Biological control of Biomphalaria glabrata by Marisa cornuarietis in irrigation ponds in Puerto Rico.

settle down at one-fifth of the initial rate. Therefore the effective concentration would be equivalent to 220 pellets to treat 330 litres/day, or 0.67 pellet per litre-day. From the first experiment, this concentration should kill newly added snails in about 5 days. In practice it was found that it was necessary to replace the snails at the rate of 10 every second day, indicating a 100% mortality in 4 days. The difference may have been due to the differences in temperatures at which the experiments were carried out; the third experiment was done at 19°C-20°C (the tap-water temperature) while the first and second experiments were at 25°C. Also, tap water was used in the third experiment whereas boiled tap water was used in the first and second. The results indicate that the pellets released molluscicide into the water continuously for at least 82 days. The rate of release dropped very quickly from its initial level and after about 8 days settled down to an almost constant release rate of about 20% of that seen on the first day. The contributioris of the individual molluscicides tributyltin oxide and niclosamide to the snail mortality in these trials is unknown. However, Goll (personal communication) found pellets 633A, which contain niclosamide only, to be inactive when given only a 24-h exposure. This would suggest that tributyltin oxide is the active ingredient in the pellets. A possible practical use of the pellet formulation is as a chemical barrier against repopulation after the existing population have been killed by a single molluscicide application. Several weeks afterwards, the pellets could be suspended in the water to prevent repopulation. The number of pellets required for this operation is difficult to gauge but from the above work where 0.67 effective pellet per litre killed snails in 4 days and from the results in the first experiment where 1 pellet/ 8 litres killed snails in 216 hours, it appears to be a very low dose, perhaps 1 pellet/4 litres or lower sufficing as a chemical barrier. At I pellet/4 litres in flowing water this would require about 29 kg of pellets per ft3/s of water, or a little over 1 kg per litre/s, for perhaps 5 snail-free years.