DECLINE OF PYRETHROID RESISTANCE IN THE ABSENCE OF SELECTION PRESSUREIN A POPULATION OF GERMAN COCKROACHES (DICTYOPTERA, BLATTELLIDAE)

The level of resistance to seven pyrethroids was followed in a laborat ory-selected population of Blattella germanica (L.) for 15 generations , during which time it was not exposed to any insecticide. As estimate d by LT50 resistance ratios (RRs), resistance declined in a stepwise m anner to preselection levels with all of the pyrethroids except pyreth rins. I also compared the levels of resistance as indicated by RRs and by gene frequency (GF) estimates. Results showed that GF estimates pr ovide a more precise indication of the changes in resistance that occu r over time than do RRs alone, especially when combined with RR and en d-point mortality data. I discuss my results from the standpoints of t he number of genetic mechanisms involved in pyrethroid resistance in t his insect, and what the declines in resistance mean for future cockro ach control.