FIELD PERFORMANCE AND BIOLOGICAL FITNESS IMPLICATIONS OF AN INSECTICIDE ROTATION STRATEGY WITH RESISTANT GERMAN COCKROACHES

Abstract:
Field and laboratory studies were initiated in 1993 to investigate the effect(s) of specific insecticide use strategies on resistant populations of German cockroach, Blattella germanica. Infestation levels were monitored using sticky traps, and these data formed the basis for the estimates of treatment efficacy at each trials site. The resistant status of each population was gauged, initially, using field test kits at a discriminating dose, and then determined by topically applying insecticides to males of the F1 generation after field-collected cockroaches had been reared in the laboratory. Further tests were done to establish the resistance of strains following various periods of time in culture, without insecticide pressure, to establish the stability of the resistance. The factor of resistance (FOR) to both pyrethroids and organophosphates (OPs) declined, markedly, over 4-12 generations in the 4 most resistant strains studied. In 3 other strains that were less resistant, the trend for declining FOR was present, but the magnitude of the change was lower. Field sites were chosen to help demonstrate the effect of continuous use of one or another active ingredient, or the alternate use of those 2 insecticides. A key feature of this study was the measurement of the biological fitness (reproductive potential) of the field strains, since a successful 'rotation' strategy requires that the resistance to any given compound (or chemical class) declines when that compound is not in use, because resistance to it carries a fitness cost. Simulation demographic studies were used to compare the fitness of different strains. In every case, insecticide resistance was associated with a significant fitness cost, such that the reproductive potential of the field strains from 2 of the sites was more than 10 times less than the reference susceptible strain. These data are discussed in terms of the rationale and justification for insecticide rotation as a practical resistance management strategy for Blattella.