Three bioassay methods fipronil toxicity in German cockroaches (Blattodea: Blattellidae)

Susceptibility to fipronil of 9 strains of the German cockroach, Blattella germanica (L.), trapped in student dormitories in Tehran was determined. Three resistance detection bioassay methods (topical application, World Health Organization glass jar and oral toxicity) were compared. Mortality data from the replicates were pooled and was assessed by probit analysis. The susceptible strain showed LD50 of 0.96 ng, LT50 of 11.3 min and LT50 of 47.1 h for topical application, World Health Organization glass jar and oral toxicity methods, respectively. Field strains showed 0.94 to 2.6 ng at LD50, 11.6 to 19.7 min at LT50, and 30.5 to 45.4 h at LT50 for topical application, World Health Organization glass jar and oral toxicity methods, respectively. Resistance ratios based on 50% mortality, obtained by comparison with the SS strain, ranged from 0.98 to 2.6 for topical application and from 1.0 to 1.7 for the glass jar and from 0.6 to 0.8 for the oral toxicity method. The results of our study demonstrate that German cockroaches are susceptible to fipronil whereas a few field strains showed 1.5 to 2.6-folds very low-level tolerant to fipronil in Iran. Comparison among the three-bioassay methods showed that there was positive and negative significant correlation between LD50 and LT50 values. The glass jar method is similar to field situations but topical application is sensitive enough to define the magnitude of resistance. Differences among the three detection bioassays and the factors governing the choice of bioassay in monitoring resistance in German cockroaches are discussed.