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The author analyzes the positive and negative results of a year of biological control by predation on 49 stations of Lymnaea truncatula in Haute-Vienne, France. The negative results can be explained by the effects of 6 natural factors: 1--the rainfall over the habitats during the first 10 days of the experimentation. The rate of natural mortality and predation of L. truncatula is reduced at values above 8 mm rainfall. The predatory snails leave the sodden soil and do not consume L. truncatula; 2--the presence of stone blocks on the habitat. These blocks constitute shelters for L. truncatula; 3--the quality of the plant cover. Rushes and graminaceae preferentially must be used; 4--the previous treatment of the habitat by a molluscicide (copper sulphate) just before the application of biological control. The predators preferentially consume the more poisoned L. truncatula and forsake the more healthy snails; 5--the presence of phoretic acaridae on the predators which die; 6--the behaviour of sheep on the habitats situated on pastures with steep slopes. Their repeated passages cause the squashing of the predators. The proposed solutions are discussed in the limits of the region of experimentation.

Le contrôle biologique de Lymnaea truncatula Müller en Haute-Vienne, France. A propos de quelques facteurs modulant son application