7. 5 Plant molluscicides

Of the numerous plants whose products kill snails, the molluscicide extract best investigated in the field is the ethyopic Endod derived from the berrys of Phytolacca dodecandra, a climbing plant. The genus is present in all regions of Africa, parts of Asia and South America. The active ingredient changes in the various species. A watery solution of the dried powdered berries of P. dodecandra kills Bulinus truncates and Biomphalaria pfeifferi at 15-30 mg/l in 24 hours. The solution is stable against UV rays at pH 5-9. In mammals and plants, it is of low toxicity but, at concentrations used in the field, it is toxic for tadpoles and fishes. The active ingredients are triterpenoide saponins and a glycoside of oil acid, both of which are biologically and probably microbiotically decomposed quickly. The powdered berries and an extract by butanol are 100% active at 3-6 ppm over 24 hours. Spread out in waters, the molluscicide activity disappears in a few days. This may be regarded as both an advantage and a disadvantage. On the one hand, the fauna of non-target snails e.g. those living hidden between plants, is not threatened or only to a limited extent and the quality of the water for the human population is impaired only for a short time. On the other hand, the contact time with the target snails is limited.

In a supraregional experiment in Adwa/Ethiopia with 17,000 inhabitants, the prevalence and incidence of schistosomiasis caused by S. mansoni was remarkably reduced by Endod. No disadvantages to the biocoenosis were observed. The plants were cultivated in the same region by the native population, which harvested and processed the berries. If well organized, this kind of campaign promises the most lasting effects and is even more to the good as Endod is active against the larvae of Anopheles, Culex, Aedes and Simulium. Plant-derived molluscicides produced from the seeds, fruits or berries of different plants might be active against snails or cercariae. They can be tested in the laboratory for their effective concentrations and contact times (tab. 7.2, page 278).

For biological control, an ampulariid snail Marisa cornuaretis was used on the Caribbean island of Puerto Rico and extended areas were cleared from Biomphalaria glabrata. The rather large M. cornuaretis feeds primarily on snails and their egg butches. After the elimination of nearby all the smaller snails, it survives by feeding on plants and thereby persists in the region. The target snails can escape only in cases of extremely dense vegetation.

Tiara granifera (syn. Melanoides granifera) of the prosobranchiate family Melaniidae propagates by parthogenesis and ovoviviparity. It is resistant against desiccation by deeply retracting its operculum. Biomphalaria glabrata has been widely removed from the rivers, swamps and marshes of various Caribbean islands. However, the required density of T. granifera was twice that of the target snails. It is less sensitive to molluscicides than B. glabrata, which can therefore be reduced in numbers before a campaign with a molluscicide. Mass rearing is possible at 24°C in containers on mud covered with 4 cm water.

Duke BOL, Moore PJ (1976) The use of a molluscicide, in conjunction with chemotherapy, to control Schistosoma haematobium at the Barombi lake foci in Cameroon. I. The attack on the snail hosts, using N-tritylmorpholine, and the effect on transmission from snail to man. Tropenmed. Parasit. 27: 297-313.

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