RESUMO
Semi-field trials were carried out in Snail Research Station under simulated natural conditions to evaluate different modes of exposure to Anagallis arvensis and Calendula micrantha as plant molluscicides and bayluscide and copper sulphate as chemical molluscicides. Firstly, B. alexandrina were exposed to the tested molluscicides alone and in addition to two densities of aquatic plants. No apparent effect of aquatic plants on the activity of both plant and chemical molluscicides, this may be due that the two densities of the aquatic plants used were insufficient to interfere with the molluscicides action. Secondly, snails were pre-exposed to three sub-lethal concentrations of the plant molluscicides for 24h then to three concentrations of the chemical molluscicides and vice versa. The results indicate that the pre-exposure increases the snail mortality significantly in all treatments of bayluscide and A. arvensis [except in the highest concentration when the snails firstly exposed to bayluscide then to A. arvensis, where the two compared treatment showed 100%] and in all treatments of bayluscide and C. micrantha. Also, in one treatment of copper sulphate and A. arvensis [in the highest concentration when the snails firstly exposed to A. arvensis then to copper sulphate] and in three treatments of copper sulphate and C. micrantha, [in least and moderate concentrations when snails firstly exposed to C. micrantha then to copper sulphate and in the highest concentration when snails firstly exposed to copper sulphate then to C. micrantha]. Thirdly, snails were exposed to mixtures of six different ratios of hayluscide and each of A. arvensiS and C. micrantha. The results indicated that the snail mortality increased significantly only in the first treatient of bayluscide and A. arvensis mixtures and in treatment number 6 of bayluscide and C. micrantha