Do slug pellets pose a risk to hedgehogs?

Summary

Probably, although data are lacking. Most pellets used in domestic gardens are metaldehyde-based and we know that this chemical is toxic to most animals if ingested in high enough concentrations. In the case of the hedgehog, we think the LD50 of metaldehyde is about 500 mg/kg, suggesting it would take about 250mg to kill a 500g (1 lb.) hedgehog – about 1,000 pellets in a sitting. As unlikely as such consumption sounds at first, some veterinary pathology investigations have pointed to metaldehyde poisoning as the cause of death for hedgehogs and carers have reported drunk-like behaviour ‘wobbly’ behaviour in animals they suspect of having been poisoned by pellets. The evidence so far suggests the risk of poisoning is small, but the pellets may reduce the hedgehog’s prey base and have wider impacts in the ecosystem, so it seems prudent to err on the side of caution and look for alternative approaches to garden pest control.

The details

Traditional slug pellets can generally be divided into three groups based on their main active ingredient: those containing methiocarb; those containing thiodicarb; and those containing metaldehyde (sometime abbreviated to “meta”). The former two types are used almost exclusively in commercial agriculture, while metaldehyde pellets are used on both a commercial and domestic basis. Hence, it is meta pellets that I will focus on primarily as these are the ones most gardeners will use. Metaldehyde is used largely because of its highly-specific toxicity; it’s by no means harmless to anything other than slugs, but it’s more toxic to gastropods (slugs and snails) than anything else, so it can be used at lower doses.

Mollusc malevolence

The potential for metaldehyde as a molluscicide (something that kills slugs and snails) was discovered by accident in the 1930s. In a paper to the Annals of Applied Biology during 1940, Ministry of Agriculture, Fisheries and Food biologist C. T. Grimingham wrote:

A lady who was using ‘meta-fuel’ [metaldehyde in combustible tablet form] to heat her curling tongs and threw the remains out of the window afterwards observed an assemblage of dead slugs.”

A variety of slug-killing chemicals are available in garden centres. It is suspected that the routine application of these treatments not only removes a potential prey source for hedgehogs, but may also result in direct or indirect poisoning. - Credit: Marc Baldwin

In 1937 Grimingham joined forces with H.C.F. Newton to write a short paper to the Journal of the Ministry of Agriculture in which they discussed the effectiveness of metaldehyde-based slug baits. Based on their trials on swede and wheat plots, Grimingham and Newton found no evidence to suggest any soil invertebrates other than slugs and snails were affected by the poisoned baits. A more recent and more comprehensive study by Lise Samsoe-Petersen, Markus Bieri and Wolfgang Buchs, published in Aspects of Applied Biology, supported the findings of Grimingham and Newton. Samsoe-Petersen and his colleagues tested the impact of methiocarb and metaldehyde on earthworms (Lumbricus terrestris), rove beetles (Aleochara bilineara) and four species of carabid beetles, concluding that:

“… metaldehyde showed no or negligible toxic effects, while methiocarb was toxic to beetles and deactivated the earthworms.”

In this study, earthworm ‘deactivation’ meant methiocarb caused them to stop feeding and retreat to their burrow.

Metaldehyde breaks down to form ethanal, also known as acetaldehyde, and this appears to primarily affect gastropod mucus cells. Within a few days the ethanol breaks down to form carbon dioxide and water. For an excellent overview of metaldehyde and how it reacts in the environment, the reader is directed to Markus Bieri’s article on The Environmental Profile of Metaldehyde (opens PDF in new window).

So, what is a slug pellet and what does it do to slugs? Domestic slug pellets (i.e. those containing metaldehyde – which will be the main focus of this section) are comprised of three main components: metaldehyde at a concentration of about 3%, which equates to about 30mg per gram of pellets or roughly 0.3mg per pellet in the smaller products; bran to attract the slugs; and a blue dye. The blue dye is added in a bid to prevent birds from eating the pellets because laboratory studies have suggested that birds seem less inclined to peck at blue objects. A study on house sparrows (Passer domesticus) by Iwona Pawlina and Gilbert Proulx found that giving seeds a blue coating greatly reduced the birds’ tendency to eat them, while a similar study on red-winged blackbirds (Agelaius phoeniceus) by a team at the U.S. Department of Agriculture in Florida yielded similar results. The theory is that birds probably avoid eating blue objects because they’re novel; there’s nothing in their natural diet of a similar colour.

Slug pellets applied around a sweetcorn plant to prevent assault by slugs and snails. Such pellets are widely available in Britain and often considered the only resort by frustrated gardeners. - Credit: Marc Baldwin

Much of our understanding of how slug pellets affect gastropods comes from work by mollusc biologist Rita Triebskorn at the University of Tübingen in Germany. The precise mechanisms by which metaldehyde works are outside the scope of this article, but sufficed to say that Triebskorn and her colleagues have discovered that it makes major changes to the structure of gastropod mucocytes, the cells that manufacture and secrete mucus, which cause them to produce excessive mucus and the slug fatally dehydrates itself.

Toxicity 101

Before we can consider the effects that metaldehyde slug pellets have on hedgehogs, we must first consider how we measure toxicity (i.e. how poisonous a substance is) to animals. The most common methods of expressing toxicity are the Lethal Dose 50, or Lethal Concentration 50 for aquatic animals; often abbreviated to LD50 and LC50, respectively. Basically, these values tell you the concentration required to kill half (i.e. 50%) of the group of whatever species you’re testing it on.

A number of problems are associated with LD and LC tests, not least that they give no indication of the underlying actions of the substances being tested. There are also various ethical concerns, which have led to a change in the way the UK government issue licences for this testing, as well as more technical issues. Essentially, poisons can act in different ways when administered via different routes (e.g. if applied to the skin or swallowed); the toxicity can vary on an individual basis as well as across species, sometimes by considerable degrees; and the susceptibility of an animal to a poison can depend on the other foods in its diet. Despite the problems and ethicality, it is the LD50 data for metaldehyde that is most abundant.

The 3D structure of metaldehyde. Black = Carbon; White = Hydrogen; Red = Oxygen. It is created by treating acetaldehyde with various acid catalysts and mixed with a wheat bait to produce slug pellets. - Credit: Jynto (Wikimedia)

According to the World Health Authority, metaldehyde LD50 values range widely. In dogs, the LD50 values can be anywhere from 100 to 1,000 mg/kg (i.e. 100 milligrams administered per kilogram of dog) depending on breed, while cats average 207 mg/kg and rats fall in the range of 227 to 690 mg/kg. Human (generally children) deaths following metaldehyde ingestion have been recorded and, according to a 1958 paper in the journal Schweizerische Medizinische Wochenschrift (Swiss Medical Weekly), it has been used in at least two murders! In humans, symptoms of metaldehyde poisoning include stomach cramps, sickness, diarrhoea, fever and convulsions – ultimately, the symptoms can terminate in coma and death.

One prominent feature of metaldehyde poisoning in humans is the loss of memory. In a 1982 paper to the Western Journal of Medicine, medical doctors W.T. Longstreth Jr. and David Pierson reported on the case of a female school teacher who swallowed 470mL of liquid slug bait in an apparent suicide attempt. The woman’s recovery was hampered by “prolonged memory and psychomotor dysfunction” – in other words, she had problems with her memory and coordinating her movements. In a personal communication to Nigel Reeve (mentioned in Hedgehogs), Les Stocker explained that meta-poisoned hedgehogs display increased hypersensitivity and excitability. In The Complete Hedgehog, Stocker elaborates, explaining that in his experience the “classic symptoms of metaldehyde poisoning” are extreme excitement and tremors, with some muscle stiffening and even partial paralysis.

Hogging slug pellets

It is generally assumed that direct consumption of slug pellets by hedgehogs is unlikely, and if they were to consume slugs that had been killed by metaldehyde pellets, they would need to eat considerable quantities before death. Despite such assumptions, there is actually little data on metaldehyde toxicity in hedgehogs. Pat Morris, in his The New Hedgehogs Book, told how, after writing to several slug pellet manufacturers, he was directed to the research of Professor Christian Schlatter in Switzerland, who seems to have been the first person to investigate the problem. According to Morris, Schlatter found that the LD50 of metaldehyde for hedgehogs was about 500 mg/kg, suggesting it would take about 250mg to kill a 500g (1 lb.) hedgehog.

Based on the Schlatter’s estimate, it has been calculated that a hedgehog would need to eat somewhere in the region of 5,000 poisoned slugs before it reached the LD50 level. Similarly, working on the basis that smaller domestic slug pellets typically contain about 3% metaldehyde (i.e. 1g pellets contains 0.03g, or 30mg of ‘meta’) then a hedgehog of 500g would need to eat 8.3 grams in one evening to accumulate 250mg of ‘meta’. If we consider that the smaller domestic pellets weigh about 10mg each, the hedgehog would need to eat 830 pellets to reach this LD50 dose. Both scenarios seem highly unlikely, and it is not difficult to see why death as a direct result of metaldehyde poisoning is considered equally improbable. None of this, however, precludes hedgehogs from consuming some slug pellets and potentially suffering the effects, albeit on presumably relatively rare occasions.

Having a hedgehog-friendly garden means avoiding pesticides, particularly slug pellets. - Credit: Marc Baldwin

In a paper to The Veterinary Record during 1991 veterinarians Ian Keymer, E.A. Gibson and Debby Reynolds presented their analysis of the causes of death for 74 hedgehogs recovered from 65 localities around East Anglia. Thirty-five carcasses were tested for metaldehyde and the vets found three individuals had probably died from metaldehyde poisoning. One individual had a ‘meta’ concentration of 80 mg/kg in the liver, suggesting it had been absorbed into the bloodstream, while the other two were found to have 40 mg/kg of acetaldehyde (a meta breakdown product) in their stomachs. Four other hedgehogs were found to have sub-lethal (i.e. less than 20 mg/kg) concentrations of metaldehyde in their stomach or kidneys. Referring to a discussion with Mark Fletcher at the Central Science Laboratory in York, Keymer and his colleagues wrote:

Fletcher believes that hedgehogs are poisoned by eating metaldehyde based pellets rather than by eating slugs poisoned by the chemical.”

During his studies at the German Federal Research Centre in Münster, Hubert Gemmeke found that hedgehogs would eat slug pellets; methiocarb pellets were apparently three-times more palatable than metaldehyde ones. Methiocarb is significantly more toxic than meta and Gemmeke calculated that, were a hedgehog to eat all the methiocarb pellets it came into contact with while foraging, it could reach the LD50 in just two square-metres (about 22 sq-ft). It has been suggested that hedgehogs may stand a greater chance of reaching the LD50 if they eat slug pellets and poisoned slugs while foraging. This seems plausible, although some authors have noted that despite a proclivity to eat carrion, hedgehogs often treat moribund prey with suspicion. In addition, it may be argued that hedgehogs eating slug pellets in a laboratory doesn’t necessarily mean they’ll actively choose them while foraging naturally or as an alternative to normal prey. Nonetheless, I know several wildlife rescues who receive hedgehogs showing signs of metaldehyde poisoning each year and, in The Complete Hedgehog, Les Stocker wrote:

All the same I believe that hedgehogs, especially youngsters, will pick up, lick and chew slug pellets especially as a stimulus to that peculiar hedgehog trait, self-lathering.”

Speculate to accumulate

While a hedgehog may not eat 830 slug pellets in one sitting, or even over the course of one night, some authors have suggested that toxic accumulation could be a problem. When an animal or plant takes in certain chemicals, particularly fat-soluble ones, over time they can build-up within the tissues if the chemicals cannot be excreted or broken down as fast as they’re taken in – this process is called bioaccumulation.

A dead slug, apparently killed by pellets. Slug pellets tend only to be effective during dry weather because they work by causing the slug to over-produce mucus (visible in right of frame) and dehydrate itself. In wet conditions slugs can re-hydrate faster than the metaldehyde dehydrates them, making the pellets ineffective. - Credit: Marc Baldwin

So, for example, although a single hedgehog may never come close to consuming the 5,000 slugs required to get the full LD50 in one night, it may eat an average of 10 poisoned slugs per night over two years, which could lead to the accumulation of 7,300 slugs-worth of ‘meta’; more than enough to reach the LD50. Obviously, this is a very simple scenario that doesn’t consider that hedgehogs, or slugs, may breakdown ‘meta’ into harmless or less harmful components, or that hedgehogs may not absorb all the ‘meta’ present in a dead slug; but it’s an example of what we mean by bioaccumulation.

Despite concerns, there is currently no evidence to suggest that metaldehyde bioaccumulates in the environment. Indeed, in his review of metaldehyde, Markus Bieri pointed out that it’s broken down to form ethanal, and subsequently water and carbon dioxide. In terms of bioaccumulation, it seems that methiocarb is considerably more problematic, though. In her fascinating review of the impacts of molluscicides on hedgehogs, produced in collaboration with The Mammal Trust and Royal Holloway University, Jo Brunner wrote:

Irrespective of the accuracy of the LD50 values, it remains that toxic residues [of methiocarb] are found in slugs and worms which can be passed on to the foraging hedgehog.”

Furthermore, it has been suggested that if hedgehogs are put off eating slugs by the mucus they produce, poisoned slugs may be more attractive because they reduce their mucus secretion. In an interesting series of experiments by Jacqueline Mair and Gordon Port at the University of Newcastle upon Tyne assessed the effect of mucus production on the ability of two species of beetle to tackle slugs and, writing in the journal Biocontrol Science and Technology during 2002, they conclude:

Results indicate that these generalist beetle species are unable to overcome the defence mucus production of healthy slugs. Slugs sub-lethally poisoned by molluscicides may be a more suitable prey item due to a reduction in defence mucus production.

Fierce & subtle poison

Concerns have also been raised that metaldehyde may pose a long-term threat to hedgehogs in terms of their reproduction and changes in their behaviour. I have been unable to find any data on the impact of ‘meta’ on hedgehog reproduction, although it has been shown to adversely affect reproduction and reduce pup survival rates in rats. Slightly more data exist for possible behavioural effects. During the early 1990s, Hubert Gemmeke presented dead slugs, poisoned with slug pellets, to six hedgehogs. Gemmeke didn’t observe any obvious physiological or behavioural effects in his subjects, even in those animals that ate nearly all of the 200 slugs offered.

More recently, Uwe Plümer, at the same institution in Münster, tried to establish whether exposure to metaldehyde had any discernible impact on hedgehog behaviour – his results were published in a German contribution to Mitteilungen aus der Biologischen Bundesanstalt in 2006. It seems that Plümer tested 21 hedgehogs with varying concentrations of metaldehyde and found that, on average, a dose of 70 mg/kg led to half the subjects showing a noticeable decrease of locomotor activity; the hedgehogs moved around much less than those which weren’t exposed. In essence, direct exposure to ‘meta’ does seem to have a detrimental effect on hedgehogs, although the jury is still out as to whether hedgehogs are likely to be subjected to sufficiently high concentrations during their day to day activities.

A "wobbly" hedgehog, displaying symptoms consistent with poisoning. - Credit: Toni Bunnell.

I have seen some suggestion that mollusicides may have a more indirect impact on hedgehogs, by reducing their potential prey base. We have already seen, however, that there is little evidence to suggest metaldehyde, unlike methiocarb, poses a significant threat to most non-molluscan invertebrates. It therefore seems unlikely that other soil invertebrates would be badly hit by the application of domestic slug pellets. Furthermore, although slugs and snails do not represent a significant part of the diet of most hedgehogs, even if they did there is no evidence that the use of slug pellets has any impact on slug populations. Indeed, because slug pellets work by causing slugs to essentially desiccate, in damp or humid conditions the gastropods can rehydrate sufficiently quickly as to make the slug pellets ineffective. I should be clear that this is the situation for meta slug pellets, and the same does not necessarily hold true for other pellets or pesticides.

Finally, do we have any idea how many hedgehogs might be killed by slug pellets each year? In short, no we don’t. The Wildlife Incident Investigation Scheme (WIIS) was setup by DEFRA and is coordinated by the Central Science Laboratory’s Wildlife Incident Unit in York with the aim of investigating “the deaths of wildlife, including beneficial insects and some pets, throughout the UK where there is evidence that pesticide poisoning may be involved.” Hedgehogs appear very infrequently in the WIIS reports, and from all the reports available via their website (1998-2007), there are no records of hedgehogs having been poisoned with slug pellets. In fact, only one record of pesticide poisoning in a hedgehog appears; two animals were poisoned by bromadiolone (rat poison) in Norfolk during September 2004 after they apparently got access to an improperly secured trap. The relative scarcity of hedgehogs in these reports is not unexpected: hedgehogs are small, solitary, secretive, nocturnal animals and as such they are likely to be overlooked.

So, after all this, we still cannot say for sure how much of a threat domestic slug pellets represent to hedgehogs.  The evidence so far suggests it is small, but it seems prudent to err on the side of caution and look for alternative approaches to garden pest control.