Nearly 20 Years of Bt Hybrids: What Have We Learned and Where Do We Go from Here?.

In 1996, a seismic event took place in the evolution of agriculture – transgenic corn hybrids were commercialized in the United States. These so-called “Bt” hybrids expressed a high dose of a crystalline (Cry1Ab) protein derived from a ubiquitous soilborne bacterium Bacillus thuringiensis (Bt). The primary target for this new generation of hybrids was the European corn borer, Ostrinia nubilalis (Hübner). To prolong the usefulness of the technology, producers were required to implement a 20% structured refuge that was planted to a non-Bt hybrid. This high dose/refuge strategy has proven very effective – to date, the evolution of field resistance by European corn borers has not occurred. In 2003, Bt hybrids entered the marketplace that offered root protection against the western corn rootworm, Diabrotica virgifera virgifera LeConte. Unlike Bt hybrids used to control the European corn borer, these transgenic hybrids expressed the Cry3Bb1 protein at lower doses. Regardless of the differences in Cry protein expression levels (low dose vs. high dose), insect biology, mating behavior, and dispersal characteristics, the same structured refuge (20%) requirement was established for Bt hybrids aimed at both of these major insect pests of corn. Perhaps not surprisingly, 6 years following commercialization, western corn rootworm resistance to the Cry3Bb1 protein was confirmed (Gassmann et al. 2011 PLoS ONE) by conducting plant-based bioassays on the offspring of adults collected from severely damaged Bt cornfields (2009 and 2010) in northeastern Iowa. Looking ahead – transgenic hybrids will remain the foundation for insect management in the Corn Belt; however, the use of planting-time soil insecticides has escalated due to mounting concerns regarding corn rootworm resistance to Bt hybrids. Additionally, the widespread use of neonicotinoid insecticidal seed treatments (corn and soybeans) is receiving increased scrutiny due to environmental concerns. By the current decade’s end, the use of transgenic hybrids that pyramid various Cry proteins with the RNAi technology will likely become more common place to achieve insect control. Even this approach will need to become part of an integrated pest management (IPM) program to endure.