Western Corn Rootworm Densities Far Below Average in Illinois Corn and Soybean Fields

As part of a USDA National Institute for Food and Agriculture (NIFA) sponsored grant, surveys of insect pests were conducted in randomly selected corn and soybean fields across 28 counties throughout Illinois this summer. Fields were sampled during two discrete periods: July 27 – July 31 and August 12 – August 24. Within each county, five cornfields and five soybean fields were sampled. Within a cornfield (away from the field edge and end rows), 20 consecutive plants were examined carefully for northern and western corn rootworm adults. In soybean fields, 50 sweeps of a sweep net were made in the field interior and another 50 sweeps were taken at the edge of a field (between the last two border rows). Within each cornfield, one leaf was obtained from one of the 20 corn plants examined for rootworm beetles. Each leaf was tested for the presence of Cry (Bt) proteins that are expressed to limit corn rootworm larval injury to root systems. Provided are the key findings.

  • After examining 5,600 corn plants, only 62 beetles were found. These findings include both sampling periods. The greatest density observed was in Whiteside County where an average of 0.1 beetles per plant was found during the second sampling period. This per plant average is far below the economic threshold of 0.75 to 1.0 beetle per plant commonly used to help make corn rootworm management decisions for the following season if a producer elects to plant corn in that same field.
  • After making 14,000 sweeps in the border rows of soybean fields, no brown marmorated, redshouldered, or redbanded stink bugs were detected. These are species of stink bugs that are likely to increase their numbers in years to come. Fortunately, we have good news to report for 2015.
  • Remarkably, only 5 western corn rootworm beetles were found in the interior of soybean fields after making 14,000 sweeps!
  • Across four sampling regions of Illinois (northwest, northeast, west central, and east central), 81% of the leaf samples tested positive for the expression of at least one Cry (Bt) protein targeted at corn rootworms. Of these positive leaf samples, 75% expressed more than one Cry (Bt) protein for corn rootworms and are often referred to as pyramided hybrids. In southeastern and southwestern Illinois, only 25% of leaf samples tested positive for the expression of a corn rootworm Cry (Bt) protein.

Why are western corn rootworm densities so low?

  • There are several factors that can help explain these low densities such as the heavy spring rains that led to saturated soils at the time of larval hatch (late May and early June) in many fields. Secondly, the significant use of highly-effective pyramided Bt rootworm hybrids is taking a toll on corn rootworm larval survival as well. These two factors along with the use of planting-time soil insecticides on top of Bt hybrids is surely having an effect in the overall suppression of the corn rootworm population in Illinois.

Based on these survey data, can I safely assume corn rootworm pressure in my fields will be lower next year?

  • No.
  • Keep in mind there are no effective rescue treatments available for corn rootworms (larval feeding) in most field situations across Illinois.
  • Secondly, corn rootworm management decisions need to be made on a field-by-field basis. Unless you’ve scouted your fields this season, don’t assume that western corn rootworm densities in your field mirror those described as regional averages in this article.

One of the key take home messages from this report should be the importance of regularly scouting your fields for key insect pests, especially the western corn rootworm. Having reliable scouting information on a field-by-field basis this year could pay dividends next season as producers face input decisions for 2016 in the coming months.

I offer my thanks to Nick Tinsley (Postdoctoral Research Associate, Department of Crop Sciences) and Alex Kaluf (Graduate Research Assistant, Department of Crop Sciences) for their leadership in the planning and conduct of this statewide survey. I also thank Luke Harvey and Tate Estes (Summer Interns, Department of Crop Sciences) for their hard work in sampling fields.

Mike Gray, Professor and Extension Entomologist and Nick Tinsley, Postdoctoral Research Associate