Comparison of two bioassay methods for determining deltamethrin resistancein German cockroaches (Blattodea : Blattellidae)

Susceptibility to deltamethrin of 10 strains of the German cockroach, Blatt ella germanica (L.), trapped in hotel kitchens in Singapore was determined. Two resistance detection bioassay methods (topical application and World H ealth Organization glass jar method) were compared. Resistance ratios based on 50% knockdown, obtained by comparison with the S strain, ranged from 17 .7 to 4,235 for topical application and from 2.2 to 22 for the glass jar me thod. A field strain, with consistently low resistance ratios (topical meth od = 17.7, glass jar method = 2.2), was identified as a potential held stra in to be used as a baseline for comparison with other field strains. Resist ance ratios for the other field strains obtained by comparison with the R5 strain ranged from 24.5 to 239 for topical application and from 1.2 to 9.8 for the glass jar method. The results of our study demonstrate that deltame thrin-resistant German cockroaches are numerous in Singapore. Comparison be tween the two bioassay methods showed that there was significant correlatio n between KD50 and KT50 values. The glass jar method is similar to field si tuations but topical application is sensitive enough to define the magnitud e of resistance. Differences between the two defection bioassays and the fa ctors governing the choice of bioassay in monitoring resistance in German c ockroaches are discussed.