A synergist increases short-term efficacy of long-lasting insecticide-incorporated netting against pyrethroid-resistant maize weevil, Sitophilus zeamais

Principal active ingredients used in chemical control tactics after harvest are pyrethroids, including for long-lasting insecticide-incorporated netting (LLIN). However, pyrethroid resistance by stored product insects has become widespread. Thus, the aim of our study was to evaluate whether a synergist, piperonyl butoxide (PBO), could rescue efficacy of alpha-cypermethrin LLIN against a field strain and pyrethroid-resistant strain of maize weevil, Sitophilus zeamais (Motschulsky) in the laboratory. Adult S. zeamais were first exposed to the vials treated with PBO or acetone (solvent control) for 1 h or 3 h, then exposed to either alpha-cypermethrin LLIN or control netting for 1 h or 3 h. Immediate mortality was recorded directly after exposure, as well as delayed mortality at 24, 48, 72, and 168 h later with adult conditions recorded as alive, affected, or dead. At 1-h exposure, the addition of PBO significantly reduced the percentage of field strain S. zeamais adults alive by 7–42% after subsequently exposed to LLIN compared to the control, but PBO did not significantly affect the percentage of alive pyrethroid-resistant adults. After a 3-h exposure, there were significantly fewer field strain (by 24–47%) and pyrethroid-resistant (by 13–36%) individuals alive when exposed to PBO compared to the control. PBO elicited quicker mortality for the pyrethroid-resistant strain. We confirmed in a separate assay that our susceptible laboratory strain was more susceptible than our field and pyrethroid-resistant strain of S. zeamais. Our results suggest that the addition of a synergist to LLIN formulations may improve efficacy against stored product insects and support resistance management.