Field trials are demonstrating the valuable suppressive effect of interspecific competition by Marisa cornuarietis on Australorbis glabratus, the snail intermediate host of Puerto Rican schistosomiasis. Marisa colonies were established easily in ten of sixteen farm ponds planted manually with the snail. Eight of the ten ponds originally contained Australorbis. By about 2 years after planting, Australorbis had disappeared from all eight of the ponds, in an average time of 8 months. Widespread planting of Marisa in one endemic watershed has practically eliminated the schistosome host there in less than 2 years. Marisa is now being used as an adjunct to other types of Australorbis control by the insular government.
Among other snails under study Helisoma duryi eudiscus shows definite promise of useful competition in irrigation tanks. Tarebia granifera manuensis appears to be effectively crowding A. glabratus in some areas, while Ampullaria sp. has been ineffective.
Other than snails, there apparently are few natural enemies of Australorbis in Puerto Rico.
Puerto Rico Field Station, Communicable Disease Center, U. S. Public Health Service, Box 52, San Juan, Puerto Rico.
University of Puerto Rico, School of Medicine, Department of Microbiology, San Juan, Puerto Rico.
Bilharzia Control Unit, Department of Health, Commonwealth of Puerto Rico, San Juan, Puerto Rico.