## 5 Resistance to Bt maize in the Agricultural Landscape  
Initial characterization of Bt resistance focused on fields with high levels of rootworm feeding injury to Cry3 maize (i.e., Cry3 Bb1 maize and mCry3A maize) [38; 39; 40; 41; 42; 43]. However, this raised the question of how common Cry3 resistance was within the broader agricultural landscape, and how cropping practices might in turn influence patterns of pest abundance and pest injury. Landscape-level patterns of Cry3 resistance were examined for western corn rootworm populations in Nebraska by [PERSON] et al. [5]. This study considered populations at a spatial scale of 2 to 10 Km, and examined two areas of intensive maize production. The authors found significant spatial variation among populations in the level of resistance to Cry3 Bb1 maize and mCry3A maize, with a few populations showing no difference in survival from the susceptible controls. This work also looked at the association of various field-history variables with the level of Bt resistance, and found that the use of a Bt trait within a field showed a positive relationship with the level of resistance, which in turn highlights the importance of diversified management in delaying resistance [5]. Studies of Cry3 resistance in the agricultural landscape in Iowa include work by [PERSON] et al. [6; 7]. [PERSON] et al. [6] tested for resistance in fields with a history of high levels of feeding injury to Cry3 maize (i.e., past problem fields) and fields that were in close proximity (within \(<\) 2.2 km) to past problem fields. This study found that both field types harbored Cry3 Bb1-resistant populations of western corn rootworm [6]. Similarly, [PERSON] et al. [7] compared counties in Iowa with and without a known history of past Cry3 problem fields. Bioassay data revealed the presence of Cry3 Bb1 resistance in both types of counties and found similar levels of resistance. In both studies, there was some variability in the level of resistance, with some populations displaying complete resistance while others had incomplete resistance (i.e., survival or larval development was lower on Cry3 Bb1 maize compared to non-Bt maize). These studies suggested widespread resistance to Cry3 Bb1 maize in Iowa, although there was variation among populations in the level of resistance. In a study examining landscape-level patterns of resistance and the effects of cropping practices on resistance, [PERSON] et al. [4] measured Cry3 Bb1 resistance in western corn rootworm from fields in Iowa with a variety of management histories, including (1) fields in continuous maize production, (2) rotated fields, (3) past Cry3 problem fields, and (4) current Cry3 problem fields. Data from plant-based and diet-based bioassays illustrated that all field types harbored Cry3 Bb1-resistant western corn rootworm [4]. However, larval development on Cry3 Bb1 maize was significantly reduced compared to non-Bt maize for rotated fields and past problem fields, but not for continuous maize fields or current problem fields, suggesting that crop rotation may help delay the development of Cry3 Bb1 resistance [4]. [PERSON] and [PERSON] [76] analyzed landscape-level patterns of maize cultivation in past problem fields and in areas surrounding past problem fields in Iowa during the timeframe when these fields failures occurred. These patterns were compared with randomly selected agricultural fields in Iowa during the same time period [76]. This study found that, not only were past problem fields characterized by higher levels of continuous maize cultivation compared to randomly selected fields, but also that the local landscape around these past problem fields had more continuous maize than randomly selected points [76]. The local landscapes around past problem fields with elevated percentages of continuous maize cultivation included an area within 3.2 km of past problem fields. Furthermore, available data indicate that 57% of fields in continuous maize production during the timeframe examined contained Cry3 Bb1 maize [22].