Data from: A synergist increases efficacy of long-lasting insecticide-incorporated netting against pyrethroid-resistant maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae)

Experimental Insects

Four to eight-week mixed-sex adults of a field strain from Eastern Kansas (collected in 2022, hereafter FS-22) and pyrethroid-resistant strain collected from Juiz de Fora County in the state of Minas Gerais, southeastern Brazil in 2006 (hereafter, Brazil-resistant) S. zeamais were used in this study. The Brazil-resistant strain exhibits high pyrethroid resistance and low fenitrothion resistance and has been used in prior studies (Guedes et al., 2006a). These strains were reared and maintained on tempered organic maize at 25–27.5 °C, 65% RH, and 16:8 (L:D) h photoperiod.

Synergist-coated glass vials

In this study, we used one of the most effective synergists, piperonyl butoxide (PBO, Tokyo Chemical Industry Co. Ltd., Tokyo, Japan). Briefly, each of 20-ml glass scintillation vials was coated with 0.5 ml of PBO solution in acetone (solvent) at 0.1 mg/ml by using a Roto-Torque Heavy Duty Rotator (Model 7637, Cole-Parmer Instrument Company, Vernon Hills, IL, USA). For the control, vials were treated with 0.5 ml of acetone (solvent) only.

LLIN treatment

We used 0.34% alpha-cypermethrin based LLIN (63.2 mg/m2 active ingredient (a.i.), 40 deniers, 100 holes/cm2; Carifend®, BASF, Ludwigshafen, Germany) and a netting physically identical but without insecticide (Casa Collection, Mesh White, 1721-9668; Jo-Ann's, Hudson, OH, USA) as a control netting in our study.

Effects of synergists on LLIN against S. zeamais

A cohort of 20 mixed-sex S. zeamais adults was first pre-exposed to each scintillation vial coated with PBO or acetone (control) for 60 min (1 h) or 180 min (3 h). The pre-exposed adults were then transferred to each plastic Petri dish (9 × 9 cm square) containing either LLIN or control netting and were exposed for 60 min or 180 min. The inside walls of the dishes were coated with a polytetrafluoroethylene (PTFE) preparation (e.g., fluon, 60 wt% dispersion in water, MilliporeSigma GmbH, Steinheim, Germany) to prevent insects from escaping. After exposure, insects were placed in an environmental chamber under constant conditions (30°C, 65% RH, and 16:8 L:D). A total of n = 5 replicates were performed per treatment combination of strain, exposure time, netting type, and synergist. Immediate mortality was recorded directly after exposure, as well as delayed mortality at 24, 48, 72, and 168 h later. Insect conditions were recorded as alive, affected, or dead as described by Ranabhat et al. (2022). Specifically, insects moving normally were considered alive, whereas they were considered affected if they moved in an uneven pattern and/or exhibited twitching of tarsi or antennae or showed lethargic or drunken movements. On the other hand, insects were considered dead if no visible movement was observed after disturbance with a fine brush.

Lethal exposure assay to determine the susceptibility of S. zeamais

For this assay, a cohort of 20 mixed-sex adults of laboratory (pyrethroid-susceptible, FS-22) or Brazil pyrethroid-resistant strain of S. zeamais was exposed each 20-mL glass scintillation vial coated with a 2 mg/ml deltamethrin solution in acetone (solvent) or acetone only (control) at constant conditions (27.5° ± 0.1 C, 65% RH, 16:8 L:D) in an environmental chamber. Each of the three insect conditions including alive, affected, or dead as described above was recorded at each of 12 time points (i.e., 1, 2, 4, 6, 24, 48, 72, 96, 144, 168, 192, and 216 h) after the exposure. To examine the insect’s conditions, the exposed adults from each vial were transferred to each plastic Petri dish (90 mm in diameter; 59.4 cm2 bottom surface area) with a lining of a filter paper (85 mm D, Grade 1, GE Healthcare, Buckinghamshire, United Kingdom) that was adhered to the bottom using double-sided tape. The inside walls of the dishes were covered with a polytetrafluoroethylene preparation (Fluon, 60 wt% dispersion in water, MilliporeSigma GmbH, Steinheim, Germany) to prevent insects from escaping. The insect conditions were assessed under a microscope (SMZ18, Nikon Instruments Inc., Minato City, Japan). We performed a total of n = 4 replications per treatment combination for each strain of S. zeamais.