Data from: Type of fitness cost influences the rate of evolution of resistance to transgenic Bt crops

The evolution of resistance to pesticides by insect pests is a significant challenge for sustainable agriculture. For transgenic crops expressing Bacillus thuringiensis (Bt), crystalline (Cry) toxins resistance evolution may be delayed by the high-dose/refuge strategy in which a non-toxic refuge is planted to promote the survival of susceptible insects. The high-dose/refuge strategy may interact with fitness costs associated with resistance alleles to further delay resistance. However, while a diverse range of fitness costs are reported in the field, they are typically represented as a fixed reduction in survival or viability which is insensitive to ecological conditions such as competition. Furthermore, the potential dynamic consequences of restricting susceptible insects to a refuge which represents only a fraction of the available space have rarely been considered. We present a generalized discrete time model which utilizes dynamic programming methods to derive the optimal management decisions for the control of a theoretical insect pest population exposed to Bt crops. We consider three genotypes (susceptible homozygotes, resistant homozygotes and heterozygotes) and implement fitness costs of resistance to Bt toxins as either a decrease in the relative competitive ability of resistant insects or as a penalty on fecundity. Model analysis is repeated and contrasted for two types of density dependence: uniform density dependence which operates equally across the landscape and heterogeneous density dependence where the intensity of competition scales inversely with patch size and is determined separately for the refuge and Bt crop. When the planting of Bt is decided optimally, fitness costs to fecundity allow for the planting of larger areas of Bt crops than equivalent fitness costs that reduce the competitive ability of resistant insects. Heterogeneous competition only influenced model predictions when the proportional area of Bt planted in each season was decided optimally and resistance was not recessive. Synthesis and applications. The high-dose/refuge strategy alone is insufficient to preserve susceptibility to transgenic Bacillus thuringiensis (Bt) crops in the long term when constraints upon the evolution of resistance are not insurmountable. Fitness costs may enhance the delaying effect of the refuge, but the extent to which they do so depends upon how the cost is realized biologically. Fitness costs which apply independently of other variables may be more beneficial to resistance management than costs which are only visible to selection under a limited range of ecological conditions.

昆虫害虫对农药的抗性演化是可持续农业面临的重大挑战。对于表达苏云金芽孢杆菌（Bacillus thuringiensis, Bt）的转基因作物，可通过高剂量-庇护所（high-dose/refuge）策略延缓晶体（Cry）毒素抗性的演化：该策略通过种植非毒性庇护所作物，促进敏感昆虫存活。高剂量-庇护所策略可与抗性等位基因相关的适合度成本相互作用，进一步延缓抗性演化。然而，尽管野外已报道多种类型的适合度成本，但现有研究通常将其描述为存活率或生存力的固定降幅，且不受竞争等生态条件影响。此外，将敏感昆虫限制在仅占可用空间一小部分的庇护所中所产生的潜在动态后果，极少被纳入考量。本研究提出一种广义离散时间模型（discrete time model），借助动态规划（dynamic programming）方法，推导针对暴露于Bt作物的理论害虫种群的最优防控管理决策。模型考虑三类基因型（genotype）：敏感纯合子、抗性纯合子与杂合子，并将Bt毒素抗性的适合度成本设置为两种形式：一是降低抗性昆虫的相对竞争能力，二是对繁殖力施加惩罚。针对两类密度依赖场景重复开展模型分析并进行对比：一类是在整个农田景观中均匀作用的均匀密度依赖，另一类是竞争强度随斑块面积反向变化、且分别针对庇护所与Bt作物地块确定的非均匀密度依赖。当Bt作物种植决策为最优时，相较于降低抗性昆虫竞争能力的等效适合度成本，以繁殖力为表现的适合度成本允许种植更大面积的Bt作物。仅当每季节种植的Bt作物比例为最优决策且抗性并非隐性时，非均匀竞争才会对模型预测结果产生影响。总结与应用。当抗性演化的约束条件并非不可突破时，仅依靠高剂量-庇护所策略无法长期维持昆虫对转基因苏云金芽孢杆菌（Bt）作物的敏感性。适合度成本可增强庇护所的抗性延缓效果，但具体增强程度取决于该成本在生物学上的实现方式。独立于其他变量的适合度成本，相比仅在有限生态条件范围内才会被选择识别的成本，或许更有益于抗性管理。