Which strategy delays insect resistance development?
Resistance evolution to Bt toxins accelerated when transgenic crops grown continuously on large acreage without alternative hosts for susceptible insects. Homozygous resistant insects emerging from Bt fields mating with each other rapidly increase resistance allele frequency. Refuge strategy maintains pool of susceptible individuals by planting non-transgenic crop nearby or mixing seeds. Susceptible moths from refuge mate with rare resistant survivors from Bt field producing heterozygous offspring. High dose of toxin expressed in transgenic plants kills heterozygotes because resistance generally recessive, diluting resistant alleles and delaying fixation. EPA mandates 20 percent structured refuge for cotton and 5 to 20 percent for corn depending on region. Data from Arizona pink bollworm program shows refuge compliance helped preserve Bt efficacy for over decade. Planting pattern may include block refuge, strip refuge, or seed mix. Hence mixing GM and non-GM crops implements high-dose refuge principle, primary tactic delaying insect resistance development to Cry toxins in commercial agriculture. Implementation details include 20 percent structured refuge for cotton and 5 to 20 percent for corn, planted within 0.5 mile of Bt field to ensure random mating. Monitoring of resistance allele frequency by F2 screen detects early resistance evolution allowing proactive management including increased refuge
Ref: Gould Annu Rev Entomol 1998 refuge strategy; Tabashnik Annu Rev Entomol 2013 mixing; NCBI NBK131103 insect resistance management; EPA refuge guidance 2001.