During the last decade, corn hybrids that express Bt toxins have been widely adopted by growers as a primary tactic to control corn rootworms. During 2011-2013, greater than expected root injury by western corn rootworm larvae to hybrids expressing the Cry3Bb1 protein has been reported in some Nebraska fields. In Cry3Bb1 problem fields severe root pruning was present in parts of each field and was often accompanied by plant lodging. Before 2013, most reports were in northeast and southwest Nebraska. In 2013, unexpected injury was also observed in central Nebraska.
A consistent pattern was observed at all Cry3Bb1 problem sites: All sites had been in continuous corn production and hybrids expressing the Cry3Bb1 toxin had been planted for multiple years (often three to six consecutive years).
In 2012-2013 UNL conducted lab and field research to determine if shifts in rootworm susceptibility to Cry3Bb1 had occurred at some Nebraska locations.
Western corn rootworm beetle collections were made at Cry3Bb1 problem sites in northeast and southwest Nebraska during 2012 (six populations) and larval offspring from each collection were screened during 2013 for susceptibility to Cry3Bb1, mCry3A, and Cry34/35 proteins. (See the Handy Bt Trait Table for companies and trait names that express Cry proteins.) First instar offspring from fields that had not experienced unexpected injury to any Bt protein (three populations) or lab colonies that had not been previously exposed to Bt toxins (six populations) were used as control populations. For each Bt protein, survival of problem field and control populations were compared on a hybrid that expressed the Bt protein (i.e., Cry3Bb1) and a near isoline hybrid without the Bt protein (i.e., no Cry3Bb1).
Lab Bioassays: Results
Trends from lab bioassays were fairly consistent across the six Cry3Bb1 problem sites. Survival (corrected for survival on the near isoline hybrid) on Cry3Bb1-expressing plants ranged from 61% to 90% for problem field populations and 0% to 14% for control populations. A similar trend was observed when larvae were reared on mCry3A and near-isoline plants. Survival on mCry3A-expressing plants ranged from 59% to 100% for problem field populations and 4% to 42% for control populations. In contrast, a different pattern was observed with Cry34/35-expressing plants as survival ranged from 14%-37% for problem field populations and 0%-34% for control populations.
Field trials were conducted on three Cry3Bb1 problem sites (Clay, Perkins, Keith counties) and a control site (Saunders County, no control problems with any Bt event) during 2013 to evaluate the efficacy of single or pyramided Bt traits with and without an at-plant soil insecticide application (see Table 1 for treatments). The 0-3 node injury scale was used to evaluate root injury in each treatment. Even though rootworm pressure was variable across sites, a similar trend was apparent at each Cry3Bb1 problem site when efficacy of treatments was placed on a relative control basis (i.e., treatment root rating/appropriate near-isoline root rating was compared between Cry3Bb1 problem sites and the control site). Trends observed in field trials were similar to those observed in lab bioassays.
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.