The environmental profile of metaldehyde

Since 1937 the dry alcohol metaldehyde has been known to act as a molluscicide. Detailed studies have shown that the main effect on slugs is that the mucus cells are irreversibly damaged. Mucus cells are part of important physiological structures typical of molluscs. Metaldehyde is not phytotoxic and in no cases have negative effects been recorded on the carabid beetles Poecilus cupreus, Carabus granulatus, Pterostichus melanarius, Harpalus rufipes, the staphylinid beetle Aleochara bilineata, the honey bee (Apis mellifica), the aphid parasitoid Aphidius rhopalosiphi and the predatory mite Typhlodromus pyri. No adverse effects were found on three earthworm species (Lumbricus terrestris, Allolobophora chloroti and Eisenia fetida). Studies with wild living mammals, hedgehogs (Erinaceus europaeus) fed with metaldehyde-contaminated slugs and wood mice (Apodemus sylvaticus) exposed to metaldehyde slug pellets did not show any signs of disturbance. No adverse effects on tilapia (Tilapia mossambicus), carp, milkfish (Chanos chanos) and on Crustacea were found in aquatic systems. Metaldehyde does not show any tendency to accumulate in soils, water bodies, plants and mammals. Under natural conditions it completely degrades to CO2 and H2O. THE ACTIVE INGREDIENT METALDEHYDE The first systematic reporting of Metaldehyde as a slug control agent goes back to Gimingham & Newton in 1937. From this year on it attracted the attention of agricultural research as a molluscicide and the first commercial formulations appeared on the market, offered to farmers, vegetable and ornamental growers. Metaldehyde was first discovered by von Liebig in 1835. It is the cyclic tetramer of acetaldehyde (Fig. 1), forming tetragonal prisms. Figure 1. Chemical structure of Metaldehyde. It can be considered as a solid alcohol, a pure hydrocarbon that degrades to acetaldehyde, then to acetic acid and thereafter into water and carbon dioxide (CO2). Acetaldehyde is a naturally occurring substance and an intermediate in the degradation chain of ethanol in mammals. The mode of action of metaldehyde on slugs was investigated in detail in several studies by Triebskorn et al. (Triebskorn, 1989; Triebskorn & Ebert, 1989; Triebskorn & Schweizer; 1990; Triebskorn et al. 1998). The main findings of these studies were that the mucus cells of slugs, typical for land molluscs, essential for land-life, are irreversibly destroyed. Metaldehyde acts very fast in slugs or snails inducing severe alterations and ultrastructural destructions in mucocytes independent of the exterior conditions, independently also at low temperatures and high precipitation rates. These findings also enlighten the fact that Metaldehyde is a highly specific molluscicide active ingredient, as will be shown in this paper. DEGRADATION AND FATE OF METALDEHYDE IN THE ENVIRONMENT In top soils under aerobic conditions Metaldehyde completely get degraded within a few days. In average German agricultural soils a DT50 of 5.3 to 9.9 days are observed (Lonza proprietary data). Similar observations can be made in water sediments under moderate temperature conditions (Fig. 2). Metaldehyde is completely degraded with a DT50 of about 12 days. It’s only metabolite acetaldehyde is formed transiently and finally mineralised to CO2. The original carbon from metaldehyde is recovered as carbon dioxide. Figure 2: Degradation of Metaldehyde in water sediment over a 100 day period at moderate temperatures (Lonza proprietary data). More rapid degradation of Metaldehyde is reported by Calumpang et al. (1995) in rice paddies in Philippines. The maximum concentration measured in the water body reaches 1.58 mg/l, whereas the Metaldehyde content fell below the detection limit within 9 days after treatment. The calculated halflife time is 0.27 day. A complete degradation of Metaldehyde in the soil was observed. The Metaldehyde content with a peak of 0.127 mg per kg soil fell in the soil below the analytical detection level within 22 days after treatment. The authors concluded that the use of Metaldeyhde as a molluscicide in rice production does not lead to persistent residues in the various components of a rice paddy ecosystem. Similar results were found by Coloso et al. (1998), where the Metaldehyde content constantly fell to 1 % of its original concentration in the sediment of fish ponds within 15 day. In the water body the Metaldehyde content decreased to 16 % of the maximum concentration in the same time range. 0 20 40 60 80 100