Abstract
Sexes and nymphal age classes of insecticide-susceptible and resistant strains of the German cockroach, Blattella germanica (L.), were evaluated for susceptibility to bendiocarb, chlorpyrifos, and cypermethrin. In all cases, late-stage nymphs had higher LC50s (µg per insect) and LD50s (µg/mg) than adult males. In most cases, these nymphs were also more difficult to kill than adult females. This reduced susceptibility to insecticides in late-instar nymphs occurred irrespective of weight differences, type of insecticide, or strain of cockroach. Females had significantly higher LC50s (µg per insect) than males for bendiocarb (resistant strain), chlorpyrifos (susceptible and resistant strains), and cypermethrin (susceptible strain). However, LD50s (µg/mg) for males and females were the same for bendiocarb (susceptible and resistant strains), chlorpyrifos (susceptible strain), and cypermethrin (susceptible and resistant strains). Resistance ratios for bendiocarb, chlorpyrifos, and cypermethrin were higher for some life stages and sexes than for others. Resistance ratios were 6-10 for bendiocarb in adult males and females, but ratios for 4- to 6-wk-old nymphs were 5-100. For chlorpyrifos, the resistance ratio was 7 for males but 20 for females. Males were 32-fold resistant to cypermethrin but 3- to 6-wk-old nymphs were 44-94-fold resistant.
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