Sandy’s Garden ... Metalhyde – The Jeykll and Hyde of Pesticides

It read: “A ban on the outdoor use of metaldehyde, a pesticide used to control slugs in a range of crops and in gardens, is to be introduced across Great Britain from Spring 2020.”

The decision to prohibit the use of metaldehyde, except in permanent greenhouses, follows advice from the UK Expert Committee on Pesticides (ECP) and the Health and Safety Executive (HSE) that metaldehyde poses an unacceptable risk to birds and mammals.

Environment Secretary Michael Gove added: “I recognise that significant effort has been put into encouraging growers and gardeners to use this pesticide responsibly by the Metaldehyde Stewardship Group. However, the advice is clear that the risks to wildlife are simply too great – and we must all play our part in helping to protect the environment.”

So what is metaldehyde? What is it used for? And does this announcement matter to amateur gardeners? ‘Metaldehyde (CH₃CHO)₄ is an ether, formed from a cyclic tetramerization of acetaldehyde,’ I learn from a scientific paper published by White Rose Research Online at the University of York. No, me neither! ‘Metaldehyde was initially used as a solid fuel firelighter ‘Meta-fuel’ but its major contemporary use is as a molluscicide in agriculture and horticulture. It is now widely used in both agricultural fields and domestic gardens and is applied as a pelleted bran bait that inhibits slug feeding after exposure.’ Ah, now I know that we’re talking about the active ingredient in the most widely-used slug killer pellets. I have a pack right beside me. ‘Contains metaldehyde. For use only as a home garden slug and snail killer,’ the label says. ‘Scatter pellets thinly. Do not leave heaps on the soil; they can be fatal to pets if eaten in quantity. Pellets must not be applied shortly before rain.’

Metaldehyde kills slugs and snails by disrupting their ability to produce mucus. This reduces their digestion and mobility and makes them susceptible to dehydration. Slugs and snails eat a lot of rotting vegetation, consuming up to forty times their own weight in a single day: but as well as serving this very useful role, they also have a penchant for many flowers and vegetables and are the bane of the lives of crop farmers and keen gardeners. Metaldehyde pellets are by far the most common way of controlling their numbers and do a first-class job. However, wildlife like hedgehogs and thrushes are partial to slugs and snails and ingest the metaldehyde when they gobble up those that have eaten the poison. This is not good.

And what happens to uneaten pellets when the rain comes? (‘Pellets must not be applied shortly before rain.’) They dissolve, making it possible for metaldehyde to drain away to water courses and, possibly, to end up in our tap water. The oft-maligned European Commission restricts metaldehyde levels to one ten-millionth of a gram in a litre of drinking water… 0.1μg/litre … that’s one part in a billion, say one drop in a bathtub full of water; and, although this was not a factor in the advice from ECP and HSE, a ban on the use of metaldehyde will help water companies continue to meet our robust drinking water standards.

Some other ways of controlling slug and snail numbers are useful to gardeners but far too labour-intensive to be used by farmers. The likeliest mass-use alternative to metaldehyde pellets will be ferric phosphate-based pellets, which don’t harm pets or other wildlife and are approved for use by organic growers. And none of this has anything to do with Christmas!