Permethrin resistance ratios compared by two methods of testing nymphs of the German cockroach, Blattella germanica
2000
Ladonni, H.
For the German cockroach, Blattella germanica L. (Dictyoptera: Blattellidae), the permethrin resistance ratio (RR) was assessed by topical application and by tarsal contact tests, using first-instar nymphs of five strains from Tehran, Iran. Each test was replicated three or four times with 10 nymphs aged 2-3 days; mortality was scored 24 h post-treatment. The reference susceptible strain showed LD50 permethrin 0.0175 microliter/nymph from topical application, KT50 of 8.41 min and LT50 of 12.82 following tarsal contact with permethrin 15 mg/m2. In four wild strains (F1 generation) the RR varied from 4.14 to 4.7 for mortality after topical application, from 4.2 to 6.45 for mortality after topical application, from 4.2 to 6.45 for mortality and 17-27 for knockdown following tarsal contact tests. Hence, overall knockdown results gave much higher RRs than for mortality data. Resistance ratios based on both methods of treatment were very similar: one strain showed a slightly higher value by topical application (RR 4.6 vs. 4.2, i.e. 1.1-fold difference) whereas the other three strains gave slightly greater RR (1.2-1.4 fold) by tarsal contact. Resistance was abolished by cotreatment with the synergist piperonyl butoxide plus permethrin (ratio 3:1 required for full efficacy), indicating that mixed-function oxidases were inhibited as a major metabolic pathway in all four resistant strains.
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