Multi-Year Regional-Scale Efficacy Assessment of Two Herbicides for Treatment of Invasive Floating Aquatic Vegetation in a California Estuary

Invasive aquatic macrophytes are a major threat to estuarine ecosystems globally. This study examined the efficacy of a program for treating invasive Floating Aquatic Vegetation (FAV) using two herbicides, glyphosate and 2,4- Dichlorophenoxyacetic acid (2,4-D), over two five-year periods. The efficacy of both these herbicides has rarely been studied in tidal aquatic environments that are much more challenging to treat. This study combined multi-year image datasets, spray location and herbicide application data to examine the efficacy of the treatment program at the scale of the full ecosystem in a tidally dynamic estuary. The objectives of the study were 1) to determine if the herbicides type, application rate and frequency had an impact on efficacy, 2) if efficacy depended on the kind of habitat that was treated and the treatment history of the site, and finally, 3) if the impact lasted years after treatment. The results show that within the year of application, probability of FAV presence reduced as herbicide application rate increased. Both herbicide treatments were most effective in slow-flow shallow open-water habitats, and least effective in fast-flowing channel habitats. Treating with glyphosate had legacy effects that were detectable even three years after treatment but only in some habitats. For either herbicide, treating at a higher concentration in fewer visits was more effective than treating at a lower concentration over multiple visits to the same site.