Examining long-term trends in insecticide resistance for Western tarnished plant bug Lygus hesperus in cotton

In western cotton production, the Western tarnished plant bug, Lygus hesperus Knight, is a key pest. It is primarily managed with insecticides. As a result, the development of insecticide resistance is likely inevitable. Whether the development of insecticide resistance is similar across multiple locations and sampling times yet to be explored. Here, we make use of a long-term dataset of resistance bioassays to lygus bug. We collected adults from three locations (West Side, Shafter, and Kearney in California) twice a year (spring: May-June and fall: August-September) from 2008 to 2020. Two time points had been selected to detect shifts in susceptibility within a growing season. For the bioassay, we used a floral-foam vial assay (foam soaked with honey water) for Transform® and Belay® and zip-lock bag assays for Capture® and Vydate®. We used six concentrations for each insecticide, including an untreated control. For each concentration, we used 30 lygus adults and assessed the mortality 8-h or 24-h after adding the adults depending upon the active ingredients (AI). We analyzed the adult mortality trend over the sampling years using generalized linear model (glm) and lethal dose (LC50) using dose-response curves (drc) package of each AI by locations. We found the adult mortality rate treated with Belay® and Transform® decreased through time (years). Three out of four insecticides had a greater LC50 in West Side. These results provide a foundation for insecticide selection for the management of lygus bugs in cotton and for understanding shifts in insecticide susceptibility for this pest.