The impact of Bt maize as a natal host on adult head capsule width in field populations of western corn rootworm

Abstract

The success of the current resistance management plan for transgenic maize, Zea mays L. (Poaceae), targeting the rootworm complex hinges upon high rates of mating between resistant and susceptible beetles. However, differences in the fitness of adult beetles could result in assortative mating, which could, in turn, change the rate of resistance evolution. Adult head capsule widths of naturally occurring populations of western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), were examined in a variety of refuge configurations. Beetles were classified into treatments based on the hybrid – non‐Bt refuge or Bt maize targeting larval rootworms (hereafter Bt‐RW maize) – and location – proximity to other Bt‐RW or refuge plants – of the natal host plant. Treatments included the following: a refuge plant surrounded by other refuge plants, a refuge plant located near a Bt‐RW plant, a Bt‐RW plant surrounded by Bt‐RW plants, and a Bt‐RW plant located near a refuge plant. The mean head capsule width of males emerging from Bt‐RW plants was significantly smaller than the mean head capsule width of males emerging from refuge plants. These results indicate that males emerging from Bt‐RW maize plants may be exposed to sublethal doses of the Bt toxin as larvae. No differences were detected between females emerging from refuge plants compared with Bt‐RW plants. Overall mean head capsule width decreased as the season progressed, regardless of treatment. The diminished head capsule width of western corn rootworm males emerging from Bt‐RW maize may act to enhance resistance management, particularly in a seed mix refuge system.

Field-Based Assessment of Resistance to Bt Corn by Western Corn Rootworm (Coleoptera: Chrysomelidae)

“…The sublethal effects of Bt crops can influence the growth, development, and reproduction of insect pests (Stewart et al 2001, Binning et al 2010, Murphy et al 2011. The presence of sublethal effects of a Bt trait can also delay the evolution of Bt resistance (Tabashnik et al 2004).…”

Scientific Opinion on an application from Pioneer Hi‐Bred International and Dow AgroSciences LLC (EFSA‐GMO‐NL‐2005‐23) for placing on the market of genetically modified maize 59122 for food and feed uses, import, processing and cultivation under Regulation (EC) No 1829/2003

“…Storer et al (2006) and Rudeen and Gassmann (2012) reported a seven-day delay in initial emergence of WCR from maize 59122, compared with the near-isogenic line. Further, males have been shown to prefer larger females under laboratory conditions (Kang and Krupke, 2009b), which could result in assortative mating too (Murphy et al, 2011). (4) Resistance alleles are partially or fully recessive: If resistance is completely recessive, then heterozygous offspring resulting from crosses between resistant and susceptible individuals are expected to be susceptible to the Bt-protein, thus preventing or slowing resistance evolution .…”

“…This increased proximity in both space and time may facilitate random mating between adults emerging from Bt-maize and refuge plants compared with block refuges (Murphy et al, 2011). However, the advantages of seed blends may be offset by the potential for larval movement between roots of Bt-maize and refuge plants (Hibbard et al, 2003Zukoff et al, 2012;Schumann and Vidal, 2012), and the exposure of later instars to sublethal doses of the toxin (Goldstein et al, 2010;Murphy et al, 2011;Onstad et al, 2011;Razze and Mason, 2012;Zukoff et al, 2012). For Bt-crops that are truly high dose, Mallet and Porter (1992) indicated that the movement of larvae between Bt-crop and refuge plants may lower the selective differential between susceptible and resistant genotypes, and increase the effective dominance of resistance by producing more heterozygote individuals (Glaum et al, 2012;Siegfried and Hellmich, 2012; but see Tabashnik, 1994).…”

Applying an Integrated Refuge to Manage Western Corn Rootworm (Coleoptera: Chrysomelidae): Effects on Survival, Fitness, and Selection Pressure

“…Few Þeld studies have examined Þtness of western corn rootworm in a block vs. integrated refuge, or effects on susceptibility to Bt maize in the next generation (but see Hibbard et al 2010a;Murphy et al 2010Murphy et al , 2011Zukoff et al 2012). In this study, survival and Þtness of western corn rootworm adults were measured after exposure in the Þeld to pure stands of Bt maize (event 59122), pure stands of refuge (non-Bt maize), and an integrated refuge.…”