The molluscicidal properties of Apodytes dimidiata (Icacinaceae): geographical variation in molluscicidal potency.

Synthetic molluscicides have proved too expensive for most countries wanting to include snail control in their anti-schistosomiasis programmes. An alternative, which is not only cheaper but also promotes self-reliance and empowerment of the affected communities, is the use of molluscicidal plants. An often-cited limitation to using such natural products is the geographical variation in the toxicity of candidate species. The geographical variation in the molluscicidal activity of Apodytes dimidiata was investigated in South Africa. Leaves of this plant were collected from six, widely separate localities within the areas of the country where schistosomiasis is endemic. The results of bio-assays using the intermediate host snail, Bulinus africanus, clearly showed that variation in toxicity did exist and appeared to be correlated with the range in mean annual temperatures and altitude where the plants grew. Whether the variation was a phenoplastic response to the environment or genetically determined still has to be investigated. Nevertheless, a thorough knowledge of the geographical variation in the level of the active compound(s) in the candidate plants in endemic areas will be needed prior to the implementation of plant-propagation and snail-control programmes.