The Rise of the Triple-Stacks: Staying a Step Ahead of Resistance Development in Transgenic Crops

Transgenic corn hybrids are rapidly becoming dominant players in Indiana cropping systems. This shift is the product of several forces, including the convenience of “in-plant protection” that allow producers to avoid handling insecticides for treating key insect pests such as the western corn rootworm. Although this shift is welcome in many ways - and has resulted in increases in profitability and ease of on-farm pest management - we are attempting to look forward and anticipate some of the problems with this shift to a new technology. Specifically, we are interested in quantifying the risks of resistance before it occurs. The western corn rootworm is tremendously abundant because it is a highly adaptable insect, and there are many crucial unknowns about how it responds to the new Bt world that is now forced to navigate.
           
It is important to understand that, unlike granular and liquid insecticides, the active ingredient in Bt corn is not acutely toxic. That is, between 5-10% of larvae exposed to the toxin may survive to adulthood. With this in mind, EPA has mandated a refuge strategy that involves planting Bt and refuge corn in an 80/20 ratio. There are several ways to plant this refuge – most commonly the refuge is planted as a solid block at one end of the field, or often by “splitting the planter” – creating strips of refuge corn as the planter moves across the field.

Data generated in 2006 and 2007 show that these refuge strategies may not function equally well as refuges. Although adult beetles move long distances, the refuge mandate is best served by minimizing the distance that refuge beetles must travel to mate with their counterparts in the transgenic areas of the field. All things considered, the strip refuge is the best available option for refuge planting.

However, with a move toward increasing the numbers of insecticidal traits in a single plant (i.e. SmartStax technology, expected in 2009), there may soon be a move to alternative refuge strategies. To determine some of the potential pitfalls of future refuge designs, we developed information regarding some experimental (currently not EPA-approved) refuge designs, including comparing an 80/20 refuge with a 90/10 refuge, and a mixed-seed or “refuge-in-a-bag” approach to the refuge problem. In 2007, we planted small plots with the following treatments:

1) 80/20 with a refuge of non-Bt Poncho 1250 seed treated in a block;
2) 80/20 with a refuge of non-Bt Poncho 1250 planted in a strip;
3) 90/10 with a refuge of non-Bt Poncho 1250 planted in a block;
4) 90/10 with a refuge of non-Bt Poncho 1250 planted in a strip;
5) 80/20 with a  refuge of non-Bt Poncho 1250 in a seed mix;
6) 90/10 with a refuge of non-Bt Poncho 1250 in a seed mix; and
7) a full refuge (non-Bt P1250 corn)

In each plot we measured the following parameters: insect emergence date, root damage and plot yield. Insect emergence was suppressed in all non-seed mix Bt plots. The seed mix had the effect of synchronizing emergence between the Bt and refuge areas. Root damage effects were similar – Bt roots in the seed mix that were adjacent to the refuge plants suffered significantly more damage than plants within a continuous Bt refuge. Finally, yield appeared to suffer (approx. 5% lower) in the seed mix plots – possibly as a result of the heightened root damage we observed.

            A final, and perhaps unforeseen, component of the rise of the triple-stacks has been the emergence of corn as a weed in soybean fields. With most current Bt-hybrids also containing glyphosate-tolerance genes, there is potential for growers rotating corn with soybean to experience high levels of volunteer corn that is not susceptible to glyphosate.

This is an obvious problem for weed management, but may also present challenges for longer-term resistance management of rootworm beetles. During 2007, we conducted on-farm surveys of volunteer corn abundance at 8 farms throughout north-central Indiana, where rootworm pressure and corn-soybean rotations are both common. We identified several fields planted with “triple-stack” (VT3) corn in 2006, and rotated to soybeans in 2007. Volunteer corn plants were removed from these fields in June and July of 2007, and taken to a laboratory for root rating and assessment of prevalence of glyphosate-tolerance and/or Bt genes. After assessment of over 700 plants from these 8 fields, 87% of the volunteer corn plants were found to possess the glyphosate-tolerance gene, meaning they would likely require supplemental herbicide application to control. Moreover, 64% of these volunteer corn plants possessed the insecticidal Bt gene for rootworm, albeit at lower than typical levels: Many of these volunteer Bt plants exhibited moderate to high levels of root damage due to corn rootworm feeding. This is a concern, as it indicates a sub-lethal dose of Bt toxin to these larvae. Sub-lethal levels of any insecticide can promote the development of resistance in target insects, and the long-term implications of this information for resistance management in western corn rootworms in the Bt-corn system will be discussed.

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Christian KrupkeAssistant Professor of Entomology
Purdue University
ckrupke@purdue.edu


Dr. Christian Krupke is an Assistant Professor of Entomology at Purdue University, with a primary responsibility in field crops extension. Dr. Krupke received his B.S. from the University of Guelph, his M.S from Simon Fraser University and his Ph.D. from Washington State University in 2004. His research program at Purdue focuses on the management of key pests of field crops, primarily corn and soybeans. The broad philosophy of the program is develop studies of insect behavior in agronomic systems to pinpoint areas where existing pest management strategies (including monitoring and control) could either be improved, or perhaps replaced altogether with novel strategies.