Evaluating Resistance Management Strategies for Multiple Toxins in the Presence of External Refuges

The use of 5 resistance management strategies in the presence and absence of external refuges (refuges that are spatially isolated from treated fields) was examined in a stochastic model. The 5 strategies were sequential introduction of 2 toxins, rotations and mosaics of toxins, and half-and full-rate mixtures of the toxins. The ability of the strategies to delay resistance was examined first with scheduled sprays, then using an integrated pest management (IPM) component and treating only when populations exceeded an economic threshold. When sprays were scheduled and in the absence of an external refuge, 4 of the strategies resulted in similar rates of resistance evolution. The 5th strategy (full-rate mixtures) provided for better insect control but resulted in resistance to both toxins in approximately the same time it took resistance to develop to the 1st toxin in the sequential use strategy. When an economic threshold (IPM strategy) was adopted, all strategies performed approximately the same in the absence of refuges. When a 5%spatially segregated refuge was included in the IPM simulations, resistance was delayed 4.4 times in the sequential introductions, 15-20 times for rotations, mosaics and half-rate mixtures, and 34.8 times for full-rate mixtures (compared with sequential introductions in the absence of refuges). Simulations comparing monogenic versus polygenic inheritance of resistance suggested that both types of resistance evolved at approximately the same rate in the absence of refuges, but resistance took 2.2times longer to evolve in the polygenic simulations in the presence of a refuge. Resistance also evolved more rapidly at low rate of gene flow, indicating that the assumption of random mating in resistance models should be examined carefully when external refuges are present. Symmetrical, partial cross-resistance between toxins at 2 resistance loci had a negative impact on all strategies. However, at all levels of cross-resistance, full-rate mixtures continued to delay resistance longest. These simulations demonstrate that full-rate mixtures used in the presence of refuges have the potential to effectively delay resistance evolution. In the absence of refuges and an IPM strategy, however, the full-rate mixture strategy resulted in the most rapid resistance development. Half-rate mixtures were less effective than full-rate mixtures under ideal conditions, but they did not have a negative impact on resistance development in the absence of refuges and an IPM strategy and may be less risky than the full-rate strategy over a broader range of likely scenarios.