Toxicity evaluation of Bayluscide and malathion to three developmental stages of freshwater snails

Laboratory studies were conducted to assess the conditions under which the use of malathion in ricelands of Cameroon may impact the transmission of schistosomiasis. Helisoma trivolvis and Biomphalaria havanensis were selected as test organisms due to the lack of intermediate snail hosts in the U.S. Using Bayluscide as a reference molluscicidal compound, malathion was tested against snail eggs, juveniles, and adults. Snail eggs were more susceptible to Bayluscide and malathion than juvenile snails which in turn were more susceptible than adult snails. A Bayluscide concentration of 0.200 mg/L caused 100% mortality to adults of both snail species after 24 h exposure. This relatively high toxicity of Bayluscide to freshwater snails is one of the reasons why it has been recommended by the World Health Organization as the molluscicide of choice for control of schistosome-bearing snails. The concentrations of malathion resulting in 100% kill of adult snails after 24 h exposure were 1,200 mg/L for H. trivolvis and 500 mg/L for B. havanensis. After 48 h exposure, these concentrations were reduced to 500 mg/L and 300 mg/L, respectively. Therefore it is expected that the use of malathion for insect control in ricelands of Cameroon may affect the survival of freshwater snails including the intermediate hosts of bilharziasis.