Extended evaluation of Bt protein cross‐pollination in seed blend plantings on survival, growth, and development of Helicoverpa zea feeding on refuge ears

Abstract

BACKGROUND A seed blend, also called refuge in a bag (RIB), is used as a strategy to provide refuge for planting pyramided Bt corn in the U.S. Corn Belt. A major concern for the RIB strategy is cross‐pollination between Bt and refuge plants, which may reduce susceptible populations of ear‐feeding insects like Helicoverpa zea and affect the evolution of resistance. Previous studies showed a 5:95% (Non‐Bt: Bt) RIB would be unlikely to provide sufficient refuge populations of H. zea for resistance management. In this study, we extended our research and conducted multiple trials to determine if three RIB plantings with greater refuge sizes (10, 20, and 30% refuge) could provide sufficient H. zea susceptible populations to delay resistance development. RESULTS Experimental results showed that cross‐pollination in 10:90%, 20:80%, and 30:70% RIB plantings still significantly reduced larval survival, delayed larval development and decreased the pupal size of H. zea on refuge plants. Regression analysis showed that the percent reduction of neonate‐to‐adult survivorship, relative to the survivorship on pure non‐Bt ears, was significantly negatively correlated with the refuge percentage. CONCLUSION These findings suggest that, approximately a 15% non‐Bt refuge plants in RIB plantings could produce a similar number of susceptible adult moths as a 5% structured refuge planting, while an approximately 30% non‐Bt refuge plant in RIB plantings could provide a similar number of susceptible moths as a 20% structured refuge planting. Information generated from this study should be useful in refining resistance management strategies for Bt crop technologies. © 2019 Society of Chemical Industry

Effects of gene flow between Bt and non‐Bt plants in a seed mixture of Cry1A.105 + Cry2Ab corn on performance of corn earworm in Arizona

“…Nevertheless, development time was shorter on ears from Bt plants in the seed mixture than block. Here as in other studies investigating natural H. zea populations, 25–27 larval performance appeared mostly similar on ears from Bt plants in the seed mixture and block. Larval weight during the 3–4 week period studied, the percentage of ears with exit holes, and ear damage did not differ significantly between these ear types.…”

“…24,27 However in previous studies of seed mixtures of Cry1A.105 + Cry2Ab + Cry1Fa or Cry1Ab + Cry1Fa + Vip3Aa corn in which larval development was monitored over longer periods (up to fourth to fifth instars, exit holes, or pupa and adult), both slower development and increased mortality was observed on ears from non-Bt plants in the seed mixtures relative to blocks. 22,25,26 As in our previous laboratory study with laboratory strains, 19 here we did not find slower larval development on ears from non-Bt plants in the seed mixtures relative to blocks. This difference from results of other studies may have occurred because of differences between Table 2.…”

“…In previous studies of early larval stages (first to fourth instars) in seed mixtures of Cry1A.105 + Cry2Ab + Cry1Fa or Cry1Ab + Vip3Aa corn, development of H. zea was slower on ears from non‐Bt plants in the seed mixtures relative to blocks, but survival did not differ significantly between the ear types 24,27 . However in previous studies of seed mixtures of Cry1A.105 + Cry2Ab + Cry1Fa or Cry1Ab + Cry1Fa + Vip3Aa corn in which larval development was monitored over longer periods (up to fourth to fifth instars, exit holes, or pupa and adult), both slower development and increased mortality was observed on ears from non‐Bt plants in the seed mixtures relative to blocks 22,25,26 . As in our previous laboratory study with laboratory strains, 19 here we did not find slower larval development on ears from non‐Bt plants in the seed mixtures relative to blocks.…”

“…[13][14][15][16][17][18][19] Gene flow between Bt and non-Bt plants in seed mixtures results in a mosaic of Bt and non-Bt kernels in ears of non-Bt plants, [19][20][21][22] and may reduce the proportion of Bt kernels and the concentration of Bt toxins in ears of Bt plants. 19 For pests with ear-feeding larvae, seed mixtures could accelerate the evolution of resistance by killing susceptible individuals feeding on ears of non-Bt plants, [23][24][25][26] or increasing the relative fitness of individuals with resistance alleles on ears of non-Bt or Bt plants. 17,19 In a recent laboratory study in Arizona, 19 we evaluated effects of a seed mixture of 10% non-Bt corn and 90% Bt corn producing Cry1A.105 and Cry2Ab versus blocks of these crops on evolution of resistance to this pyramid in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae).…”

“…Gene flow between Bt and non‐Bt plants in seed mixtures results in a mosaic of Bt and non‐Bt kernels in ears of non‐Bt plants, 19–22 and may reduce the proportion of Bt kernels and the concentration of Bt toxins in ears of Bt plants 19 . For pests with ear‐feeding larvae, seed mixtures could accelerate the evolution of resistance by killing susceptible individuals feeding on ears of non‐Bt plants, 23–26 or increasing the relative fitness of individuals with resistance alleles on ears of non‐Bt or Bt plants 17,19 …”