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

Authors: RANABHAT, SABITA - Kansas State University; QUELLHORST, HANNAH - Kansas State University; BLACK, BRANDON - University Of Nebraska; ANDERSEN, JAYCOB - University Of Nebraska; BINGHAM, GEORGINA - University Of Nebraska; ZHU, KUN YAN - Kansas State University; Morrison Iii, William

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2025
Publication Date: 4/10/2025
Citation: Ranabhat, S., Quellhorst, H.E., Black, B., Andersen, J., Bingham, G.V., Zhu, K., Morrison Iii, W.R. 2025. A synergist increases efficacy of long-lasting insecticide netting against pyrethroid-resistant maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae). Journal of Economic Entomology. https://doi.org/10.1007/s10340-025-01884-4.
DOI: https://doi.org/10.1007/s10340-025-01884-4

Interpretive Summary: Pyrethroids are widely used insecticides to control stored product insects and are often incorporated into long-lasting insecticide netting (LLIN). However, resistance to pyrethroid insecticides by stored product insects has become widespread. Chemicals known as synergists can be added to insecticides to increase their effectiveness and overcome resistance. In this study, we sought to evaluate whether exposure to the synergist piperonyl butoxide (PBO) could increase the efficacy of an LLIN incorporated with a pyrethroid against two different pyrethroid resistant strains of maize weevil, a widespread post-harvest pest of corn. We found that exposure to PBO for 1 hr increased mortality to LLIN by up to 42% for only one of the two strains. However, exposure to PBO for 3 hr increased mortality to LLIN for both strains, by up to 47% for one strain and up to 36% for the second strain. In addition, mortality was faster for both strains when PBO was added. Our results suggest that the addition of a synergist to LLIN formulations may improve efficacy against stored product insects and support resistance management.

Technical Abstract: A principal active ingredient used in chemical control tactics after harvest are pyrethroids, including for long-lasting insecticide-incorporate 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 and pyrethroid-resistant strain of S. zeamais in the laboratory. Sitophilus zeamais adults were first exposed to the vials treated with PBO or acetone 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 acetone, 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 an acetone solvent. 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.