## 4 Factors Affecting Resistance Evolution
Strains with laboratory-selected resistance to Cry3 Bb1 maize have displayed fitness costs in some cases [68; 69] but not in others [63; 70; 71]. Additionally, fitness costs appeared to be absent in strains with laboratory-selected resistance to eCY3.1 Ab and mCry3A [58; 72]. Patterns of fitness costs associated with field-evolved Cry3 Bb1 resistance were similar to results for strains with laboratory-selected resistance, with costs present in some cases but not in others. However, costs appear to be more common in strains with field-evolved resistance compared to laboratory-selected resistance (Table 2). [PERSON] and [PERSON] [65] identified costs affecting larval development, survival to adulthood and fecundity in one strain (Cresco) but costs were absent in another strain (Hopkinton). In a study of two additional strains, [PERSON] and [PERSON] [66] found a fitness cost in one strain (Elma) but not in a second strain (Monona). [PERSON] and [PERSON] [67] studied several field populations and detected a negative relationship between adult size and the level of Cry3 Bb1 resistance, indicating a fitness cost of resistance affecting adult size. St. Clair  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type of** & **Train** & **Resistant to** & **Cost Present? ** & **Trains** & **Reference** \\
**Resistance 1** & & **Toxin 2** & **3** & **Affected 4** & **Reference** \\ \hline Laboratory & Brookings Moderately & & & & & \\ Selected & Selected & & & & & \\ Laboratory & Brookings Moderately & & & & & \\ Selected & Selected & & & & & \\ Selected & & & & & \\  & ( Strain 1) & & & & & \\ Laboratory & Brookings Moderately & & & & & \\ Selected & & & & & \\ Selected & & & & & \\ Laboratory & Brookings Internely Selected & & & & & \\ Selected & & & & & \\ Laboratory & Brookings Internely Selected & & & & & \\ Selected & & & & & \\ Laboratory & Brookings Internely Selected & & & & & \\ Selected & & & & & \\ Laboratory & Data Presented as & & & & & \\ Selected & Composite of Three & & & & & \\  & Resistant Strains & & & & & \\ Laboratory & Brookings Moderately & & & & & \\ Selected & Selected & & & & & \\ Laboratory & Brookings Moderately & & & & & \\ Selected & Selected & & & & & \\ Laboratory & Brookings Moderately & & & & & \\ Selected & Selected & & & & & \\ Laboratory & & & & & & \\ Laboratory & & & & & & \\ Selected & & & & & & \\ Laboratory & & & & & & \\ Selected & & & & & & \\ Laboratory & & & & & & \\ Selected & & & & & & \\ Selected & & & & & & \\ Selected & & & & & & \\ Selected & & & & & & \\ Selected & & & & & & \\ Field Evolved & Oregon3 Bb1 & & & & & \\ [PERSON]  
et al. [73] assessed fitness costs of Cry3 Bb1 resistance in Hopkinton and Cresco through a selection experiment, which tested for a loss of resistance over time in the absence of exposure to Cry3 Bb1 maize, and this study found evidence of fitness costs in both strains. The contrasting results between [PERSON] and [PERSON] [65], which measured individual life-history characteristics, and [PERSON] et al. [73], which used a selection experiment, likely arose because selection experiments provide a more comprehensive metric for assessing fitness costs and are therefore more sensitive [34]. However, it is important to note that [PERSON] et al. [73] also found that Cry3 Bb1 resistance persisted for at least six generations in the absence of exposure to Cry3 Bb1 maize, which translates to 6 years in the field, because western corn rootworm has one generation per year. Consequently, to the extent that fitness costs do accompany Cry3 Bb1 resistance, it is likely that Bt resistance currently present in the agricultural landscape may remain for several years, even if farmers were to discontinue planting of Bt maize [4; 5; 6; 7]. Taken together, the available data suggest that minimal fitness costs may often be associated with Bt resistance in western corn rootworm, and consequently, fitness costs may do little to delay the evolution of Bt resistance.