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 pyrethro id insecticides-cypermethrin, cyfluthrin, and esfenvalerate-in a field population (RHA) of German cockroaches, Blattella germanica (L.), in urban apartments buildings. Applications of all py rethroids to RHA-in fested apartments stopped in July 1990, and control was provided prima rily by chlorpyrifos. By using resistance ratios (RR) at LC(50), we us ed the level of cypermethrin, cyfluthrin, and esfenvalerate resistance in the RHA population periodically to monitor field responses of coll ected 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 1 991 declined to 4.6 in 1993. The percentage reduction of apartment inf estations of RHA 2 wk after application of 0.2% (AI) cypermethrin incr eased from 20% in 1990 to 76% in 1993 after 3 yr without exposure. Per cent reduction of RHA infestation provided by cyfluthrin increased fro m 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 indicati ons of the return of limited pyrethroid susceptibility to the RHA popu lation.