Effects of methoprene and synergized pyrethrin aerosol applications on Tribolium castaneum (Herbst) populations

• Hidden refugia enable Tribolium castaneum to escape exposure to aerosols.
• Horizontal transfer of the IGR can occur but may be limited with established populations.
• Repeated applications may be necessary for complete control.

The occurrence of horizontal transfer of the insect growth regulator (IGR) methoprene on confined populations of Tribolium castaneum (Herbst), either with or without hidden refugia, was determined through a series of experiments. Multiple applications were made with the IGR alone or combined with synergized pyrethrin, and compared to untreated controls which received no insecticide applications or were treated with the carrier Isopar M that was a component of the pyrethrin formulation. The total number of living beetles from test colonies inoculated with adults that were treated with Isopar M or with no adults (control) was significantly greater than those colonies that were inoculated with adults treated with either synergized pyrethrin or methoprene (P < 0.05). There was no difference in the number of living individual in the larval, pupal, and adult stages and the instantaneous rate of increase (ri) in established populations treated with methoprene and containing a hidden refugia compared to those which received the pyrethrin applications (P ≥ 0.05). Sanitation levels of two different flour quantities nested inside the treatment groups also had no effects (P ≥ 0.05), suggesting that populations in hidden refugia can persist even with multiple applications of methoprene and synergized pyrethrin. Populations with an accessible hidden refugia that were exposed to synergized pyrethrin and methoprene had a lower number of living adult and a lower ri value than populations that were exposed to synergized pyrethrin alone (P < 0.001). Additionally, populations which received one, two, or three aerosol applications had similar numbers of living adults and ri but were significantly different from populations which received four aerosol applications. Results suggest multiple applications of methoprene and synergized pyrethrin could be more effective than synergized pyrethrin alone for control of T. castaneum.