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In a study of the relationship between molluscicidal activity and exposure time, Biomphalaria boissyi were exposed to copper sulfate in pond water at 20°C. for various lengths of time. Expressed in terms of CuSO4·5H2O, LD50 decreased from 55 p.p.m. following a 6-hour exposure period to 1.35 p.p.m. after an exposure of 72 hours. From a comparison of these data with the results of other workers it is concluded that the use of tap water in copper sulfate evaluation tests results in a considerably higher mortality than the use of natural waters from sites favorable for snail colonization. This effect is not due to the presence in natural water of factors which rapidly remove copper sulfate from solution.
It is suggested that the use of low copper concentrations maintained for an extended period of time might be more effective and less expensive than the high concentrations which result when solid copper sulfate is dissolved directly in canal water.
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