Issue No. 18, Article 2/August 5, 2011
Resistance to Bt Corn by Field Populations of Western Corn Rootworms Confirmed in Iowa
An Iowa State University team led by Aaron J. Gassmann has reported the development of resistance by a field population of western corn rootworms in northeastern Iowa. The journal article, "Field-Evolved Resistance to Bt Maize by Western Corn Rootworm," was published July 29 in PLoS ONE (www.plosone.org; Vol. 6, Issue 7, pp 1-7). Decreased susceptibility to the Cry3Bb1 protein was reported for progeny whose parents had been collected in cornfields that had significant larval injury.
Producer interviews confirmed that Bt hybrids expressing the Cry3Bb1 protein had been used for 3 consecutive years in fields with excessive root pruning, the same fields where western corn rootworm adults were collected. Progeny obtained from corn rootworm adults collected in these problem fields remained susceptible to the Cry34/35Ab1 protein, suggesting an absence of cross-resistance. The authors offered this statement on the significance of their findings: "This is the first report of field-evolved resistance to a Bt toxin by the western corn rootworm and by any species of Coleoptera. Insufficient planting of refuges and non-recessive inheritance of resistance may have contributed to resistance. These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary."
I commend these authors for this important scientific contribution to the literature. I also agree with them about the need for a more integrated approach to managing insect pests of corn. Our current approach is insurance-based, fueled by high commodity prices. The results of the western corn rootworm survey described in my other article attest to the fact that we are applying enormous selection pressure to this insect species. The pressure comes in multiple forms: increasing use of Bt hybrids, neonicotinoid insecticidal seed treatments, and broadcast treatments to corn and soybean fields of pyrethroid insecticides that are frequently tank-mixed with fungicides.
Let's hope that cross-resistance does not surface among the various Cry proteins targeted against corn rootworms. If it does, the longevity of the pyramided Bt hybrids may be compromised. Let's also not forget that crop rotation no longer affords us a pest management alternative for the western corn rootworm in many areas of the north-central region.--Mike Gray