Relevance of Resistance Ratios to Operational Control in the German Cockroach (Dictyoptera: Blattellidae)

A simulated crack and crevice method of exposure to insecticides was developed to ascertain the relationship between mortality produced by field-application rates and resistance ratios obtained by the time-mortality method. Nymphal German cockroaches, Blattella germanica (L.), from strains of widely varying resistance backgrounds were tested with chlorpyrifos, acephate, cyfluthrin, and cypermethrin. I observed an inverse relationship between resistance ratios and 72-h crack and crevice mortality in all 4 cases. Gene frequency estimates paralleled resistance ratios for cyfluthrin and cypermethrin. I concluded that time mortality resistance ratios provide a good indication of the level of resistance in a given strain, and that resistance ratios >3.0 signal potential control failures in the field.