Stage-Dependent Tolerance of the German Cockroach, Blattella germanica for Dichlorvos and Propoxur

tage-dependent dichlorvos and propoxur tolerance in a field population of the German cockroach, Blattella germanica Linnaeus (Blatodea: Blattellidae), was investigated in the laboratory using a topical application bioassay. The results showed the 6 week-old nymphs were more tolerant to dichlorvos and propoxur than the other ages tested. LD₅₀ values of dichlorvos and propoxur for the 6 week-old nymphs were 2.003 µg per insect and 5.296 µg per insect, respectively. Tolerance ratios of 18.55-fold and 4.98-fold for LD₅₀ were obtained from 6-week-old nymphs compared to 4 week-old nymphs. The specific activity of acetylcholinesterase (AChE) from 1 week-old nymphs was the highest among all tested developmental stages of nymphs and adult males and females. The specific activity of AChE decreased significantly with increasing age. The sensitivity of AChE to dichlorvos was the highest with a k(i) value of 3.12 × 10⁴ mol⁻¹min⁻¹ in the last nymphal stage of B. germanica (about 6 weeks-old). The AChE from 4 week-old nymphs was the most sensitive to propoxur, with the highest k(i) value being 2.63 × 10⁵ mol⁻¹ min⁻¹. These results indicated that the different developmental stages and sexes of B. germanica affected the inhibition of AChE by dichlorvos and propoxur.