Insecticide resistance in German cockroaches (Blattella germanica) from hospitals and households in Taiwan

Since a high percentage of cockroach infestation has been determined in hospitals in Kaohsiung, South Taiwan, insecticide resistances may have developed in the cockroach populations. To determine the resistances in cohorts of German cockroach (Blattella germanica) from hospitals and households to organophosphates, carbamates and pyrethroids, cockroaches were trapped from 30 hospitals and 30 households in this area. Bioassays by topical application were then carried out on the first generation bred in the laboratory. Resistance ratio (RR) was determined by comparing the LD50 of the collected cohorts with that of a susceptible cohort. RR ranges of chlorpyrifos, propoxur and cypermethrin in the hospitals were 2.04 - 28.80, 2.86 - 30.86 and 1.95 - 14.05 and those in the household were 1.92 - 17.72, 6.93 - 62.50 and 2.80 - 27.35 respectively. Significant correlations among the RRs of the three insecticides were separately found in the hospitals and households, indicating the existence of cross resistance among the three groups of insecticides. The patterns of propoxur > chlorpyrifos > cypermethrin in hospitals and propoxur > cypermethrin > chlorpyrifos in households reflect that chlorpyrifos is more frequently used in hospitals than in households. In contrast, cypermethrin is used more frequently in households.