Abstract In food facilities the majority of insect populations typically occur within hidden locations with limited direct exposure to insecticides, but there is potential for dispersing insects to transport insecticides into hidden areas and transfer insecticide to other individuals (i.e., horizontal transfer). In a series of laboratory experiments, the potential for transfer of the insect growth regulator methoprene between treated and untreated Tribolium castaneum (Herbst) and Tribolium confusum (Jacquelin du Val) individuals was evaluated. In the first experiment, late-instar larvae were exposed to methoprene or water treated individuals in flour and their ability to develop successfully was determined. For T. castaneum, survival time for individuals exposed to methoprene treated individuals was shorter than those exposed to control individuals, but the difference was not significant for T. confusum. For both T. castaneum and T. confusum, the number of individuals that were able to successfully survive exposure and complete development without externally visible deformities was significantly reduced for individuals exposed to methoprene treated pupae or larvae relative to controls, but not when exposed to methoprene treated adults. In the second experiment, early-stage larvae, late-stage larvae, and pupae were similar in susceptibility to horizontal transfer of methoprene, even though they were exposed as immatures for different lengths of time. In a third experiment, the potential for sublethal effects due to horizontal transfer of methoprene was evaluated, but no difference in reproduction and oviposition was found for normal appearing adults that developed when exposed to methoprene treated or and control individuals. This is the first report indicating that methoprene can be transferred between stored-product insects and further research is needed to determine the impact this potentially has on pest populations.