The chemical and biological control of

The snails present in 17 natural watercress beds have been treated during 1 or 2 years by a control associating the use of a molluscicide (СuCl2) and the introduction of a predatory snail, Zonitoides nitidus. Lymnea truncatula was eliminated in only one year of treatment in 14 stations, and in two years in the three others. Elimination of other aquatic pulmonate snails was effected in two years for L. glabra, in 3 years for L. palustris, and in only one year for Physa acuta. Z. nitidus has disappeared from 12 watercress beds during the 4th year after treatment ; in the five others, the numbers of this predator increased during 1 or 2 years and then decreased before their disappearance