Microcosm assessment of potential molluscicides for control of the rice snail Isidorella newcombi sens. lat. (Gastropoda: Basommatophora: Planorbidae)

Twenty-seven pesticides were evaluated for their toxicity to mature Isidorella newcombi (Adams & Angas), a serious pest of irrigated rice in NSW, Australia. Evaluations were conducted using microcosms in which typical field conditions were simulated. Only 3 compounds, niclosamide, n-tritylmorpholine, and nicotinanilide, provided >95% mortality at an initial screening rate of 3 mg/L. These compounds were assessed at a range of rates using both immediate and delayed-exposure bioassays to determine LC values and comparative persistence. Niclosamide (as Bayer Bayluscide 250 g/L EC) and n-tritylmorpholine (as Shell FresconB 165 g/L EC) were the most toxic (LC90, immediate exposure, 0.19 mg/L for both compounds), whilst the corresponding LC90 value for nicotinanilide (laboratory grade in DMSO) was 0.53 mg/L. Persistence (reflected by a slower increase in LC90 values over time in delayed exposure bioassays) was strongest in nicotinanilide and weakest in n-tritylmorpholine. Although niclosamide is the most promising compound for the control of I. newcombi, further development of nicotinanilide is also recommended, as it represents a potentially valuable tool for use in situations where low fish toxicity is required.