Risk Assessment for <I>Helicoverpa zea</I> (Lepidoptera: Noctuidae) Resistance on Dual-Gene Versus Single-Gene Corn

Abstract

Recent Environmental Protection Agency (EPA) decisions regarding resistance management in Bt-cropping systems have prompted concern in some experts that dual-gene Bt-corn (CrylA.105 and Cry2Ab2 toxins) may result in more rapid selection for resistance in Helicoverpa zea (Boddie) than single-gene Bacillus thuringiensis (Bt)-corn (CrylAb toxin). The concern is that Bt-toxin longevity could be significantly reduced with recent adoption of a natural refuge for dual-gene Bt-cotton (CrylAc and Cry2Ab2 toxins) and concurrent reduction in dual-gene corn refuge from 50 to 20%. A population genetics framework that simulates complex landscapes was applied to risk assessment. Expert opinions on effectiveness of several transgenic corn and cotton varieties were captured and used to assign probabilities to different scenarios in the assessment. At least 350 replicate simulations with randomly drawn parameters were completed for each of four risk assessments. Resistance evolved within 30 yr in 22.5% of simulations with single-gene corn and cotton with no volunteer corn. When volunteer corn was added to this assessment, risk of resistance evolving within 30 yr declined to 13.8%. When dual-gene Bt-cotton planted with a natural refuge and single-gene corn planted with a 50% structured refuge was simulated, simultaneous resistance to both toxins never occurred within 30 yr, but in 38.5% of simulations, resistance evolved to toxin present in single-gene Bt-corn (CrylAb). When both corn and cotton were simulated as dual-gene products, cotton with a natural refuge and corn with a 20% refuge, 3% of simulations evolved resistance to both toxins simultaneously within 30 yr, while 10.4% of simulations evolved resistance to CrylAb/c toxin.

Evolution of Resistance byHelicoverpa zea(Lepidoptera: Noctuidae) Infesting Insecticidal Crops in the Southern United States

“…(2013) , who based their parameters on the opinions of eight entomologists. For the three insecticidal traits acting alone, our means and standard deviations and the estimates of Edwards et al. (2013) are (0.77 ± 0.16, 0.69) for Cry1F, (0.54 ± 0.25, 0.58) for Cry1Ab, and (0.02 ± 0.01, 0.07) for Vip3A, respectively.…”

“…We calculated the mean survival by dividing the mean for all replicates for each treatment by the mean for all replicates observed in refuge corn. The means are similar to those used by Edwards et al. (2013) , who based their parameters on the opinions of eight entomologists.…”

“…For vegetative-stage corn, productivity is believed to be less than that on reproductive-stage corn ( Gustafson et al. 2006 , Edwards et al. 2013 ).…”

“…2013 ). Therefore, we use the following survival rates for homozygous susceptible larvae on vegetative corn: 0.49 on Cry1F and 0.36 on Cry1Ab (both based on expert opinions reported by Edwards et al. 2013 ), and 0.01 on Vip3A, which is less than the 0.02 observed in the field in reproductive-stage corn.…”

“…(2008) used a simple population genetics model to study the influence of refuge levels on evolution of corn earworm to Cry1Ac cotton. Edwards et al. (2013) extended the work of Caprio et al.…”