Variation in glyphosate effects and accumulation in emergent macrophytes

Abstract

Invasive aquatic plants can disrupt native biodiversity with considerable ecological impacts. Glyphosate-based herbicides provide one effective option to manage invasive macrophytes, but the variation in glyphosate sensitivity and accumulation in both target and non-target macrophytes is unclear. We performed an outdoor microcosm concentration-response study in which we applied seven glyphosate concentrations (0.1–8% of the Roundup WeatherMAX®formulation, corresponding to 0.5–43.2 g L-1glyphosate) in order to compare sensitivities and accumulation amounts of glyphosate and aminomethylphosphonic acid (AMPA; one degradation product) within and among two invasive emergent plants in North America (Phragmites australis and Typha × glauca), plus a native co-occurring plant (Typha latifolia) over a period of 27 days. Phragmites australis (dose lethal to 50% of the plant population, LC50= 0.34 ± 0.03%) exhibited four to five times higher glyphosate sensitivity than T. latifolia (LC50 = 1.37 ± 0.13%) and T. × glauca (LC50 = 1.70 ± 0.17%). Invasive T. × glauca and native T. latifolia exhibited a similar glyphosate response, although individual variation was high within both taxa. 27 days after treatment with 5% glyphosate, a concentration that mimics many real-world applications, P. australis retained more glyphosate (348 ± 27 mg kg-1dw) than either T. latifolia (102 ± 20 mg kg-1dw; P < 0.001) or T. × glauca (92 ± 12 mg kg-1dw; P < 0.0001). Our results suggest that glyphosate response varies among and within emergent aquatic macrophyte taxa, independent of taxon invasiveness. When the goal is to minimize glyphosate exposure of the environment, managers could consider variation in glyphosate response at both individual and taxonomic levels. Moreover, managers should be aware that glyphosate accumulates in macrophytes.