Shift in corn susceptibility to rootworms in Nebraska
2013 Field Trials: Key Results
Relative level of root protection (Cry3Bb1 root rating/near-isoline root rating) was significantly lower at Cry3Bb1 problem sites (average of 45.9% control) than at the control site (92.8% control).
The relative level of control (mCry3A root rating/near isoline root rating) followed a similar pattern as described for Cry3Bb1. The relative level of root protection provided by mCry3A was significantly lower (average of 44.9% control) at Cry3Bb1 problem sites than at the control site (86.3% control).
The relative root protection obtained with Cry34/35Ab1was not significantly different across all four sites (three problem sites and the control site; relative rootworm control averaged 90.6% across sites). The greatest root protection was obtained with single trait Cry34/35 or a pyramid of Cry34/35 with either Cry3bb1 or mCRY3a.
The at-plant soil insecticide provided a significant root protection benefit only when applied to non-Bt near-isoline hybrids or single trait hybrids exhibiting greater than expected relative injury. Little root protection benefit was obtained by adding soil insecticide to single trait Cry34/35 hybrids or pyramids containing Cry34/35.
Bioassay results demonstrate that there are heritable differences in susceptibility of some Nebraska western corn rootworm populations to rootworm Bt proteins.
Bioassay and field trial data jointly support the conclusion that a level(s) of resistance to Cry3Bb1 has evolved in some populations after repeated use of single trait hybrids over time. This has reduced the effectiveness of Cry3Bb1 in the field.
Data suggest that a possible cross-resistance relationship exists between Cry3Bb1 and mCry3A which reduced the effectiveness of mCry3A hybrids in Cry3Bb1 history/problem fields. However, there was no apparent cross resistance relationship between Cry3Bb1 or mCry3A- and Cry34/35-expressing corn.
An at-plant soil insecticide provided a significant root protection benefit only when applied to non-Bt near-isoline hybrids or single trait hybrids exhibiting greater than expected relative injury.
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