Insecticide Two insecticides were used in this study: an existing formulation (tradename: Diacon IGR+ R ; Central Life Sciences, Schaumberg, IL, USA), and a new formulation with synergist (tradename: Gravista ). Diacon IGR+ contains 11.4% methoprene and 4.75% deltamethrin, with a label rate of 0.12 kg AI/L and 0.05 kg AI/L. The label rate as a residual surface treatment gives a range of 28.5 mL AI/L−171 mL AI/L H2O to cover 94 m2 for both compounds. We used the maximum labeled rate of 24 mg AI/m2 for deltamethrin and 57 mg AI/m2 for methoprene. This corresponded to 0.3 ml of the formulation in 25 ml H2O, sprayed at the rate of 0.3 ml per 50.3 cm2 arena, using an artist’s air brush (Badger 100 series, Badger Corporation, Franklin Park, IL, US) for each treatment. Each replicate was evenly applied to the concrete dish using a compressor pump. The new Gravista formulation has one labeled rate of 684 ml formulation/L H2O to cover 92.9 m2. To achieve this, we mixed 0.5 ml of the new formulation in 10 ml H2O. This was sprayed at the same rate as the other compound. Distilled water was used for the control arenas at 0.3 mL per arena. The arenas were given 8 h to dry prior to use in experiments. Insects (20 of each species per replicate) were exposed on the insecticide-treated petri dishes for either 4 or 24 h. After exposure, individual Prostephanus truncatus and Sitophilus zeamais were removed and placed into clean Petri dish arenas and evaluated for condition. Using a stereomicroscope (SMZ-18, Nikon Inc., Tokyo, Japan) under 60× magnification, P. truncatus and S. zeamais were classified as alive (moving normally, is able to right itself when flipped over, no twitching), affected (moving sluggishly or erratically, unable to right itself, twitching of antennae or legs may be present), or dead (completely immobile even after prodding) according to prior published definitions (Ranabhat et al., 2022). Dispersal and Mortality To test dispersal capacity to new food patches, a dispersal apparatus was employed. Species-specific cohorts of 20 adults (P. truncatus or S. zeamais) were exposed to Gravista, IGR+, or an untreated control as above for 4 or 24 h, then given 48 h to disperse across 30 or 70 cm standardized sections of PVC pipe (3.175 cm ID). After exposure to insecticide formulations, insects were evaluated for condition after exposure before placing them in the dispersal apparatus. The ends of both sides of the PVC pipe were sealed with mesh (425 μm) to prevent escape. At the far end of the pipe, a hole (2 cm D) was drilled and centered over a glass jar (5 × 6.5 cm D:H) to create a pitfall trap design. The glass jar contained 20 g of whole maize kernels, representing a novel food patch, to induce insects to disperse with food kairomones. Untreated, clean, and uninfested yellow maize was used in the experiments. Grain was sourced from Heartland Mills (Marienthal, KS, USA), and frozen for 72 h prior to use to ensure no prior insect infestation was present. At the end of the sampling period, the number of insects in the jar and their mortality was scored as alive, affected or dead. In addition, the position of each individual was recorded as residing in zone 1 (at the release point), zone 2 (in first half of tube), zone 3 (in second half of tube), or zone 4 (collection jar with maize). In total, there were n = 12 replicate cohorts for each species and combination of distance and treatment. In total, 1,440 P. truncatus and 1,440 S. zeamais were tested in this experiment.
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