Abstract: A method and composition for destroying disease transmitting trematode hosts over a sustained period of time by treatment with a controlled release toxicant composition. The composition comprises a water-soluble, inorganic copper salt capable of releasing copper ions in water which is substantially uniformly dispersed in a moderately crosslinked elastomer in which the copper salt is insoluble. The composition, when contacted with water, allows for the controlled and sustained release of copper ions to cause chronic intoxication and eventual destruction of the molluscan target.