Studies were conducted across nine U.S. states, over 5 yr, to characterize the efficacy of transgenic corn (Zea mays L.) hybrids producing insecticidal proteins derived from Bacillus thuringiensis (Bt) for control of western corn rootworm, Diabrotica virgifera virgifera Le Conte. Hybrids tested had the same genetic background, contained one of two single events (DAS-59122-7 expressing Cry34Ab1/Cry35Ab1 or MON 88017 expressing Cry3Bb1) or a pyramid consisting of both rootworm-active events (SmartStax traits) and were compared with a non-Bt near isoline. Frequency analyses of root feeding data showed that hybrids containing both events sustained less root damage (0–3 node injury scale) than hybrids containing either event alone. The levels of root protection provided by MON 88017 and DAS-59122-7 were not different from each other. Efficacy was also evaluated based on consistency of protection, based on the proportion of plants with root ratings of either ≤0.25 or <1.00 on the node injury scale. The combination of two modes of action in SmartStax provided greater product consistency over a single mode of action at the 0.25 level and all hybrids producing Bt proteins provided equally high consistency at the 1.00 level. Overall these data show single and multiple mode of action hybrids provided high, consistent protection over the past 5 yr across the trial geography; however, pyramiding the rootworm Bt events provided greater and more consistent root protection. These findings also support that pyramided traits like SmartStax (Cry3Bb1 Cry34Ab1/Cry35Ab1) remain a viable strategy for delaying resistance to either trait.
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Vol. 106 • No. 2