Comparative toxicity of Euphorbia milii latex and synthetic molluscicides to Biomphalaria glabrata embryos

Plant molluscicides have been regarded as possible alternatives to the costly and environmentally hazardous molluscicides currently available. This study was undertaken to compare the developmental toxicity of a plant molluscicide (Euphorbia milii latex, LAT) with that of three synthetic molluscicidal compounds. Biomphalaria glabrata egg masses (0–15 h after spawning) were exposed to molluscicides for 96 h and thereafter examined up to the 14th day after spawning. Embryo deaths, abnormal embryo development (malformations) and the day of hatching were recorded. Although exhibiting a weak ovicidal effect, LAT markedly impaired the development of snail embryos at concentrations ⩾1000 μg L−1 and produced anomalies (EC50 = 2040 μg L−1) such as abnormal shells, hydropic embryos, cephalic and non-specific malformations. Embryolethal potencies of molluscicides were as follows: triphenyltin hydroxide (TPTH; LC50 = 0.30 μg L−1) > niclosamide (NCL; LC50 = 70 μg L−1) > copper sulphate (CuSO4; LC50 = 2190 μg L−1) ⋙⋙ LAT (LC50 = 34 030 μg L−1). A few malformations were recorded in embryos exposed to concentrations of TPTH within the range of lethal concentrations, while almost no anomalies were noted among those treated with NCL or CuSO4. A hatching delay (hatching on day 10 after spawning or later) was observed among LAT-exposed embryos. The effects of NCL, TPTH and CuSO4 on hatching were to some extent masked by their marked embryolethality. The no-observed effect concentrations (NOEC) for embryotoxicity were as follows: TPTH, 0.1 μg L−1; NCL, 25.0 μg L−1; CuSO4, 500.0 μg L−1 and LAT, 500.0 μg L−1. Results from this study suggest that, although LAT was not acutely embryolethal after a short-term exposure, it markedly disrupted snail development. The marked embryotoxicity of E. milii possibly contributes to its effectiveness as a molluscicide.