The interaction of population dynamics and movement among two habitat types (toxic transgenic fields and nontoxic refuge fields) on the evolution of insecticide resistance was examined in two different simulation models. The two models were developed to test the hypothesis that increasing habitat grain from fine-grained to coarse-grained, and the resultant increase in nonrandom mating, would increase the rate of local adaptation, here the evolution of resistance. The first model, a complex, stochastic spatially explicit model, altered habitat grain by varying adult dispersal rates between habitat patches. In contrast to the expectation that increasing patch isolation and increasing the coarseness of the habitats would increase the rate of resistance evolution, intermediate levels of dispersal actually delayed resistance by as much as fivefold over the range of dispersal levels observed. Source-sink dynamics related to ovipositional patterns and the related population dynamics appear to explain the results. A simple deterministic model was developed to abstract out the separate impacts of mating and ovipositional behaviors. This model showed qualitatively the same results, although under similar assumptions it predicted much longer delays in resistance evolution. In this model, nonrandom mating alone always increased the rate at which insects adapted to transgenic crops, but nonrandom mating in combination with nonrandom oviposition could significantly delay resistance evolution. Differences between the two models may be due to the population regulation incorporated in the spatially explicit model. The models clearly suggest that resistance management programs using untreated refuges should not over-emphasize random mating at the cost of making the habitat too fine-grained.
We discuss assay approaches for monitoring the sensitivity of Lepidoptera to Bacillus thuringiensis (Bt) insecticidal proteins and compare the relative sensitivity of larval feeding bioassays in which, respectively, mortality or growth inhibition were scored. Heliothis virescens (F.) and Helicoverpa zea (Boddie), major lepidopteran pests targeted for control by transgenic cotton, were used for assay comparison. Larval growth inhibition assays using sublethal CryIA(c) protein concentrations were considerably more sensitive than dose-response mortality assays. Growth inhibition assays were easy to set-up and read, and could readily deliver a diagnostic dose allowing for visual discrimination of resistant from susceptible phenotypes. The ability of a larval growth assay, combined with a diagnostic dose, to unambiguously separate resistant from susceptible insects was validated using a CryIA(c) protein resistant strain of H. virescens and F 1 hybrids derived by crossing the resistant strain to a susceptible H. virescens strain.
The primary management tactic for lepidopteran pests of cotton in the United
States of America (USA) is the use of transgenic cotton that produces
Bacillus thuringiensis Berliner (Bt)
toxins. The primary target pests of this technology are Helicoverpa
zea (Boddie) and Heliothis virescens (F.) in the
eastern and central Cotton Belt of the USA. Concerns over the evolution of
resistance in H. zea to Bt
toxins and scrutiny of the necessity of Bt crops has escalated.
We reviewed published and unpublished data from field trials of
Bt cotton in the eastern and central Cotton Belt of the USA
through 2015 to evaluate the effectiveness of Bt cotton
(Bollgard, Bollgard II, WideStrike, WideStrike 3, and TwinLink).
Bt cotton reduced insecticide usage, reduced heliothine
pest numbers and damage, and provided a yield benefit, but Bollgard II and
WideStrike efficacy declined in the Midsouth over the period evaluated. In the
Southeastern region, heliothine damage remained constant through 2015, but yield
benefits declined from 2010 until 2015. Resistance of H.
zea to several Bt toxins is the most
plausible explanation for the observed changes in Bt cotton
efficacy. The introduction of new Bt toxins such as found in
Widestrike 3 and Twinlink may preserve the benefits of Bt
crops. However, while both Widestrike 3 and Twinlink had less damage than
Widestrike, damage levels of both were similar to Bollgard II.
For highly polyphagous cotton, Gossypium hirsutum L., pests such as Helicoverpa zea (Boddie), a substantial portion of the larval population develops on noncotton alternative hosts. These noncotton hosts potentially provide a natural refuge for H. zea, thereby slowing the evolution of resistance to the Bacillus thuringiensis Berliner (Bt)-derived Cry1Ac protein present in Bollgard cotton. Here, we demonstrate how the measured contribution of such alternative hosts can be included in estimating the "effective refuge" present for H. zea and in modeling resistance evolution in this species. A single-gene, two-compartment model was used in which one compartment represented corn, Zea mays L., and cotton that express the Cry1Ac protein or similar proteins, and the other compartment was the effective refuge, made up of a weighted average of non-Bt cotton and noncotton hosts. The effective refuge was estimated for each of six generations of H. zea based upon available data on larval population densities on different hosts and cropping patterns. Model runs were performed for regions centered on three states: Georgia, Mississippi, and North Carolina. Three sets of fitness cost assumptions for the putative resistance gene were used: none, low, and moderate, with either recessive or additive inheritance for resistance and fitness costs. For Georgia and North Carolina, resistance was predicted to take >30 yr to evolve except in the absence of fitness costs. For Mississippi, results were sensitive to fitness costs: >30 yr with moderate costs, 7-14 yr with low costs, and 6-10 yr without such costs.
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