Insecticide Resistance and Dominance Levels

Dominance has been assessed in different ways in insecticide resistance studies, based on three phenotypic traits: the insecticide concentration required to give a particular mortality (DLC), mortality at a particular insecticide dose (DML), and fitness in treated areas (DWT). We propose a general formula for estimating dominance on a scale of 0 to 1 (0 = complete recessivity and 1 = complete dominance). DLC, DML, and DWT are not directly related and their values depend on genetic background and environmental conditions. We also show that pest management strategies can have the consequence to increase DWT via the selection of dominance modifiers. Studies on resistance to Bacillus thuringiensis toxins provide the ultimate example of the complexity of the definition of the concept of dominance. Almost all studies have focused on calculation of DLC, which provides little information about the efficiency of pest management programs. For instance, one assumption of the high dose/refuge strategy is that Bacillus thuringiensis resistance must be effectively recessive (i.e., DML must be close to zero). However, DWT, rather than DML, is relevant to the resistance management strategy. Therefore, we strongly suggest that the time has come to focus on fitness dominance levels in the presence and absence of insecticide.