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

The effect of metaldehyde on brackish water pond snails, Cerithidea cingula ta, was tested in 250 m(2) ponds, and in semi-commercial and commercial mil kfish ponds. The field trials were conducted at three locations, namely Luz on, Visayas, and Mindanao, Philippines. Three application rates (80, 100, 1 20 kg/ha) of 10% metaldehyde formulation, an untreated control and a refere nce standard 25% niclosamide EC (1.0 L/ha) were tested during the dry seaso n in a pond with heavy snail infestation. Seven days after application (DAA ), snail mortality rates (86-87%) did not differ significantly among the va rious metaldehyde treatments but were significantly higher than those in th e control (6%) and the reference standard (29%). In the field trials, in he avily infested ponds (more than 2000 snails/m(2)), a dose of 120 kg/ha was effective under both dry and wet conditions. In moderately infested ponds ( less than 2000 snails/m(2)), a dose of 80 kg/ha of a 10% metaldehyde formul ation 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% meta ldehyde 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 steadil y to approx. 16% of the initially detected amount at 15 DAA. In the pond se diment, the metaldehyde concentration steadily declined to approx. 1% of th e initially detected amount at 15 DAA. As metaldehyde is rapidly degraded i n aquatic systems its efficacy in controlling brackish water pond snails de pends on a high initial dosage. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.