Tarsal exposure is considered the most common route of insecticide uptake by the German cockroach, Blattella germanica (L.), in the field. To monitor resistance, we developed and used a procedure for making topical applications to the tarsal pads. We also examined the more conventional topical-thoracic and jar methods of insecticide exposure. Chlorpyrifos and cypermethrin were used to test these exposure methods on one susceptible and two resistant strains of the German cockroach. Two statistically based ratios (able to indicate significant differences) were used to compare probit analysis results from these exposure methods. First, the resistance ratio (at LD or LT(50) and LD or LT(95)) was used to make comparisons for all three exposure methods. Second, the tarsal-thoracic ratio was used to specifically compare lethal doses (at LD(50) and LD(95)) from tarsal applications to thoracic applications. An analysis of variance (ANOVA) procedure was also used to analyze three factors contributing to the production of the resistance ratios. Results of resistance ratio by strain and insecticide showed a rank of tarsal > thoracic > jar for all but three comparisons. Tarsal-thoracic ratios for cypermethrin indicated no significant differences between tarsal and thoracic LD(50)s and LD(95)s, but did indicate significant differences for chlorpyrifos, Finally, ANOVA of resistance ratios showed that the effects of strain had the most variation, followed by bioassay (i.e., exposure method) in the production of resistance ratios in this study. We discuss the; implications of the resistance ratio, tarsal-thoracic ratio, and results of ANOVA, its well as the benefits of accurately determined body weight bioassays. We emphasize relevant uses for the tarsal application and statistically based ratio methods, and how our results affect the process of selecting insecticide exposure methods to be used for monitoring resistance in the German cockroach.
