Identification of Bt-Resistance in Corn Rootworms

Identification of Bt-Resistance in Corn Rootworms 2017-11-02T13:03:53+00:00

Janet J. Knodel, NDSU Department of Plant Pathology
Joel Ransom, NDSU Department of Plant Sciences
Mark Boetel, NDSU Department of Entomology
Veronica Calles-Torrez, NDSU Department of Entomology

Northern corn rootworm (NCR), Diabrotica barberi, and western corn rootworm (WCR), D. virgifera virgifera, are major insect pests of corn in the Midwest. Corn rootworm (CRW) larvae damage plants by feeding on roots, which results in plant lodging and reduced yields. Many corn producers have adopted the strategy of planting hybrids expressing Bt (Bacillus thuringiensis) endotoxins to manage corn rootworms. Rootworm Bt-corn hybrids express Bt proteins that are specifically toxic to corn rootworm larvae. Western corn rootworm resistance to transgenic corn expressing Bt endotoxins has been confirmed in some states; however, the status of sensitivity to this technology has not been previously investigated in NCR or WCR populations in North Dakota.

The goal of the 2016-2017 research was to determine if North Dakota populations of NCR and WCR have developed resistance to any of the Bt proteins.
Northern and western corn rootworm adults were collected from five corn fields in North Dakota during 2016. Collection of beetles were from July 24 through September 20th, 2016. A grand total of 30,372 NCR beetles were collected from the five corn fields. The WCR was found only in Ransom Co., and a total of 304 beetles were collected from this field.

Collected beetles were transported to the NDSU greenhouse. Beetles were caged by field site and reared in a growth chamber, which was set at temperature of 25°C, 65% R.H., and at photoperiod 16:10 (L:D). Beetles were fed with artificial corn diet, and agar was provided as source of water. In each cage, one to two petri dishes containing fine/aggregate soil were provided for oviposition eggs. Oviposition substrate dishes were changed once a week. When egg dishes were taken out of the rearing cage, they were left outside of cages for two weeks. After two weeks, eggs were washed and counted.
Up to 26 November 2016, a total of 110,790 eggs of northern corn rootworm were counted from the five corn fields and 51,050 eggs of western corn rootworm from Ransom County. Eggs were stored in a growth chamber set at temperature of 8°C, 65% R.H., and dark (no light). Eggs need to be stored for a minimum of five months to complete egg diapause period. After five months, eggs can be used for testing corn rootworm resistance in Bt corn hybrids in the greenhouse.

Controls for the experiment were obtained from WCR and NCR laboratory colonies at the USDA ARS laboratory in Brookings, SD that had never been exposed to any Bt endotoxins. Currently, larvae of WCR and NCR are being screened for potential resistance to Cry3Bb1, Cry34/35 and pyramided (Cry3Bb1 + Cry34/35Ab1) Bt corn endotoxins at the NDSU Research Greenhouse. Due to the length of time required to complete the rearing of corn rootworms and the Bt bioassay, we asked and received an extension on this grant until December 31, 2017.

Previous results of the 2015 bioassays indicated that the field collected populations of WCRs had significantly greater survival on Cry3Bb1 compared to the laboratory (control) colony progeny. For NCR, there were no significant differences in sensitivity to any of the Bt endotoxins between a susceptible laboratory colony and the field population of NCR; however, the bioassay used was developed for WCR. In 2016-2017, we are repeating this bioassay with modifications for NCR. Results also indicated that pyramided Bt corn had the lowest larval survival from ND field-collected populations: WCR at 16% larval survival and NCR at 10% larval survival.

Peer-reviewed papers: We submitted the following corn rootworm paper to the Journal of Economic Entomology for publication as result of this NDCUC funded project: “Transgenic Bt Corn, Soil Insecticide, and Insecticidal Seed Treatment Effects on Corn Rootworm (Coleoptera: Chrysomelidae) Beetle Emergence, Larval Feeding Injury, and Corn Yield in North Dakota.” It is in the process of being modified for re-submission.
Poster: A poster titled Effects of Transgenic Bt Corn, Soil Insecticide, and Insecticidal Seed Treatment on Timing of Diabrotica barberi Beetle Emergence in North Dakota, which was presented by Ms. Veronica Calles Torrez at the 2nd Annual Asa Fitch Entomological Symposium in Fargo, ND.

NDSU Extension Publications: A new publication E1852 Integrated Pest Management of Corn Rootworm in North Dakota was recently published during June 2017. Two older NDSU Extension publications are in the process of being updated: “Corn Insects of North Dakota Affecting the Crop after Emergence;” and “Corn Insects of ND Affecting Planting Decisions.”

Graduate Student: This funding also partially supported the research/training of two graduate students in the field of Plant Sciences (Kellie Podliska, M.S. degree confirmed in Spring 2015), and Entomology (Veronica Calles-Torrez, Ph.D. degree to be completed in Fall Semester 2017).

Summary: The preliminary results of this research suggest that, although WCR populations have declined in North Dakota in recent years, they appear to be in the process of developing resistance to CRW-specific Bt endotoxins. The 2016-2017 research will reevaluate these research findings. As such, grower adherence to insect resistance management practices in relation to Bt hybrid use for CRW management is critical to sustaining the effectiveness of this technology in North Dakota.