Acute and chronic toxicity of the herbicide benzofenap (Taipan 300) to Chironomus tepperi Skuse (Diptera : Chironomidae) and Isidorella newcombi (Adams and Angas) (Gastropoda : Planorbidae)

Acute and chronic toxicity tests were conducted on the herbicide benzofenap (Taipan(R) 300) using two Australian freshwater invertebrates. The commerc ial formulation of benzofenap and a blank formulation containing only the a djuvants of Taipan(R) 300 were tested against final instar larvae of the mi dge Chironornus tepperi and adults of the aquatic snail Isidorella newcombi . In 24-h acute bioassays the midge larvae did not show significant mortali ty at a nominal concentration of 1.2 mg/L active ingredient (AI), double th e maximum notional concentration expected in rice fields after application of Taipan(R) 300 at the permit rate of 2 L/ha. No significant snail mortali ty was recorded in acute bioassays (24-h exposure, 48-h recovery) at nomina l concentrations up to 76 mg/L AI, which is over 120 times the maximum noti onal field concentration. In chronic assays, the pupation and emergence of C. tepperi was monitored after a 4-h pulse exposure of final instar larvae to Taipan(R) 300 (nominal concentrations 0.001 to 0.1 mg/L AI) and adjuvant -only (0.1 mg/L equivalent) treatments. No statistically significant effect s were observed, although emergence appeared to be delayed by higher benzof enap concentrations and by the adjuvant-only treatment. During Snail bioass ays, egg and feces production were monitored for 21 days after 24-h exposur e to Taipan(R) 300 (nominal concentrations 1.2 mg/L to 60 mg/L AI) and adju vant-only treatments (60 mg/L equivalent). No significant chronic effects w ere proven against I. newcombi, despite a decline in egg mass production fo llowing exposure to all treatments and a reduction in the total numbers of eggs produced at the highest nominal concentrations tested (60 mg/L AI and adjuvant-only treatments). Similarly, feces production by snail pairs expos ed to the majority of treatments declined, but these differences were not s tatistically significant. It is concluded that Taipan(R) 300 does not repre sent a significant risk to mature C. tepperi larvae or adult I. newcombi in downstream environments when applied to rice fields at the permit rate of 2 L/ha.