Effects of atrazine and metolachlor on the survivorship and infectivity of Echinostoma trivolvis trematode cercariae

Parasites play important roles in ecosystems and can be impacted by chemical inputs. In a series of experiments, we examined the impact of two common herbicides, metolachlor and atrazine, on a host-parasite system consisting of the trematode, Echinostoma trivolvis and its two intermediate hosts, the snail Planorbella trivolvis and larval Rana spp. tadpoles. Metolachlor and atrazine are two widely used agricultural herbicides that inhibit the growth of pre-emergent vegetation. Residues of these pesticides are commonly found in water bodies near agricultural areas. In our first experiment in the laboratory, we examined changes in survivorship when free-living trematode cercariae were exposed to a low concentration (10 ppb: 15 ppb) and high concentration (85 ppb: 100 ppb) mixture of metolachlor and atrazine, respectively. These exposure levels were chosen to represent the higher end of levels that have been documented in aquatic systems. There was a significant decline in cercarial survivorship in the high concentration treatment at 14 hours. In our second experiment, we exposed the parasites, the second intermediate host tadpoles, or both the parasites and the tadpoles, to the pesticide mixtures for a maximum of 10 hours prior to infection and examined subsequent tadpole infection levels. The atrazine and metolachlor mixtures had no significant effects on parasite load, although newly shed cercariae were more likely than 10-hour-old cercariae to infect tadpoles. In our final experiment, we utilized outdoor mesocosms to expose parasites, infected snail hosts, and Rana sylvatica tadpoles to the pesticide mixtures for two weeks and examined differences in tadpole parasite loads. The pesticides had no significant effect on tadpole parasite loads in the mesocosms. Overall, our findings suggest that atrazine and metolachlor mixtures at the doses we examined do not significantly alter the short-term dynamics of Echinostoma trivolvis infection in aquatic systems.