Lessons from a decade of lake management: effects of herbicides on Eurasian watermilfoil and native plant communities

Eurasian watermilfoil (Myriophyllum spicatum) is a non-native and invasive aquatic macrophyte with a broad North American distribution. It can have significant negative effects on invaded waterbodies, including decreased native macrophyte diversity, formation of recreational nuisances, and lowered lakefront property values. Previous research suggests that M.spicatum decreases in response to herbicide treatment, but most studies are spatially and temporally limited, usually focusing on a single waterbody for a single year. The long-term effects of herbicides remain relatively unknown. Here, we share the results of an 11-yr observational study of aquatic macrophyte diversity, dynamics, and response to herbicide treatment on 28 Wisconsin lakes (15 of which were adaptively managed with herbicide for M.spicatum and 13 of which acted as unmanaged reference lakes). We found that overall, adaptive management decreases M.spicatum abundance over time, but that the efficacy of individual herbicide treatments can vary. We also found that lakes with relatively new M.spicatum populations (discovered within the last decade) treated smaller areas with lower frequency than lakes with established populations, and were able to maintain lower M.spicatum abundance. This suggests that using adaptive, science-based aquatic plant management strategies, including early detection and response, may increase invasive species management success. Finally, we show that the effect of herbicide treatment on native macrophytes is variable and can be significant. Overall, our results suggest that while herbicide treatment can be an effective adaptive management tool, particularly in lakes with relatively recent M.spicatum invasions, the specific effects of individual treatments can be unpredictable. This study allows lake stakeholders to better understand the efficacy of herbicide treatment, in addition to the possible non-target effects on native macrophyte species.