The Influence of Selection Pressure on the Development of Resistance in Populations ofTetranychus telarius (L.)1

Abstract

A program of building strains of the two-spotted spider mite, Tetranychus telarius (L.), resistant to parathion under 90 .F. constant temperature has been carried through 45 generations treating all stages by the aerosol method and allowing the survivors to interbreed. All strains were initially sampled from a highly susceptible wild population (N-8) and were distributed among two levels of selective intensity (high and low). The results of this program lead to the following conc1nsions. (1) Resistance develops more rapidly under high selection pressure; (2) Under regular periodic selection the same level of resistance is reached in time regardless of selection pressure; (S) Low selection pressure in this program appears ultimately to produce more homogeneity than high selection pressure, Greater homogeneity may be due to chance variability or a difference in the “quality” of resistance as defined in he text; (4) High levels of resistance can be demonstrated after one severe selection. Early appearance of resistance may be related to the failure of the Control Strain to demonstrate 100% mortality when exposed to selection treatments.