2.3. Leaf dip bioassay

Adults were taken from age-structured populations and were aged < 10 days old. Rice stems (10 cm cut length) were dipped into the required concentrations of formulated fiprole insecticide (ethiprole SC 200 and fipronil WG 80, Bayer CropScience, Monheim, Germany) for 20 s, air-dried and placed in a plastic specimen tube. Approximately 15 adults were aspirated directly into each tube and sealed with a ventilated lid. A small hole (3 mm diameter) was drilled in the base of each of the tubes, which were then stored vertically in a water bath (submerging only the base of each stem) at 26 °C for 72 h. Mortality was assessed and adults showing no sign of movement were scored as dead. Bioassays consisted of 3 replicates at each concentration. For synergism assays, each insect was treated upon the pronotum with 0.2 μL of 100 mg/L^− 1 piperonyl butoxide (PBO in acetone) (20 ng adult^− 1) and then transferred to rice stems dipped in fipronil. Mortality was assessed at 48 h.