Field trials to evaluate the effectiveness of the molluscicide N-tritylmorpholine in irrigation systems.

In field trials with the molluscicide N-tritylmorpholine (Frescon, WL 8008), a prolonged low-dosage technique has been developed for use in irrigation systems. In Tanzania a dose of 0.025 ppm applied for 30 days to the headworks of a 5000-acre (2025-ha) irrigation system gave effective control of Biomphalaria pfeifferi, the snail host of Schistosoma mansoni, for a period of 3-4 months. This was comparable to the effectiveness of other molluscicides and other dosage regimes but the technique had the advantages of simplicity, lower cost and lack of toxicity towards fish or other aquatic fauna. Similar trials have been carried out in Egypt and Southern Rhodesia. Theoretical and practical considerations involved in the choice of dosage regimes are discussed and a general formula is suggested for choosing dosage regimes in irrigation systems.