The systemic insecticide imidacloprid is commonly used to protect trees against attack by the Adelges tsugae (Hemlock Woolly Adelgid [HWA]), an invasive pest that threatens Tsuga canadensis (Eastern Hemlock) and T. caroliniana (Carolina Hemlock) in eastern North America. Although there have been some studies documenting the short-term (1–3 years) impact of imidacloprid on non-target arthropods in hemlock systems, almost nothing is known about the impact over longer time scales. Here, using a set of trees which were experimentally treated 3 and 9 years prior to this study, we found that while the impact of imidacloprid on HWA may be approaching the limits of detection and efficacy on trees treated 9 years ago, there is still an intermittently detectable impact on HWA density. Similarly, 9 years after application there is a subtle but detectable increase in arthropod richness and a shift in canopy-arthropod community composition. Results from the 3-year treated trees were, however, ambiguous, but may be the result of detectable cross-contamination of insecticide among trees.