EVOLUTION OF PESTICIDE RESISTANCE AND SLOPE OF THE CONCENTRATION MORTALITY LINE - ARE THEY RELATED

Researchers have often assumed that slopes of concentration-mortality lines from pesticide bioassays change predictably as resistance evolve s. In particular, many authors have used differences in slope among co nspecific populations to make inferences about the progression of resi stance and the genetic variation available for further increases in re sistance. Such inferences are based on the assumption that slope is st eepest in populations that are highly susceptible or highly resistant (i.e., such populations have less genetic variation than heterogeneous populations with intermediate levels of resistance). We tested this h ypothesis independently for each of 41 sets of bioassay data of conspe cific Variation in susceptibility to pesticides obtained from 26 artic les published in the Journal of Economic Entomology during the past de cade. These data represent 18 species of insects and mites and 25 pest icides. We used polynomial regression and a nonparametric approach to examine the relationship between slope and LC(50) for each set of bioa ssay data. We found that, for most pest and pesticide combinations, sl opes of the concentration-mortality lines were not highest in the popu lations with highest and lowest LC(50)s. This result suggests that slo pe was not a good indicator of genetic Variation in susceptibility or that genetic Variation in susceptibility was not related simply to LC( 50) The slope of the concentration-mortality line provides information about the phenotypic variation within a population, including genetic and environmental variation. If most of the phenotypic variation is n ot genetically based, variation in estimates of slope among conspecifi c populations may be caused primarily by changes in environmental vari ation and errors of estimation.