EXPERIMENTAL CONTROL OF THE SCHISTOSOME-TRANSMITTING SNAIL BIOMPHALARIA-PFEIFFERI BY THE AMPULLARIID SNAIL PILA-OVATA

Adults of the African ampullariid snail Pila ovata were examined for t heir ability to control laboratory populations of the pulmonate snail Biomphalaria pfeifferi, a widespread, intermediate host of the human p athogen Schistosoma mansoni in sub-Saharan Africa. In a 6-week experim ent conducted in large (100 x 60 x 60 cm) outdoor tanks containing flo ating macrophytes (Nymphaea caerula) and initially set up with one adu lt ampullariid for every three adult pulmonates, the numbers of B. Pfe ifferi egg masses were always about half those in similar tanks withou t P. ovata. Although, by week 6, the numbers of B. pfeifferi in the co ntrol tanks (without ampullariids) had increased 5-fold, from an initi al mean of 30 snails/tank, there was no significant increase in the nu mbers of B. pfeifferi in the experimental tanks (containing ampullarii ds). Results of experiments conducted in indoor glass aquaria indicate d that adult P. ovata rapidly attacked egg masses or neonates (< 2.5 m m shell diameter) of B. pfeifferi but had no effect on the adults. The adult ampullariids also significantly decreased cover by floating mac rophytes over a 6-week period compared with that in similar but ampull ariid-free aquaria. This decrease in plant cover is relevant to biolog ical control of the schistosome vectors as macrophytes serve as food, shelter and oviposition sites for pulmonate snails. The present result s indicate the ability of P. ovata to inhibit multiplication of B. pfe ifferi populations, at least under laboratory conditions, both directl y, through predation, and indirectly, by competition for resources. Pi la ovata may therefore prove useful in the biological control of medic ally important, pulmonate snails.