Applying a Selection Experiment to Test for Fitness Costs of Bt Resistance in Western Corn Rootworm (Coleoptera: Chrysomelidae) and the Effect of Density on Fitness Costs

Abstract

Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of corn and is often managed with transgenic corn producing insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This pest has developed field-evolved resistance to all commercially available Bt traits, beginning with Cry3Bb1 in 2009. Fitness costs may accompany Bt resistance, where individuals with alleles for Bt resistance have reduced fitness on non-Bt corn compared to Bt-susceptible individuals. In conjunction with non-Bt refuges, fitness costs can delay the evolution of Bt resistance. Importantly, ecological factors may affect the presence and magnitude of fitness costs. For western corn rootworm, available data suggest that fitness costs of Bt resistance may be present in some cases. Using two Cry3Bb1-resistant western corn rootworm strains (Hopkinton and Cresco), a fitness-cost experiment was performed by rearing rootworm in the absence of Bt for six generations to test for fitness costs of Cry3Bb1 resistance and the effect of larval rearing density on fitness costs. Fitness costs were detected for both strains; however, strains were still resistant to Cry3Bb1 corn at the end of the experiment. Cresco experienced a greater loss of resistance at low versus high density, but no effect of density was detected in Hopkinton. Our study shows that fitness costs can accompany Bt resistance in western corn rootworm and may be more pronounced under low larval density. Even though fitness costs were present, it appears that rootworm populations may remain resistant to Cry3Bb1 corn for years after resistance has evolved.

Characterizing the sublethal effects of SmartStax PRO dietary exposure on life history traits of the western corn rootworm, Diabrotica virgifera virgifera LeConte

“…In continuous corn production areas of the U.S. Corn Belt, WCR plant-incorporated protectant options are currently limited and metapopulations within an area will continuously be exposed to existing Bt protein combinations. Genetic mixing within the metapopulation due to the movement of WCR resistance alleles in the landscape [ 12 , 76 ] and the persistence of Bt resistance for multiple generations after removal of selection pressure [ 77 – 80 ] may enable WCR resistance to SmartStax ® PRO or other technologies to be maintained in an area over time.…”

Inheritance and Fitness Costs of Laboratory-Selected Resistance to Gpp34/Tpp35Ab1 Corn in Western Corn Rootworm (Coleoptera: Chrysomelidae)

“…All data were analyzed with SAS 9.4 (SAS Institute 2015). Data on proportion survival to adulthood from the experiment measuring the dominance of resistance were analyzed with an analysis of variance (ANOVA) based on a general linear model (PROC GLM) (Sokal and Rohlf 1995). Proportion survival was calculated for each replicate (i.e., seedling mat) as the number of adults that emerged from a seedling mat divided by the number of neonate larvae added to that seedling mat.…”

“…Subsequently, the full data set was analyzed with an ANOVA that included the factors of genotype (Bt-resistant, Bt-susceptible, and heterozygotes), corn hybrid, and their interaction. A significant genotype-by-hybrid interaction was present, so comparisons were made among genotypes within each hybrid and between hybrids for each genotype, with a significance level of P < 0.0056 after a Bonferroni adjustment using nine comparisons (Sokal and Rohlf 1995). Corrected survival was calculated, based on the formula of Abbott (1925), as the quotient of proportion survival on Bt corn divided by the proportion survival on non-Bt corn.…”

Resistance to Bt Maize by Western Corn Rootworm: Effects of Pest Biology, the Pest–Crop Interaction and the Agricultural Landscape on Resistance

“…Shrestha and Gassmann [ 67 ] studied several field populations and detected a negative relationship between adult size and the level of Cry3Bb1 resistance, indicating a fitness cost of resistance affecting adult size. St. Clair et al [ 73 ] assessed fitness costs of Cry3Bb1 resistance in Hopkinton and Cresco through a selection experiment, which tested for a loss of resistance over time in the absence of exposure to Cry3Bb1 maize, and this study found evidence of fitness costs in both strains. The contrasting results between Ingber and Gassmann [ 65 ], which measured individual life-history characteristics, and St. Clair 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 St. Clair et al [ 73 ] also found that Cry3Bb1 resistance persisted for at least six generations in the absence of exposure to Cry3Bb1 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 Cry3Bb1 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 ].…”