DOI: 10.5061/dryad.4s405
title: Data from: Type of fitness cost influences the rate of evolution of resistance to transgenic Bt crops

description: 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.
description: R code for fixed
 landscape partitioningThis is the code used to
 generate Figures 1, S1, S2, S3 and S4 as presented in the paper and
 supplementary materials.Constant Bt planting (Figs 1,
 S1-4).RR code for average Bt
 usageThis is the R code used
 to generate Figure 2 as presented in the paper.Figure 2
 code.RR code for uniform
 density dependenceThis is the R code used
 to generate Figures 3,4 and S5 as presented in the paper and supplementary
 materials.Uniform competition, Figs 3,4
 & S5.RR code for heterogeneous
 density dependenceThis is the R code used
 to generate Figures 5, S6 and S7 as presented in the paper and
 supplementary materials.Heterogenous competition, Figs
 5, S6 &S7.R