Janet J. Knodel, Extension Entomologist, NDSU Department of Plant Pathology
Joel Ransom, Extension Agronomist, NDSU Department of Plant Sciences
Mark Boetel, Research and Extension Entomologist, NDSU Department of Entomology

Northern corn rootworm (NCR) and western corn rootworm (WCR) are major insect pests of corn in the Midwest. Corn rootworm larvae damage plants by feeding on roots, which results in plant lodging and reduced yields. Corn producers have adopted the strategy of planting rootworm Bt hybrids to manage corn rootworm. Rootworm Bt-corn hybrids contain Bt proteins that are toxic specifically to corn rootworm larvae. Recently, researchers in several Midwestern states have documented resistance to rootworm Bt-corn hybrids in WCR populations. In the summers of 2012 and 2013, we observed unusually high numbers of WCR and increased corn lodging in Cass and Richland Counties in fields with a history of continuous corn. The goal of this research was to determine if current rootworm Bt traits control corn rootworms in problem fields in southeastern North Dakota, and to determine if resistance to rootworm Bt traits has evolved.

Objectives:

  • To evaluate Bt-traits for control of CRW in corn hybrids,
  • To determine if Cry3Bb1 resistance exists in CRW populations in North Dakota.

Findings

Objective 1:  Field experiments were established at three locations in southeastern ND for testing the performance of rootworm-resistant Bt-corn. Locations included:  Arthur, Hope and Page in Cass County. A split-plot design was used to evaluate the following treatments in two main plot environments (i.e., with and without a planting-time application of Force 3G soil insecticide at 5 oz / 1000 row ft):

In 2014, we evaluated rootworm Bt-corn hybrids with different Bt traits:  a Cry3Bb1 hybrid, a Cry34/35Ab1 hybrid, and a ‘pyramid’ hybrid that contains both Cry proteins. Comparisons were made to a non-Bt-corn hybrid with and without Poncho 1250 insecticide seed treatment. Additionally, the use of Force 3G soil insecticide applied in-furrow was evaluated across all hybrids. Emergence cages were placed in experimental plots to capture emerging adults from the various treatments. Root feeding injury and yield were measured and evaluated.

In 2014, we observed severely reduced corn rootworm pressure at all field sites due to the cold open winter, which likely caused high mortality of overwintering eggs. Under light rootworm feeding pressure we found that:

  • Rootworm Bt-corn hybrids had less root injury than the non-Bt corn hybrids.
  • There were no differences for root injury among the rootworm Bt-corn hybrids.
  • There were no differences for root injury in the non-Bt corn hybrids regardless of whether Poncho 1250 was used.

Objective 2:  The second part of this experiment is to determine if Bt resistance exists in corn rootworm populations in North Dakota. Thousands of corn rootworms were collected from corn fields in southeastern North Dakota in 2014. Research is still underway assessing whether NCR and WCR have developed resistance to any of these Bt traits used in corn hybrids. Results should be available after the greenhouse experiments are completed in the summer of 2015.

This funding also supported training of two graduate students in the fields of Plant Sciences and Entomology. Ms. Podliska successfully completed her M.S. Degree in the Spring 2015. Ms. Calles-Torrez plans to complete her Ph.D. in the Winter 2016.