Toxicity of an herbicide and adjuvant to saltmarsh invertebrates in the management of invasive grass; Comparative laboratory and field tests

• The toxicity of Fusilade Forte® and Hasten® to native marine invertebrates was assessed.
• In most cases, the chemical mixture was more toxic than the herbicide alone.
• Field application of herbicide showed no detrimental impacts on invertebrates.
• A comparative approach of laboratory and field tests yielded crucial insights.

Coastal weeds are often treated with herbicides without knowledge of non-target impacts, and toxicity data from standardized test species can have limited applicability. We evaluated toxicity to invertebrates from Fusilade Forte® and the adjuvant Hasten® in the control of invasive salt marsh grass, Spartina anglica. For 3 of 4 local invertebrates, Fusilade Forte® was moderately toxic (96 h LC50 5.4–144 mg L− 1), whereas Hasten® was less toxic (14.2–450 mg L− 1). For most species, the mixture was more toxic than the herbicide alone, with 96 h LC50 reduced 23–45%. However, a field experiment applying typical concentrations (1000 × the lowest 96 h LC50) showed low concentrations of herbicide residues and no detrimental impacts on invertebrates over 6 months. The results reveal the importance of testing locally relevant species for potential toxicity, and of comparison tests with field exposures to determine the realised toxicity in nature.