Army Cutworms in West Texas Wheat – Final results from our efficacy trial

Army Cutworms in West Texas Wheat – Final results from our efficacy trial

           

              With the outbreak of army cutworms in many of our local wheat fields for grain, we, the IPM Team on the High Plains (Dr. Pat Porter, Dr. Suhas Vyavhare, Blayne Reed, John Thobe, and Dagan Teague), took the opportunity to place an efficacy trial in a particularly heavy infestation in northwestern Swisher County.  We placed this trial in an edge of a failed field where there would be no chance of overspray but the population was more than enough to guarantee a good trial.  The repercussions of this decision would show later.  

In this research trial, we placed three labeled products which are the most likely to offer the best control for the pest.  The treatments consisted of an untreated check (UTC), two rates of Prevathon, Blackhawk, and Baythroid.  On March 12, 2020, we counted our 3 DAT (days after treatment) counts and released those results earlier.  The trial proved difficult to conduct with weather in the form of 3 inches of rain over 5 days, preventing a 7 DAT count until we reached the 10 DAT timeframe.  Then, high winds drove the larva ever deeper into the soil and tried to carry equipment and researchers away.

The nature of the pest proved difficult also.  The army cutworm stays below ground during the day and emerges at night to feed.  Our scouting became better for this sporadic pest with experience making some of our pre-treatment counts inaccurate.  The area of the field where we had placed the trial had shown in our pre-treatment and early scouting to exhibit 5.08 larva per square foot.  We learned that either the larva moved into this area and/or we had missed many of the larva by not ‘digging’ deep enough or thorough enough.  By the 3 DAT and 10 DAT counts the area around the trial and UTC exhibited about 20 army cutworms per square foot, well above the 4 larva per square foot that represents the economic threshold (ET) and more than the 5.08 we counted pre-treatment.  In addition, the larva is mobile.  There was ample evidence that the larva was moving between test plots and from outside the test plot area into our trial.  

This plainly muddied the water of our results.  Yet, we are still receiving phone calls about this pest, either from failed ‘cheap generic’ insecticide treatments and newly found economic populations threatening wheat.  The absolute results of this trial remain eminently important to this date. 

              We tried mathematically cleaning up the data by several methods to make the most sense of it.  We tried Abbott’s formula to normalize unhealthy UTC populations but it is not designed for this situation or unevenly distributed populations, which this trial had.  Then we tried the Schneider-Oreli formula that would adjust mortality of the UTC to zero, no matter how many treated worms had moved into the UTC from the treated plots or untreated worms to the treated plots.  This plainly did not take into account the number of untreated larva that moved into the plots from outside the trial area.  Then we tried the Sun-Shepard formula that normalized the treatments to the percent mortality of treated worms that moved into the UTC.  This also did not take into account the influx of larva into the trial.  

We now feel the best representation of this data is the percent average mortality of each treatment, calculated by the replications.  The reader should understand that a large contingent of untreated larva were moving into the treatments and from the treatments into the UTC, between the treatments, and even out of the trial before dying.  All of that being said, we feel very good about recommending Prevathon or Baythroid for army cutworm in wheat today.

              It is disappointing to see a treatment without what most of us would consider reaching control, or something nearing 100% mortality.  We feel many of the treatments faired far better than this data represents.  Please keep in mind healthy worms were moving into these plots from outside the treated plots and between them.  The heavy rains may have also washed many of the dead worms from plot to plot also.  It was not uncommon to find clusters of 20 or more dead worms in an area on top of the soil, but upon scouting could find just as many below the soil surface, sometimes quite a distance below. Also, these treatments were providing residual control at 10 DAT and following heavy rains and adverse conditions.  Maintaining control here would have been difficult at best and almost impossible to quantify fully with the insect movement issue.  

              The bottom line today is that we feel strongly about recommending either Prevathon or Baythriod for army cutworm control in wheat in West Texas.  At both the3 and 10 DAT counts, while counting blind (not knowing which plot we are in to ensure fairness) we could clearly tell when we were in the UTC, Prevathon, and Baythriod plots.  The number of dead worms versus live would give the plot away every time.  With treatment coverage to control entire fields, Prevathon should offer outstanding control at light rates that still offer the added benefit of saving predators for other pests and possibly other crops later this year.  With the same amount of field coverage, Baythriod, a first line pyrethroid, still offers outstanding control of the army cutworm with a touch more residual in harsh conditions such as heavy rains following treatment.

Thanks

Blayne Reed