The molluscicidal activity of two 2-benzamido-5-nitrothiazole bihalogenated derivatives and niclosamide. Influence of some environmental factors on their toxicity.

Two synthetic molluscicides, 3,4-dichloro-2-benzamido-5-nitrothiazole (3,4-dichloro-BNT) and 3,5-dichloro-BNT, were studied to determine their efficacy against the snail Lymnaea glabra. Results were compared with those of a reference molluscicide, niclosamide. Snail exposure to these chemicals markedly increased overall snail mortality during the experiment (96 h). The LC50 values slightly decreased and, at day 4, were lower for the two BNT derivatives than for niclosamide. The effect of several factors on compound toxicity was also studied using lethal concentrations near LC50. Acclimatization duration under controlled conditions had a variable influence upon overall snail mortality according to the compound. No significant variation in the overall mortality was detected in snail groups exposed to 3,4-dichloro-BNT. On the other hand, snail mortality greatly increased in the 3,5-dichloro-BNT-exposed groups when the duration increased from 24 h to 15 days; the same result was obtained in the niclosamide-exposed snails acclimatized for 8 days. Snail mortality increased in the presence of sand, whatever compound was used, and greatly decreased in the presence of marl; it was also higher with water of low calcium content. When the food supply was low, snail mortality greatly decreased in the niclosamide- and 3,4-dichloro-BNT-exposed groups, whereas it increased in the case of 3,5-dichloro-BNT. In the presence of running water, the number of dead snails with 3,4-dichloro-BNT was identical to that of controls, whereas it was greater with the other molluscicides. These experiments, carried out under controlled conditions, demonstrate that the use of a molluscicide in the field must take into account its chemical properties as well as the characteristics of the snail habitat to optimally eliminate snails.