sion where both susceptible phenotypes are delayed 6 days, the allele frequency did not change in the 99 years of using transgenic maize. When the homozygous susceptibles were delayed by 3 to 9 days with additive allele expression, resistance developed more quickly relative to the stan- dard, but the number of years required to reach 3% allele frequency did not change by more than 1 year (and then only for the lowest dose).
Storer (2003) created a stochastic, spatially explicit computer model that simulates the adaptation by WCR to transgenic maize. The model reflects the ecology of the rootworm in much of the Corn Belt of the USA.
It includes functions for crop development, egg and larval mortality, adult emergence, mating, egg laying, mortality and dispersal to simulate the population dynamics of WCR and compares alternative methods of rootworm control. The allele for resistance to transgenic maize varies from incompletely recessive to incompletely dominant, depending on the efficacy of the toxin in the crop. Validation was achieved by comparing populations from the model with field data on population dynamics, and with field data documenting WCR adaptation to cyclodienes and organophosphates. The model was used to compare the rate at which the resistance allele spread through the population under different refuge deployment scenarios, and with crops of different efficacy.
For a given refuge size, the model indicated that placing the refuge in a block within a transgenic maize field would be likely to delay WCR resistance longer than planting the refuge in separate fields in varying locations. If a portion of the refuge were planted in the same fields or the same in-field blocks each year, WCR adaptation would be substantially delayed.
Storer (2003) conducted a brief analysis of the need for insecticide use in refuges because results suggested that resistance to transgenic maize would be unaffected by soil insecticide treatment in the refuge. In this analysis, refuge insecticide treatments were warranted for the first few years of transgenic deployment until the regionwide population was reduced. The smaller the proportion of fields planted to non-transgenic maize, the smaller the proportion of them that require treatment.
In areas where the allele frequency for resistance to crop rotation is already high, either from local evolution or from invasion, it is difficult, if not practically impossible, to reverse or halt further evolution of resist- ance. However, we hypothesize that the rotation-resistant WCR cannot persist over the long term in small areas with high landscape diversity.
Uncertainty about the timing of invasion by the rotation-resistant variant and initial gene frequency makes it difficult to choose among manage- ment strategies. The costs of resistance are not limited to reductions in farmer returns. If the use of soil insecticides increases as a result of rota- tion resistance, there may be broader social costs. From this perspective, it may be desirable to subsidize farmers in the present so that they have an incentive to change practices now and delay the development of resist- ance in the future.
The history of the WCR provides several examples of its ability to evolve around our attempts to manage population densities or damage.
Future management strategies should be used cautiously. Transgenic maize, active against corn rootworms, may provide the only economical and environmentally sound control for rotation-resistant rootworms.
Given the adaptability of this pest with regard to crop rotation and previ- ous insecticides, further research must be done into alternatives or addi- tions to the refuge strategy to assess the risk of evolution of resistance to transgenic maize. Future management strategies should include biologi- cal control, host plant resistance and other feasible tactics and offset these selection pressures with regional landscape heterogeneity.
Heterogeneous landscapes are the stage on which these stories of evo- lution are played out. Natural variation in adult behaviour is acted upon by natural selection caused by society’s choices of land use and crop selection. Any pest management strategy that is very effective over the short term will cause strong selection pressure on a pest. Thus, even crop rotation and host plant resistance can be misused over evolutionary time.
Individual farmers and society must recognize that simple solutions that ignore landscapes, insect behaviour, ecology and evolution will always lead to complications.
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