Resistance management in the 21st century: an entomologist's point of view

Abstract

It is probable that pest control will increasingly emphasize biotechnology as we approach the 21st century. While biotechnology promises enormous benefits in terms of accurately placing the toxin in the field, even to the point of limiting expression of the toxin to specific plant structures at specific phenological stages, the very effectiveness of these strategies may also result in rapid adaptation by insect pests. The evolution of resistance can be delayed by tactics such as providing refuges, pyramiding toxins and combining transgenic crops with alternatives strategies, such as mating disruption. In most cases, these strategies have only been examined in models of single crop systems, but most transgenic crops will exist in a mosaic of crops that may contain either the same gene expressed at different levels or different genes with various levels of cross-resistance. Resilient resistance management strategies will need to consider insect movement and exposure to these toxins within this mosaic. Recent models of single crop systems suggest that toxin mixtures in systems with external (“out-of-field”) refuges, combined with the use of economic thresholds, can significantly delay resistance. These simulations suggest that designing external refuges to manipulate source-sink dynamics can be more important in delaying resistance than the generation of nonrandom mating by the partial isolation of the refuges. While crossresistance between toxins in these single crop simulations did speed resistance evolution, the ranking of the different strategies did not differ over large ranges of cross-resistance (e.g. cross-resistance had a negative impact on all strategies). As these simulation models begin to examine mosaics of multiple crops, they must contend with the impact of crossresistance among multiple toxins in different crops, multiple resistance management strategies for the same toxins in different crops and the changes in demographics that may alter the relative importance of specific