Evaluation of Three Methods for Detecting Chlorpyrifos Resistance in German Cockroach (Orthoptera: Blattellidae) Populations

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Abstract
Three methods for measuring German cockroach, Blattella germanica L., susceptibility to formulated chlorpyrifos (Dursban LO) were compared. Adult progeny from a cross between two field-derived colonies were compared with a susceptible laboratory strain through exposure to chlorpyrifos and xylene applied topically, and in glass jars at single and multiple doses. Resistance ratios (LX50 field ÷ LX50 susceptible) were 14.3, 7.8, and 2.0 for the topical application, multiple, and single concentration exposures, respectively. The single dose method failed to detect any differences in susceptibility (resistance ratio < 3.0). Topical application should be considered when surveying for resistance to organophosphate insecticides or when lethal time values obtained from jar test do not agree with empirical observations.