We used a mathematical model with processes reflecting larval mortality resulting from feeding on crosspollinated ears or Bt ears of corn to analyze the risk of evolution of Cry-toxin resistance in Ostrinia nubilalis (Hübner). In the simulations, evolution of resistance was delayed equally well by both seed mixtures and blocks with the same proportion of refuge. Our results showed that Bt-pollen drift has little impact on the evolution of Bt resistance in O. nubilalis. However, low-toxin expression in ears of transgenic corn can reduce the durability of transgenic corn expressing single toxin, whereas durability of pyramided corn hybrids is not significantly reduced. The toxinsurvival rate of heterozygous larvae in Bt-corn ears expressing one or two proteins has more impact on evolution of Bt resistance in O. nubilalis than the parameters related to larval movement to Bt ears or the toxin-survival rate of the homozygous susceptible larvae in Bt ears. Bt resistance evolves slower when toxin mortality is distributed across the first two larval stadia than when only the first instars are susceptible to Bt toxins. We suggest that stakeholders examine toxin-survival rates for insect pests and take into account that instars may feed on different parts of Bt corn. KeywordsBt corn, resistance management, simulation Disciplines Entomology | Plant Breeding and Genetics CommentsThis article is from Environmental Entomology; 41 (2012); 200-211; doi: 10.1603/EN11133 RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. ABSTRACT We used a mathematical model with processes reßecting larval mortality resulting from feeding on cross-pollinated ears or Bt ears of corn to analyze the risk of evolution of Cry-toxin resistance in Ostrinia nubilalis (Hü bner). In the simulations, evolution of resistance was delayed equally well by both seed mixtures and blocks with the same proportion of refuge. Our results showed that Bt-pollen drift has little impact on the evolution of Bt resistance in O. nubilalis. However, low-toxin expression in ears of transgenic corn can reduce the durability of transgenic corn expressing single toxin, whereas durability of pyramided corn hybrids is not signiÞcantly reduced. The toxinsurvival rate of heterozygous larvae in Bt-corn ears expressing one or two proteins has more impact on evolution of Bt resistance in O. nubilalis than the parameters related to larval movement to Bt ears or the toxin-survival rate of the homozygous susceptible larvae in Bt ears. Bt resistance evolves slower when toxin mortality is distributed across the Þrst two larval stadia than when only the Þrst instars are susceptible to Bt toxins. We suggest that stakeholders examine toxin-survival rates for insect pests and take into account that instars may feed on different parts of Bt corn.
The cowpea weevil, Callosobruchus maculatus F. (Coleoptera: Bruchidae), can cause up to 100% yield loss of stored cowpea seeds in a few months in West Africa. Genes expressing toxins delaying insect maturation (MDTs) are available for genetic engineering. A simulation model was used to investigate the possible use of MDTs for managing C. maculatus. Specifically, we studied the effect of transgenic cowpea expressing an MDT, an insecticide, or both, on the evolution of resistance by C. maculatus at constant temperature. Transgenic cowpea expressing only a nonlethal MDT causing 50-100% maturation delay did not control C. maculatus well. Mortality caused by a maturation delay improved the efficacy of transgenic cowpea expressing only a lethal MDT, but significantly reduced the durability of transgenic cowpea Transgenic cowpea expressing only a lethal MDT causing 50% maturation delay and 90% mortality controlled C. maculatus better than one expressing only a nonlethal MDT, but its durability was only 2 yr. We concluded that transgenic cowpea expressing only an MDT has little value for managing C. maculatus. The resistance by C. maculatus to transgenic cowpea expressing only an insecticide rapidly evolved. Stacking a gene expressing a nonlethal MDT and a gene expressing an insecticide in transgenic cowpea did not significantly improve the durability of an insecticide, but stacking a gene expressing a lethal MDT and a gene expressing an insecticide in transgenic cowpea significantly improved the durability of an insecticide and an MDT. We also discussed this approach within the idea of using transgenic RNAi in pest control strategies.
Understanding how habitat quality in heterogeneous landscapes governs the distribution and fitness of individuals is a fundamental aspect of ecology. While mean individual fitness is generally considered a key to assessing habitat quality, a comprehensive understanding of habitat quality in heterogeneous landscapes requires estimates of dispersal rates among habitat types. The increasing accessibility of genomic approaches, combined with field-based demographic methods, provides novel opportunities for incorporating dispersal estimation into assessments of habitat quality. In this study, we integrated genomic kinship approaches with field-based estimates of fitness components and approximate Bayesian computation (ABC) procedures to estimate habitat-specific dispersal rates and characterize habitat quality in twotoed sloths (Choloepus hoffmanni) occurring in a Costa Rican agricultural ecosystem.Field-based observations indicated that birth and survival rates were similar in a sparsely shaded cacao farm and adjacent cattle pasture-forest mosaic. Sloth density was threefold higher in pasture compared with cacao, whereas home range size and overlap were greater in cacao compared with pasture. Dispersal rates were similar between the two habitats, as estimated using ABC procedures applied to the spatial distribution of pairs of related individuals identified using 3,431 single nucleotide polymorphism and 11 microsatellite locus genotypes. Our results indicate that crops produced under a sparse overstorey can, in some cases, constitute lower-quality habitat than pasture-forest mosaics for sloths, perhaps because of differences in food resources or predator communities. Finally, our study demonstrates that integrating field-based demographic approaches with genomic methods can provide a powerful means for characterizing habitat quality for animal populations occurring in heterogeneous landscapes. K E Y W O R D Sagroecosystem, dispersal, genomics, habitat quality, kinship, sloths | INTRODUCTIONCharacterizing habitat quality for species occurring in heterogeneous landscapes is a key endeavour of ecology, where high-quality habitats are expected to lead to high mean individual fitness in the absence of density-dependent effects (Johnson, 2007). Theoretically, individuals will seek to promote their fitness by preferentially selecting high-over low-quality habitats (Franklin, Anderson, Guti errez, & Burnham, 2000;Morris, 2003;Orians, 1969), and estimating mean individual fitness in different habitat types is therefore considered the "gold standard" for assessing habitat quality (Johnson, 2007). Fitness, however, is also a function of the density of conspecifics within a habitat patch as intraspecific competition for resources can modulate the fitness benefits of high-quality habitats (Hassell & May, 1973;Milinski, 1979). Consequently, individuals must balance perceived trade-offs between habitat quality and intraspecific competition when selecting habitat in order to maximize their contribution to future generations...
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