Sensitivity of the stripe-faced dunnart, Sminthopsis macroura (Gould 1845), to the insecticide, fipronil; implications for pesticide risk assessments in Australia

A lack of toxicity data quantifying responses of Australian native mammals to agricultural pesticides prompted an investigation into the sensitivity of the stripe-faced dunnart, Sminthopsis macroura (Gould 1845) to the insecticide, fipronil (5-amino-3-cyano-1-(2,6-dichloro-4-trifluoromethylphenyl)-4-trifluoromethylsulfinyl pyrazole, CAS No. 120068-37-3). Using the Up-And-Down method for determining acute oral toxicity in mammals (OECD) median lethal dose estimates of 990 mg kg(−1) (95% confidence interval (CI) = 580.7–4770.0 mg kg(−1)) and 270.4 mg kg(−1) (95% CI = 0.0–>20,000.0 mg kg(−1)) were resolved for male and female S. macroura, respectively. The difference between median lethal dose estimates for males and females may have been influenced by the older ages of two female dunnarts. Consequently, further modelling of female responses to fipronil doses used the following assumptions: (a) death at 2000 mg kg(−1), (b) survival at 500 mg kg(−1) and (c) a differential response (both survival and death) at 990 mg kg(−1). This modelling revealed median lethal dose estimates for female S. macroura of 669.1 mg kg(−1) (95% CI = 550–990 mg kg(−1); assuming death at 990 mg kg(−1)) and 990 mg kg(−1) (95% CI = 544.7–1470 mg kg(−1); assuming survival at 990 mg kg(−1)). These median lethal dose estimates are 3–10-fold higher than available LD(50) values of 94 mg kg(−1) for a similarly sized eutherian mammal, Mus musculus (L. 1758) and 97 mg kg(−1) for Rattus norvegicus (Birkenhout 1769). Implications for pesticide risk assessments in Australia are discussed.