CHANGES IN AN INSECTICIDE-RESISTANT FIELD POPULATION OF GERMAN-COCKROACH (DICTYOPTERA, BLATTELLIDAE) AFTER EXPOSURE TO AN INSECTICIDE MIXTURE

Combinations of sticky trap monitoring toxicity test, and biochemical assays were conducted to study the effects of an insecticide mixture o n an insecticide-resistant field population of the German cockroach, B latella germanica (L.). Monthly treatments (6 total; month 1-6) were m ade, the 1st treatment consisted of a formulated pyrethroid insecticid e (lambdacyhalothrin)+synergist (piperonyl butoxide)+juvenile hormone analog (pyriproxyfen), and the following 5 consisted of lambdachalothr in+pyriproxfen alone. This study was monitored for 12 mo. We found red uctions in sticky trap catches (from 27.9 to 2.6 cockroaches trapped p er apartment at the beginning and end of the study, respectively); inc reases in the percentage of trapped adults having twisted wings (an in dicator of juvenile hormone analog-induced sterility); and reduced rat ios of nymphs:adults and gravid:nongravid females. For F-1 offspring o f cockroaches live-trapped before insecticide treatments, topical toxi city tests revealed the following:resistance to the active ingredient used on the population for 8 previous years (cypermethrin; LD(50) resi stance ratio [RR]=82.2), a reduction in cypermethrin resistance follow ing pretreatment with piperonyl butoxide (90.2% at LD(50)), and cross- resistance to chlorpyrifos (LD(50) RR=5.22). In surface-contact (jar) tests, significant tolerance was also found to cypermethrin (LT(50) RR =7.3), chlorpyrifos (LT(50) RR=1.2), and lambdacyhalothrin (LT(50) RR= 1.5). In F-1 offspring of cockroaches collected from the field after a n 80.0% reduction in trap catch, jar tests with lambdacyhalothrin indi cated increased tolerance over the original population (25.0% at LT(50 )). Similarly, biochemical assays of hydrolytic and oxidative enzymes indicated increases in 1-naphthyl acetate and 2-naphthyl acetate hydro lysis, and in total cytochrome P450 content, but not change in p-nitro phenyl acetate hydrolysis. We discuss results in terms of the effects of this strategy on resistance levels, suspected resistance mechanisms , and the further development of resistance management strategies.