John Tooker, Ph.D.
Assistant Professor and Extension Specialist
Penn State Extension
Department of Entomology
Penn State University
Varieties of corn that have been genetically modified to be resistant to some caterpillar or beetle species are valuable assets for insect pest management. Many growers across the United States depend on these ‘Bt corn’ varieties to control serious pests, particularly European corn borer and western corn rootworm, which can be quite challenging to control with typical insecticide applications. It is vital to realize, however, that continued utility of Bt hybrids depends on proper stewardship of the technology. This point has been driven home recently with the announcement that researchers at Iowa State University have detected field-evolved resistance by western corn rootworm to the Cry3Bb1 toxin expressed in some transgenic varieties. The study is publicly available online (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022629) and was lead by Dr. Aaron Gassmann, who collected adult western corn rootworms from damaged fields that had been under continuous corn production. Dr. Gassmann and his team found that the larvae of the field-collected beetles were able to survive far better on corn hybrids expressing the Cry3Bb1 protein than larvae from control beetles.
While some of the circumstances leading to this evolution of resistance remain to be determined, it appears that all the fields involved were in continuous corn production for at least four years and had used the same Bt toxin each of the years. Specifically, most of the fields had been continuously planted with YieldGard rootworm varieties that expressed the Cry3Bb1 toxin. Additionally, it seems that a lack of refuge acreage may have contributed to the problem. Refuge acreage is required of growers who are using Bt hybrids and is vital to maintaining populations of beetles that are susceptible to Bt toxins; these susceptible beetles would then be available to mate with any resistant individuals emerging from Bt fields, delaying the evolution of resistance. Importantly, while Dr. Gassmann’s work was conducted in Iowa, the fields he studied are not isolated incidents; indeed, similar apparent resistance has been discovered in nearby states, including parts of Illinois, South Dakota, Minnesota, and Nebraska.
So what is the relevance of this issue for Pennsylvania? Thus far, we have not heard any reports of failures of Bt hybrids in the Commonwealth, and this, of course, is good and will allow us to learn from the Midwestern experiences. In Pennsylvania, corn rootworms do not pose the same threat that they do in the Midwest. Here, many growers routinely rotate corn with soybeans and other crops, and this rotation prevents establishment of corn rootworm population and renders unnecessary transgenic varieties of corn targeting rootworms. Nevertheless, like the Midwestern fields, some Pennsylvania farmers have continuous corn acreage where rootworm-active Bt varieties may be appropriate, and I urge these growers to use the documentation of Bt resistance in the Midwest as a caution signal and re-consider their rootworm management strategy for the next field season. First, consider rotating fields that have been in corn for more than a few years. Second, if you cannot rotate, and want to continue using rootworm-active Bt hybrids, ensure that you fully comply with refuge requirements. Planting of refuges is required to help prevent the evolution of resistance and not complying simply threatens the viability of the technology. Third, consider switching to a Bt hybrid expressing a different corn rootworm-active protein than the one you have been using, including possibly stacked hybrids with more than more protein active against rootworms. This will provide a different mode of action for the field and delay of the possibility of resistance evolution. Finally, consider applying a soil insecticide at planting, the way corn rootworms were controlled prior to the introduction of rootworm Bt events. Any of these approaches will help break the path toward resistance development. The bottom line is that relying on one tactic for too long is a prescription for evolution resistance—change up your approach to stay one step ahead of this important pest.