A semi-quantitative approach to the selection of appropriate candidate plant molluscicides--a South African application.

The high cost of synthetic molluscicides, used in the control of the intermediate snail hosts of schistosomiasis (bilharzia), has resulted in renewed interest in plant molluscicides. The history of the use of plant molluscicides is reviewed. Although screening programmes have been conducted in other African countries, no efforts have been made to identify South African plants which would be suitable for use locally, using appropriate technology. The prohibitive costs (time and financial) of random surveys for activity necessitated the development of an objective selection procedure. A simple scoring system derived for this purpose is described. Of 600 plant species with potential, or recorded activity, 150 occurred in South Africa. Twenty-six taxa were active according to standards set by WHO. A further 37 species, although untested, warrant further investigation. Species were ranked on cumulative scores for: (a) coincidence of the endemic areas of the plant, snail host and disease; (b) ethnomedicinal value which would provide greater incentive for cultivation; (c) molluscicidal activity (if known, a minimum LD90 of < or = 100 ppm). Two lists resulted, those with recorded and those with potential activity. Both are important in prioritizing research on molluscicidal plants in South Africa. Problems inherent to the scoring system and to the development of plant molluscicides are discussed.