The Research Story, Part 2: An Unexpected Discovery

Figure 3: Javier collecting kochia. (A. Wingeyer, University of Nebraska-Lincoln)

Researchers at the University of Nebraska-Lincoln wanted to identify what species were at the greatest risk of developing resistance to pesticides, including insecticides, fungicides, and herbicides, before they were available for use by the public.  The goal of the project was to help develop management practices that would reduce the chance of resistance evolving.  Dr. Mark Bernards and Javier Crespo developed a research project to assess the risk of resistance in some important weeds.  Using an expert survey tool, they identified a short-list of four important weed species in Nebraska that were predicted to be at the greatest risk of evolving resistance, including kochia, waterhemp, Palmer amaranth, and horseweed (Marestail).  The second step was to understand the difference in responses to dicamba across multiple weed populations collected from across the state.  The hypothesis for this research was: 

“All populations of a given species will respond the same to a given dose of dicamba.”

If all populations responded similarly to dicamba, then Javier and Dr. Bernards’ expectation was that the genetics controlling the weed’s response to dicamba would be similar in all populations.  If the response to the dicamba dose varied across populations, then that perhaps suggested that there was genetic variability that could result in the evolution of resistant populations.  For this reading, let’s focus on kochia.

Figure 4: Kochia plant. (L. Sandall, University of Nebraska-Lincoln)

Kochia is common in corn, soybean and wheat fields in western Nebraska where the environment is drier, but can be found across the entire state. In Nebraska, many kochia populations are resistant to PS-II inhibitors and to ALS-inhibitors (2018 Guide to Weed Management in Nebraska). In addition kochia lines have been bred in the greenhouse that have elevated resistance to dicamba (Preston 2009).  So the question was, would naturally occurring kochia populations all be susceptible or will some populations contain individuals that survived dicamba applications?

Here is the research method Javier used:

  1. Travel the state and collect at least one kochia population from each county where he could find it while following a predetermined route.
  2. Plant seed and grow plants in the greenhouse.
  3. Spray plants from all populations with dicamba at a single rate similar to that used by farmers.
  4. Select the two most susceptible and two least susceptible kochia populations; plant seed and grow in the greenhouse.
  5. Spray these four selected populations with a range of dicamba rates.
  6. Collect data that documents the immediate plant response to dicamba (visual injury) and data that documents the plants eventual response (seed production and plant dry weight).
  7. Harvest seed from any plants that survived the treatments.

Figure 5: Location of sampled points and counties where kochia populations were collected across Nebraska in 2009 (red triangles) and 2010 (green stars), and four kochia populations (blue squares) chosen in the single-dose screening experiment. (R. J. Crespo, University of Nebraska-Lincoln)



Why would Javier take the time to gather seed from different parts of the state?

Looks Good! Correct: Populations of organisms can be genetically different, especially when environmental factors (i.e., insects, herbicides, etc.) are placing pressures on a particular population.