A glass of metaldehyde please


Recently I was speaking to someone involved in the distribution of agricultural chemicals who mentioned the over-use by some farmers of metaldehyde slug pellets.

These common slug controls are cheap. Very cheap at £5 to treat a hectare of land and so the tendency is to spread them as directed and then add a bit more for luck.

Six to ten thousand tonnes of slug pellets are spread each year on farms in the UK. 

The point is metaldehyde is a hazardous pesticide and when it’s applied to land around crops and then it rains some of that chemical ends up in water run-off and some will find its way into drinking water.

It is rain which adds to this problem. Our climate is becoming wetter leading to higher numbers of slugs and snails. In England and Wales farmers are no longer allowed to burn stubble, I think they still are in Scotland although it is discouraged because of environmental damage. Burning was a means of controlling large numbers of molluscs in agricultural land in the past now slug pellets are the normal means of dealing with them.

Rainwater which falls on the land must run off somewhere: onto roads or ditches, burns and eventually into rivers. We get our drinking water from these freshwater streams and inevitably some of what is sprayed over land and crops finds its way into reservoirs. This something will sometimes include metaldehyde in areas where it is used. Tests on fresh water supplies have shown alarmingly high levels of this pesticide in some parts of the UK.

Filter it out you may say but that is not easily achieved. The problem with metaldehyde is that it is very stable in water making it virtually impossible to be removed in treatment plants.


What is metaldehyde?

Metaldehyde is the commonest molluscicide. It is a cheap ingredient of slug pellets and so commonly spread on land growing cereals such as winter wheat, potatoes and oilseed rape as a means of controlling slugs and snails from wreaking havoc with crops.

Problems with metaldehyde

  • It is a stable chemical compound difficult to remove from water.
  • It is dangerous if consumed by birds and animals, including domestic pets. Slug-eating hedgehogs are very vulnerable to the pellets. Once metaldehyde is ingested by an animal the chemical attacks the liver, kidneys and heart so that the animal finds it difficult to stand. It may become blind and its breathing it affected. It may sweat and salivate and death is accompanied by seizures.
  • In the less likely instances of human exposure to metaldehyde there may be skin damage but for a young child the consequences can be fatal – organ collapse and death.

In view of possible contamination from metaldehyde the EU set limits on its levels in drinking water. Greatest risk from this kind of contamination come in areas with the highest agricultural output such as we have here in the Northeast of Scotland as well as parts of southern England and in Wales.

Scottish Government figures for 2011 state that over 10,000 hectares of Scotland’s farmland was treated with metaldehyde mainly for winter wheat. In 2010 141 out of 1,912 hectares of strawberries were treated with it.

Contaminated water in the River Ugie near Peterhead was found to be seven times over acceptable levels in autumn last year and this was three times higher than the concentration found there in 2010. Higher levels of metaldehyde were also found in the Deveron.

Scottish Water is said to be doing what it can to remove traces of metaldehyde from its plant at Peterhead but as we’ve seen this substance is very stable and does not easily break down.

A Briefing Paper by Water UK reports

‘The characteristics of metaldehyde mean that it is not effectively removed by adsorption onto activated carbon –The normal treatment for removing any pesticides that may be in raw water. In addition, the relatively simple structure of metaldehyde means it cannot be broken down by other water treatment processes using chlorine or ozone. It is therefore a very difficult compound to remove even using existing advanced water treatment processes. Further research is being carried out into other treatment methods but early indications are that even if they work they would be prohibitively expensive and energy consuming to implement.’


Despite the incidence of metaldehyde in UK rivers we are reassured drinking water is safe. In October 2012 the UK Health Protection Agency which covers England assessed metaldehyde in water as not posing risk. It stated that while levels greater than the standard for individual pesticides has been found in drinking water this standard is ‘not set on a health basis.’

The EU specifies that pesticides should not be present in drinking water. That sounds totally reasonable to me. Why should we be happy with any level of pesticides in our drinking water when it could be avoided altogether? And some eyebrows may be raised by the HPA’s description of the EU ruling as ‘a technical and political issue’. The EU is not alone, the USA has had strict regulations over the use of metaldehyde for years.

I can understand the frustration of farmers faced with the voracious appetites of slugs and snails for there is no question they can wreak havoc with crops. Farmers tell us that if metaldehyde pellets are no longer available to them the alternatives will push up food prices. However it is the cheap nature of this control which has encouraged misuse by some farmers – £5 a hectare remember. The over-application of metaldehyde appears to be growing and inevitably some irresponsible farmers will overuse the pesticide and be careless over disposal of its containers – a direct danger to animals. But it is the threat to drinking water in those areas where the use of metaldehyde is highest which pose the greatest risks specially where farmers’ interests lie solely in their yields. In such cases run-off from their land is going to be a secondary concern.

Alternative slug treatment by using different chemicals would increase the costs of treating each hectare by £13, a trifle in most farm budgets and little more than the price of a set of rubber mats for the new Range Rover.


6 Comments to “A glass of metaldehyde please”

  1. Good, well written piece about the dangers of the pesticide let down apallingly by the last paragraph. Was that really necessary? Farmers are going bankrupt due to the buying policies of the Supermarkets forcing Farmers to produce crops as cheaply as possible in order to make any money or rather to prevent them making a loss.

    Probably not even factual – I bet you can’t get accessories for a Range Rover nearly as cheap as that. 🙂

    • John

      I recognise the plight of ‘small’ farmers who are indeed struggling and have been doing so for years and I am a critic of the buying policy of supermarkets which plays fast and loose with food producers. However I stay in an area where there are farmers who receive enormous subsidies for land they own or lease in far flung areas but which can be used to claim even greater subsidies from the EU(which I expect they will criticize for its zeal over certain controls while happily receiving cash from it). My part of the world has expensive cars and 4X4s galore, driven by farmers who update them far more often than I do my wee car.

      I was careful to say some farmers in my blog as I don’t tarnish the whole industry with the recklessness of the few. It is this couldn’t care attitude which we have to be wary about. The farming industry has looked at this issue for a while and has been warning farmers that they must act sensibly when using metaldehyde because of the consequences of over-application or risk having it withdrawn from use.

      It doesn’t matter what the occupation is there are good and bad to be found in all. Misuse of metaldehyde can lead to potential health hazards and it is right that awareness of this is not confined to the industry’s own press.

      Anyway thanks for taking time to read the blog. I enjoy reading all feedback.


  2. another important story from Lena. Between this, loss of land to over-development, and the continuing insistence on using pesticides which we know kill bees, things are looking grim.

    • Thanks Suzanne. There are so many aspects of agricultural and other land use which have negative impacts on us and the natural world which should be aired. This one was brought to my notice in a casual conversation but the potential risks to water supplies around Peterhead must be an area for concern.

  3. Surely a “slug of metaldehyde”

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