FORCED CONTACT AND ARENA BIOASSAYS TO ASSESS THE PERFORMANCE OF A PYRETHROID WP DEPOSIT AGAINST ORIENTAL COCKROACHES

Abstract:
LT5Os and LT95s were determined at 28°C for adult female Blatta orientalis over deposit concentrations varying from 15-200 mg A1 m-' on a plywood surface treated with a WP formulation of cypermethrin. Mortality was determined 3 days following removal from the deposits. Regression equations for these parameters were LT50 (min) = 31.8-11.8 log [surface concentration (mg AI m-2)] (r2 = 0.896) LT95 (min) = 65.0-25.1 log [surface concentration (mg A1 m-2)] (r2 = 0.875) These results were compared with mortalities obtained when the cockroaches were allowed free movement in arenas containing a harbourage and a treated strip of the WP deposit (40 mglm2) positioned either adjacent to the harbourage, across the centre, or at the far end of the arena with food / water stations placed on the deposit. The arenas were illuminated on a 12:12 photocycle. 50 adult females were introduced into each arena at the start of a light phase and allowed to condition it for 84 h. Treated strips were introduced prior to the start of the subsequent dark phase, and cockroach emergence, distribution and knockdown monitored during this dark phase. Mortality was recorded 3 and 6 days after introduction of the plates. Positioning the deposit either at the harbourage end or the food / water end of the arena gave >95%, knockdown within 6-7 h and >95%) mortality at 3 days, consistent with the high proportion of the time which active cockroaches spent in these two locations during the first 3 h of the dark phase. Most of the cockroaches surviving the first dark phase also survived the subsequent 6 dark phases. In control arenas a maximum of 30%) of the population was active in the arena at any time during the dark phase, whereas contact with treated strips placed at the harbourage / food and water ends produced a flushing action 2-4 h into the dark phase in which >95% of the population emerged over a period of approximately 1 h. The flushing effect produced faster knockdown and a greater percentage mortality than would have been anticipated on the basis of the forced contact bioassays and initial pattern of emergence.