Evaluation of three methods for detecting permethrin resistance in adult and nymphal Blattella germanica (Dictyoptera: Blattellidae).

The insecticide resistance status of the German cockroach, Blattella germanica (L.), against permethrin was evaluated with three different bioassay methods. The LT50 and KT50 in adults were measured by glass jar test, the LT50 and KT50 in nymphs were assessed by glass petri dish-test, and the LD50 in adults and nymphs was measured by topical application. The results indicate that in all three bioassays, cockroaches showed different degrees of resistance to permethrin. Resistance ratios of 6.7-, 12.8-, and 7.5-fold for KT50, LD50, and LT50, respectively, were obtained for adult cockroaches compared with those from a susceptible strain. Similarly, resistance ratio values of 17-fold for KT50, 8-fold for LD50, and 6.4-fold for LT50 were obtained for nymphs compared with those from a susceptible strain. Although both adults and nymphs exhibited a similar level of susceptibility to permethrin, adults showed a higher degree of susceptibility to topical application than to the other exposure method. The order of sensitivity (based on resistance ratio values) was topical application (LD50) > LT50 > KT50 in adults. For nymphs, the resistance ratio in the knockdown test was higher than in other tests (resistance ratio = 17-fold). Therefore, the order of test sensitivity based on comparisons of resistance ratio values was KT50 > topical application (LD50) > LT50 in nymphs. Topical application exhibited a higher sensitivity than the LT and KT methods in adults. Thus, the results suggest that topical application and knockdown testing methods are the most sensitive assays for measuring resistance of adult and nymphal German cockroaches, respectively, to permethrin.