Metaldehyde is an organic compound with the formula (C8H16O4). It is used as a pesticide against slugs, snails, and other gastropods. It is the cyclic tetramer of acetaldehyde. Production and properties Metaldehyde is flammable, toxic if ingested in large quantities, and irritating to the skin and eyes. It has a white crystalline appearance with a menthol odor. Metaldehyde is obtained in moderate yields by treatment of acetaldehyde with various acid catalysts, such as hydrogen bromide, and cooling. The liquid trimer, paraldehyde is also obtained. The reaction is reversible; upon heating to about 80 °C, metaldehyde and paraldehyde revert to acetaldehyde. Metaldehyde exists as a mixture of four stereoisomers, molecules that differ with respect to the relative orientation of the methyl groups on the 8-membered ring. The stereoisomers have respectively the molecular symmetries C (with symmetry of order 2), C (order 4), D (order 8), and C (order 8). All have at least one plane of reflexion, so none of them is chiral. Uses As a pesticide It is sold under various trade names as a molluscicide, including Antimilice, Ariotox, Blitzem (in Australia), Cekumeta, Deadline, Defender (in Australia), Halizan, Limacide, Limatox, Limeol, Meta, Metason, Mifaslug, Namekil, Slug Fest, and Slugit. Typically it is applied in pellet form, but it is also found as a liquid spray, granules, paste, or dust. Often the pesticide includes bran or molasses to attract pests, making it attractive to household pets as well. Metaldehyde is effective on pests by contact or ingestion and works by limiting the production of mucus in mollusks making them susceptible to dehydration. Metaldehyde products were used to control the invasive African land snail population in Miami-Dade County in Florida. Experimental use permits from the U.S. Environmental Protect Agency authorized the application amount and usage in residential areas. Due to the contamination of drinking water by metaldehyde's use in agriculture, a specialist organisation was established in 2008 called "The Metaldehyde Stewardship Group (MSG)". On 19 December 2018, the British government banned the use of metaldehyde slug pellets outdoors from spring 2020; after this date it would only be legal to use it in permanent greenhouses. In July 2019, the ban was overturned after the High Court in London agreed with a challenge to its legality. Metaldehyde pellets returned to the UK market until 18 September 2020, when the British government banned the use of metaldehyde slug pellets outdoors after 31 March 2022.