Biological Control of Biomphalaria Glabrata in Major Reservoirs of Puerto Rico *

Abstract

In 1956, when a snail control program was initiated, most of the 30 major lakes and reservoirs in Puerto Rico contained populations of the planorbid snail Biomphalaria glabrata. The reservoirs had been created by building dams during the previous 50 years, and the largest had a volume of 50 million m3. About 500 ampullarid snails (Marisa cornuarietis) were introduced into each lake to displace B. glabrata, the intermediate host for transmitting schistosomiasis (bilharzia). For 20 years the snail populations in the lakes were monitored by periodic examination of the shorelines, re-seeding the ampullarids when necessary. On the average, each lake was re-seeded once or twice in the 20-year period, and about half a million snails were used. A final survey in 1976, which included analyses of water quality and observations on vegetation, showed that only five reservoirs contained the planorbid snail and that these populations were very small in number. Marisa cornuarietis was firmly established in 22 of the reservoirs. The total cost of this biological control program was small, about $15,000 in 1968 prices. This was an annual average of U.S. $0.0005 per 100 m3 of habitat protected, compared to a cost of $8 per 100 m3 treated by application of chemical molluscicides during these same years. In the lakes for which there were data on both species of snails from 1955 to 1976, the success rate for this inexpensive technique was 12/17 (70%) for elimination of B. glabrata. However, in terms of the strictly fresh-water reservoirs, if success was defined as complete or near elimination of the planorbid snail, the success rate was 14/16 (88%).