyphosate trials. Learn how the trials were prepared, planted and managed.