Effects of Individual and Combined Pesticide Commercial Formulations Exposure to Egestion and Movement of Common Freshwater Snails, Physa acuta and Helisoma anceps

—Pesticides are detected in streams at concentrations that might have adverse effects on aquatic organisms. These agrochemicals typically occur in streams as combinations, yet research has focused on the effects of individual pesticides. We studied the effects of commercial formulations of atrazine, metolachlor, carbaryl, and chlorothalonil on aquatic gastropods Physa acuta and Helisoma anceps egestion and movement. We observed an eightfold reduction in P. acuta egestion rates when exposed to individual (atrazine: 200 lg/L; metolachlor, carbaryl, and chlorothalonil: 100 lg/L) and combined (atrazine x metolachlor: 200 lg/L x 100 lg/L and carbaryl x chlorothalonil: 100 lg/L x 100 lg/L) pesticide treatments relative to controls. For H. anceps individual and combined pesticide treatments had no significant effects on egestion, highlighting differential species response. Helisoma anceps movement declined when exposed to atrazine, carbaryl, and chlorothalonil individually, though responses varied with exposure time. When combined atrazine þ metolachlor and carbaryl þ chlorothalonil reduced H. anceps movement relative to the control. In addition to pesticide physicochemical characteristics, it is important to consider exposure durations to better understand the effects of pesticides on aquatic organisms. Furthermore, future risk assessments should incorporate multiple species to better represent response diversity.