ACUTE AND CHRONIC TOXICITY OF THE RICE HERBICIDES THIOBENCARB AND MOLINATE TO OPOSSUM SHRIMP (NEOMYSIS-MERCEDIS)

Neomysis mercedis were exposed to the rice herbicides molinate and thi obencarb, which have been detected in the Sacramento-San Joaquin Delta . Continuous-flow acute and chronic toxicity studies were performed. F or thiobencarb, the 4-, 7- and 14-day LC50s were 304, 214 and 91 mug/l iter, respectively. For molinate, these values were 9910, 2530, and 82 0 mug/liter, respectively. Mortality stabilized after 18 and 28 days e xposure, respectively, for thiobencarb and molinate. The incipient let hal concentrations were 53 and 230 mug/liter, respectively. Joint toxi city studies indicated additive toxicity for the two chemicals. Chroni c no observable effects concentrations (NOECs) were estimated at 3.2 a nd 25-6 mug/liter for thiobencarb and molinate, with measurable effect s occurring at 6.2 and 45.2 mug/liter, respectively. Environmental mon itoring data suggests that concentrations of these chemicals in the De lta prior to 1985 may have exceeded the threshold for chronic toxicity , under conditions of joint exposure. However, due to continued improv ements in pesticide management practices, environmental concentrations decreased to approximately 30 percent of the chronic threshold for jo int toxicity by 1988. Baseline data on growth and reproduction relevan t to toxicity tests are also presented for this species.