Study Confirms Resistance of Western Corn Rootworm Field Populations Bt CRW Corn in Northeastern Iowa

Study Confirms Resistance of Western Corn Rootworm Field Populations Bt CRW Corn in Northeastern Iowa

Eileen Cullen, Extension Entomologist

An Iowa State University entomologist research group led by Aaron J. Gassmann reported development of resistance by field populations of western corn rootworm in northeastern Iowa to Bt CRW corn expressing the Cry3Bb1 rootworm trait. The journal article, “Field-Evolved Resistance to Bt Maize by Western Corn Rootworm” published July 29 in PLoS ONE (; Vol. 6, Issue 7, pp. 1-7) is available for open-access download.

This is the first confirmation of field-evolved resistance to a Bt corn crop by western corn rootworm, or any beetle (Coleoptera) species. The researchers collected western CRW adults from four corn fields in NE Iowa where growers had reported significant larval feeding injury on Bt CRW corn expressing the Cry3Bb1 protein, as well as from five non-problem control fields where Bt CRW root injury was not a problem. They reared larvae from these “problem” and “control” field collected adult populations and conducted greenhouse feeding assays on Bt CRW (Cry3Bb1), Bt CRW (Cry34/35Ab1), and respective non-Bt isoline corn hybrids. Results confirmed decreased susceptibility to Cry3Bb1 among larvae from the problem fields.

Resistance was not detected on Bt CRW Cry34/35Ab1 corn, indicating a lack of cross-resistance. The mCry3A CRW trait was not tested in this study. The Cry3b1 rootworm trait is pyramided with Cry34/35Ab1 rootworm trait in some Bt corn hybrids. For a fact sheet guide to the increasingly complex array of Bt corn products, Bt Cry proteins, target pests and IRM refuge requirements click here.

The IA study includes information from grower interviews about the problem and non-problem control corn fields. The four problem fields had been planted to Cry3Bb1 Bt CRW corn for at least three consecutive years. By contrast, only one of the five non-problem control fields in the study had Cry 3Bb1 corn for any consecutive seasons. The non-problem control fields were planted to a greater diversity of crops in the rotation, and employed an array of management practices to control corn rootworm.

How will this play out over the landscape level in the Corn Belt? Spread of western CRW resistance to Cry3Bb1 will depend on gene flow between insects in the western CRW population, fitness trade-offs in the insect to increased survival on Cry3Bb1, and selection intensity. Western CRW have limited dispersal distance compared to Lepidopteran pests, so we’d expect at first a localized pattern to the western CRW resistance to Cry3Bb1 where it can intensify and persist. Overall at the landscape level, some populations of western CRW may remain susceptible to Cry3Bb1.

The ISU authors of this study attribute western corn rootworm resistance in this example to insufficient planting of refuges, and non-recessive inheritance of resistance by western CRW. They also highlight the need for a more integrated approach to the use of Bt crops.

This study serves as an prominent reminder of Insect Resistance Management (IRM) and Refuge requirements for Bt corn, and the long-term benefits to incorporating an array of corn insect pest management tactics (IPM) as appropriate to farm location (e.g. field pest scouting records, econmic thresholds, pest pressure, crop rotation, soil or seed applied insecticides, and Bt crops).

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