INSECTICIDE RESISTANCE DECLINE AND SELECTION IN LABORATORY-REARED GERMAN COCKROACHES (DICTYOPTERA, BLATTELLIDAE)

The level of cypermethrin resistance was monitored to determine the hi story of a laboratory-reared, pyrethroid resistant strain of German co ckroach, Blattella germanica (L.). The LC(50) (mu g [Al] per insect) o f cypermethrin decreased from 2.29 mu g (AI) per insect to 0.68 mu g ( Al) per insect within 23 mo in laboratory culture when no selection pr essure was applied. Subsequently, newly hatched nymphs (<24 h old) col lected from the last unselected generation (parent generation) were se lected with residual cypermethrin in a glass jar for 6 generations. Mo rtality (24 h) of newly hatched nymphs exposed to residual deposits of 2 mu g (AI)/cm(2) decreased from 61% of the parent generation to 38% of the selected F-6 generation. The LC(50) of the selected Fs generati on adults was elevated 3-fold compared with the parent generation. Whe reas our selection of Ist instars was unique, resultant increase in re sistance was similar to previous selection processes.