A comparative study of insecticide resistance assays with the German cockroach

Abstract Females of a diazinon‐resistant strain of German cockroach exhibiting cross resistance to propoxur were evaluated for resistance to diazinon and propoxur by six assays: LD50s for topical application and injection; LC50, KT50 and LT50 for deposits on wood and KT50 for deposits on glass. Also investigated were certain variables in the topical application technique and the experimental conditions for topical synergism studies. Among all assays, diazinon resistance ratios (resistant:susceptible) ranged from 3‐ to 145‐fold; propoxur varied from 1‐ to 14‐fold. Resistance ratios based on KT50 data were consistently low (<4x) for both insecticides. Resistance to diazinon was 13x when applied topically and 6x when injected. Resistance to propoxur was 14x by injection compared to 8x for topical application. The LC50 assay exhibited the greatest difference in resistance to the two insecticides: namely, 145x to diazinon and 1x(no resistance) to propoxur. By LT50 analysis, resistance to propoxur was 10x compared with 100x resistance to diazinon. Thus, the method used had a significant effect on the final value of the resistance ratio. Even though resistance ratios may vary widely among different techniques, the final choice of a method depends upon many factors. The LT50 method may provide a more realistic appraisal of resistance in wild populations of German cockroaches especially when experimental residues are similar to those used in control programmes.