Instability of Cypermethrin Resistance in a Field Population of the German Cockroach (Orthoptera: Blattellidae

We initiated a program to reduce the level of resistance to 3 pyrethroid insecticides-cypermethrin, cyfluthrin, and esfenvalerate-in a field population (RHA) of German cockroaches, Blattella germanica (L.), in urban apartments buildings. Applications of all pyrethroids to RHA-infested apartments stopped in July 1990, and control was provided primarily by chlorpyrifos. By using resistance ratios (RR) at LC 50 , we used the level of cypermethrin, cyfluthrin, and esfenvalerate resistance in the RHA population periodically to monitor field responses of collected cockroaches for 13 generations (F 1 -F 13 ). The resistance ratio for cypermethrin declined from 180 in July 1990 to 123 in March 1991 (F 2 ) to 66 in October 1991 (F 5 ), to 41 in October 1992 (F 9 ), and to 2.5 in October 1993 (F 13 ). For cyfluthrin resistance, the 27 RR (1990) declined to 3.3 in 1993. For esfenvalerate resistance, the 17 RR in 1991 declined to 4.6 in 1993. The percentage reduction of apartment infestations of RHA 2 wk after application of 0.2% (AI) cypermethrin increased from 20% in 1990 to 76% in 1993 after 3 yr without exposure. Percent reduction of RHA infestation provided by cyfluthrin increased from 49% in 1991 (F 1 ) to 74% in 1993 (F 13 ). The resistance ratio based on LD 50 and percentage reduction data appear to be reliable indications of the return of limited pyrethroid susceptibility to the RHA population.