Monitoring and adaptive resistance management

An adaptive management strategy can be used to hedge against failures in the scientific assumptions underlying the high-dose plus refuge strategy for managing the resistance of the European corn borer to Bt corn. This will require sensitive monitoring coupled with effective adaptive management interventions. Using a theoretical model, we suggest that recessive resistance alleles must be detected at frequencies of ≤5 × 10^-3 to provide enough time to adapt management. Analysis of the costs of monitoring indicates that an F₂ screen is the most cost effective method for monitoring recessive resistance to Bt corn. If resistance is dominant, an in-field screen is the most cost effective method. Both methods can detect and measure resistance at frequencies of ≤5 × 10^-3 for ∼$5000 (U.S. dollars) per site. Two kinds of adaptive responses can be taken: (1) reducing the selective advantage of the resistance allele or (2) modifying the mating system so fewer resistance alleles are passed on to future generations. Assuming there is a two-year time delay between detecting resistance and taking an adaptive response, we found that either increasing refuge size to 66% (from 20%), or decreasing survival and reproduction of European corn borers from Bt corn fields by 90% can prolong susceptibility by 10 generations. Modification of the mating system by changing movement rates and attracting susceptible males into Bt corn fields could prolong susceptibility for >20 generations. These results suggest that adaptive resistance management could increase the durability of Bt corn.