The effects of fluctuations in water level and pressure for the control of Biomphalaria glabrata were studied in the laboratory. Daily fluctuations in water pressure resulted in doubled fecundity of the snails, but 6-day fluctuations had no effect. On a submerged slope, snails migrated downward more slowly on gentle slopes than on steep; speed was also reduced by lack of light, by low temperatures, and by roughness of the surface. Snails moved more slowly as they migrated more deeply into the water, and small snails moved faster than large ones. It was calculated that during the day snails can be stranded with a vertical rate of drawdown greater than 23 cm per hour on a 5:1 slope and 0.1 cm per hour on a 100:1 slope. At night the rate of drawdown can be reduced 82% or even greater, as snails are thought to reverse their migration at night. In an analysis by mathematical model, a symmetrical pattern of fluctuation of water level (best suited to reservoirs on rivers of medium size) was estimated to strand snails most rapidly if the periods of fluctuation were 5 to 10 days. An asymmetrical pattern of fluctuation, which would obtain in reservoirs in small streams controlled by automatic siphon, would best control snails with periods of 5, 10, or 20 days. The asymmetrical pattern appeared to be more effective than the symmetrical for fluctuations of the same frequency. The periodic, rapid discharge from such a reservoir might also flush snails from downstream habitats.
Tropical Disease Section, Ecological Investigations Program, National Communicable Disease Center, Health Services and Mental Health Administration, U. S. Public Health Service, San Juan, Puerto Rico 00902.