MEASURING HYDRAMETHYLNON RESISTANCE IN THE GERMAN COCKROACH, BLATTELLA GERMANICA (L.)

Abstract:
In a field population (RHA, Roanoke VA, USA) of Blattella germanica a decline in the efficacy of hydramethlynon has been documented for eight years. In 1987 >85% reduction of apartment infestations was achieved 4 wk after application, in 1990 reduction declined to 73%, and in 1995 reduction declined to 60%. The susceptibility to five concentrations of hydramethylnon in RHA was compared to a susceptible strain (VPI). The RHA strain demonstrated a low to moderate level of resistance to hydramethylnon. Delivery of an equal dose of insecticide is important when using a feeding bioassay to determine resistance to insecticides formulated as baits. Resistance ratios at KT50 or KT90 based on feeding may accurately depict resistance at concentrations of hydramethylnon that provide for a nearly equal amount of bait consumed by the field and susceptible strain. Following starvation there may be a nearly equal amount of bait and active ingredient consumed so that RRs based on LT50s accurately depict the level of resistance. Bioassays based on topical application were free of problems associated with feeding. A discriminating dose of 5 pg provided consistent RRs based on LT50 and KT50 estimates