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The effect of several methods of aquatic plant control on two Bilharzia-bearing snail species
Affiliation:1. Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA;2. Laboratory of Integrative Brain Sciences, Department of Biology, Waseda University, Tokyo 162-8480, Japan;3. Division for Arts and Science, Faculty of Arts and Science, Kyushu University, Fukuoka 819-0395, Japan;4. Department of Bioresource Sciences, Faculty of Agriculture, Kyushu University, Fukuoka 812-8581, Japan;5. Suntory Foundation for Life Sciences, Bioorganic Research Institute, Osaka, 618-8503, Japan
Abstract:The effects of four methods of aquatic plant control (manual, chemical, mechanical and biological) on Bilharzia-bearing snails were compared in the period from March–July 1982 in the canals of the irrigation district Mansuriya in Egypt. Grass carp (Ctenopharyngodon idella (Val.)) was used as a biological control agent. Chemical control was affected by means of the herbicide acrolein (2-propanal) and a dragline was used for mechanical control. In biologically controlled waterways, the number of the two vectors of Bilharzia, the snails Biomphalaria alexandrina (Ehrenberg) and Bulinus truncatus (Audouin) was significantly lower (36 and 64%, respectively) than in waterways with one of the other methods of aquatic plant control.Biological control results in the lowest total plant coverage in the snails' habitat. Moreover, the number of snails per unit of vegetation appeared to be lowest in biologically controlled plots. The reduction of the snail population in waterways with biological control may be due to the fact that grass carp feed on the leaves of macrophytes on which egg masses are deposited. This study revealed that the habitat preference of the two snail species and the feeding preference of grass carp for plant species coincide to a certain extent. Increased predation by omnivorous fish in biologically controlled canals may play an additional role.
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