An Evaluation of a Non-Vector Snail Melanoides Tuberculata ( Muller) as An Agent for the Biological Control Of Bulinus Trunca Tus ( Audouin ) A Snail Host Of Urinary Schistosomiasis

Abstract

Preliminary studies earned out in the Weija Lake in Ghana gave indications of competitive interactions between Melanoides tuberculata and three snail hosts of schistosomiasis ie Bulinus truncatus, Bulinus globosus and Biomphalaria pfeifferi. Subsequent laboratory investigations involving Melanoides tuberculata and Bulinus truncatus carried out under four different conditions ie sandy gravel ( Normal and Heat treated) and sandy clay loam ( Normal and Heat treated) showed that Melanoides tuberculata is able to suppress the growth rate of Bulinus truncatus to varying extents depending on the type and condition of the sediment. Of the two sediments used, it was noted that Melanoides tuberculata competes more effectively on sandy gravel than on sandy clay loam mixture as the adverse effects of the presence of this snail on the rate of weight of Bulinus truncatus using sandy gravel sediment was found to be more severe than the results obtained when the sandy clay loam was used. Normal sediments were found to enhance the competitive advantage of Melanoides tuberculata over Bulinus truncatus more than the Heat treated sediments. In fact Heat treated sediments tended to reverse the competitive interactions in favour of Bulinus truncatus at least in some cases. The intensity of the competitive effects of Melanoides tuberculata over Bulinus truncatuswas also found to be directly proportional to the ratio of Melanoides tuberculata to Bulinus truncatus. Thus M15B5 (ie 15 M. tuberculata to 5 Bulinus truncatus) combination had a greater effects on Bulinus truncatus than M10B10 (ie 10 M. tuberculata to 10 Bulinus truncatus) and M5B15 (ie 5 M. tuberculata to 15 Bulinus truncatus) respectively. When the feeding behaviour and apparatus of the two snails were considered critically, to ascertain the possible influence of food on this ecological interaction it was established that, the radula of the two snails are different morphologically and also function differently. Food selection or preference among the two snail species was also found to be different (P(T=t)=0.001), though some similarities do exist . This suggests that, the basis of the observed competition between Melanoides tuberculata and Bulinus truncatus might not under normal circumstances be due to food However, the competitive ability of Melanoides tuberculata is directly or indirectly linked under normal circumstances to food stress. The findings from these studies suggest the need to extend our biological control studies to include other environmental factors which might be critical in determining the outcome of the competitive interactions between the host and the non host snails of schistosomiasis