Use of metaldehyde as a molluscicide in semi-commercial and commercial milkfish ponds

Abstract

The effect of metaldehyde on brackish water pond snails, Cerithidea cingulata, was tested in 250 m2 ponds, and in semi-commercial and commercial milkfish ponds. The field trials were conducted at three locations, namely Luzon, Visayas, and Mindanao, Philippines. Three application rates (80, 100, 120 kg(ha) of 10% metaldehyde formulation, an untreated control and a reference standard 25% niclosamide EC (1.0 L/ha) were tested during the dry season in a pond with heavy snail infestation. Seven days after application (DAA), snail mortality rates (86-87%) did not differ significantly among the various metaldehyde treatments but were significantly higher than those in the control (6%) and the reference standard (29%). In the field trials, in heavily infested ponds (more than 2000 snails/m2), a dose of 120 kg/ha was effective under both dry and wet conditions. In moderately infested ponds (less than 2000 snails/m2), a dose of 80 kg/ha of a 10% metaldehyde formulation was effective under dry conditions but a dose of up to 120 kg/ha was needed under wet conditions. In a pond dosed with 120 kg/ha of the 10% metaldehyde formulation, the concentration increased in pond water, until 3 DAA relative to the initial level, indicating that the active ingredient had dissolved from the granules. From day 3, the concentration declined steadily to approx. 16% of the initially detected amount at 15 DAA. In the pond sediment, the metaldehyde concentration steadily declined to approx. 1% of the initially detected amount at 15 DAA. As metaldehyde is rapidly degraded in aquatic systems its efficacy in controlling brackish water pond snails depends on a high initial dosage.