Slugs, slug pellets and predatory ground beetles

September 22, 2011

in Birds, Gardening For Wildlife, Slugs & snails

Predatory beetles, slugs and metaldehyde slug pellets

Slugs, slug pellets and predatory ground beetles, what have they got to do with the decline of song thrushes? Read on and all will be revealed!

Why were song thrushes declining?

When I was a part time mature student at university, I needed a topic for my MSc thesis. I decided to explore reasons as to why the beautiful song thrush was in decline. So where do you start? Were numbers declining because not enough eggs were being laid, not all eggs were fertilised, not all hatched, youngsters were dying in the nests or were being abandoned by the parents or simply lack of food ? Could the position of the nest cause them to be abandoned because of disturbance by humans, traffic or what? Lack of suitable nest sites? I narrowed it down and decided to look at nesting sites. After speaking with Dr. Mark Langan, my mentor, who stated that to gather enough data to analyse, I would have to find about 40-50 song thrush nest sites to monitor and note their actual position. Where they nesting in a hedge, tree, ivy covered fence? What height was the tree, hedge, what species of tree were they nesting in, was the nest in a field or garden, etc!!  I started the declining numbers aspect of my research via scientific journals etc,. bearing in mind I had to time my searching for nests and nests with eggs to the nesting season and I started to be more specific about my research…. During the nesting season, after many weeks of searching, I could not find one song thrush nest! Things were looking bad for my thesis, the written research was coming together nicely, but no nests for stats ! At this time in my life with 3 young children, embarking upon a completely new career as a part time tutor at an FE college, with a pending Ofsted inspection (!) pressure was mounting…..I must admit it was tiring but I loved every minute, except the statistics! Time was running out and I needed to find a topic to research and write about.

The predatory black found beetle

The predatory black ground beetle will feast on your slugs!

One evening I went out into my back garden to fill up my bird feeder which contained black sunflower seeds. Underneath it was the detritus of black sunflower seed shells. In amongst them were different species of slugs, both large and small. Looking more closely I saw the black ground beetle, (Pterostichus madidus), also feeding amongst the detritus and NOT feeding upon the slugs which I knew they ate. Why was the beetle not attacking and eating the slugs? This was it….I abandoned all my song thrush research and led me to researching ground beetles, slugs and slug pellets instead for my MSc thesis, a small part of which was published in a scientific journal, ( see below) and started a whole new area for researchers to explore!

As I started to delve into the academic papers and journals, one thing struck me. Academics had noted that because many farmers had enlarged their fields by grubbing out their hedges, this deprived predatory beetles of a breeding ground and a place to live. They recommended creating strips of land sown with wildflowers, now known as beetle banks…… or wildflower margins. Thus beetles would be given habitats to colonise, live and breed in these undisturbed mounds and move out into the surrounding areas adjacent to the strips to feed upon pests such as carrot root fly larvae, aphids and slugs. This makes sense!

Beetle banks as pest reservoirs!

The undisturbed islands in a sea of agricultural green soon became a breeding ground for predatory ground beetles and enhanced their populations. But slugs also increased. Why? The actual physical construction of the beetle bank by cultivation of the land opened up the soil structure creating a more favourable condition for slugs as opposed to a compacted soil surface as created by continuous cultivation by heavy machinery. The undisturbed areas, not ploughed or cut provided ideal damp areas for slugs to thrive. Slugs are restricted to habitats where conditions remain moist, such as the dense vegetation of beetle banks and wildflower strips! Cultivating the soil around such areas tends to dry the soil out, kills slugs by physical or mechanical damage and also disturbed slug populations. Such areas would also provided organic matter as a food resource for slugs. By providing food resources, daytime shelter, shade, undisturbed areas and overwintering cover within them, provided ideal conditions for both predatory beetles and slugs! In fact beetle banks and wildflower margins actually enhanced the attractiveness to slugs and beetles and concentrated them in areas that they would not normally find as favourable. Prior to the beetle bank, the land simply formed part of a large field. In general, pests breed quicker and in greater numbers than their predators…..

Consequently farmers noticed an increase in the damage caused to their crops by slugs adjacent to the beetle banks. Scientists recommended increasing the slug killing chemicals,(molluscicides) around the edges of the beetle banks to counter this damage. But would this affect the beetles? What happens in reality? This intrigued me and I wanted to see if the academics recommendation would have the desired affect in reducing slugs and therefore slug damage.

The ferocious devil’s coach horse beetle (Ocypus oleos )

I thought I would collect a range of predatory beetles found in woodland, farms and gardens. I have always had a healthy respect for the devils coach horse ( Ocypus olens). They aggressively arch their backs and open their powerful mandibles if you touch them. I had even filmed one eating a large cabbage white butterfly caterpillar which are supposed to be distasteful to predators. Not in the least to this predator. I took some time to collect about 25 or so of these beetles placing them in a sealed cardboard shoe box and late in the evening took them to the laboratory at the university, opened the box to find a load of severed heads, dismembered beetle corpses, legs and severely injured beetles! Ok, so I could n’t use them for my research! Absolute killers!

The lovely looking violet ground beetle (Carabus violaceus)

Another potential candidate for my research was the lovely looking violet ground beetle. I quite like the look of violet ground beetle (Carabus violaceus) with its purple tinges and could only find 4 specimens of these in woodlands so they would not be part of my plan!

So back to my organic gardens at home where I found numerous black ground beetles and went hunting in my local parks and waste land until I collected about 40 of them but using just 30 or so giving the remaining beetles their freedom back in my garden to eat slugs! The lucky remaining beetles were fed at the university labs with tubifex blocks and then starved for 5 days before being offered a selection of slugs!

Grey field slug ( Deroceras reticulatum) a hugely damaging agricultural and garden pest

Collecting the slugs was so easy! Over a period of a few days I collected several hundred grey field slug specimens many of which were found on my garden lawns and on the grass in my nearby park. Mow your lawns in the darkness of evening and you will kill many slugs! I later had to weigh all of these slugs and lot of them were not up to weight or were too large and were not needed for the research, being released into the university grounds, (better than my garden! ). The chosen slugs were kept alive at specific temperatures and fed upon organic carrots. Nearer the time for the trials, some were frozen, some were fed slug pellets containing metaldehyde.

Metaldehyde slug pellets

The slugs pellets used were those are sold under various trade names and easily bought by amateur gardeners from garden centres. Whereas insecticides kill insects, molluscicides kill molluscs, i.e. snails and slugs. Because it is difficult to penetrate a slug’s slimy coating, most molluscicides are used as poisons and put into baits to be consumed by the slug. Basically they contain a mixed cereal matrix, such as wheat or barley flour, binders, fungistatic agents to stop mould and an active ingredient called metaldehyde.

Metaldehyde was originally used as a solid fuel (meta-tablets). Its slug killing properties were accidentally discovered by farmers in southern France who noticed dead and dying slugs at picnic sites where meta-tablets had been left on the ground. Introduced as a slug killing bait, in 1936 and first used in slug baits in the early 1940’s it is still the most commonest molluscicide used. They have no effect on the predatory ground beetle, Pterostichus madidus. Pellet formulations are constantly being updated to include adding feeding attractions and to improve pellet integrity, which slows them from degrading when wet.

Metaldehyde is more often effective as a contact poison. However, its formulation as a bait makes it difficult to establish whether the poisoning results primarily from ingestion, contact or both, as the mucus on the slugs body may prevent direct contact with the metaldehyde when applied directly to the skin. Personally I found that if you drop a few pellets on an unsuspecting specimen of this particular slug, then mucus is produced.

Song thrush with snail- never did research why song thrushes were declining!


Four types of slug was placed in separate quadrants into a petri dish: a dead frozen slug, a dead poisoned slug, a poisoned and dying slug and a living un-poisoned slug. Under red lights I and others introduced a predatory black ground beetle into the petri dish, its behaviour and prey selection was noted and observed for 10 minutes. This was replicated a number of times to create the required stats.


The idea was to see if, when presented with a dead slug, a dying slug or an alive slug, which one would the beetle prefer? It turned out that the beetles preferred the dead or dying poisoned slugs. When threatened, as an anti predatory trait, these slugs produce vast amounts of milky white mucus, in a very short time. Contact with metaldehyde slug pellets cause these slugs to produce their mucus as if threatened.  For a beetle, the mucus is obviously messy and sticky when released around its jaws and fouls them up as it attacks and has to be cleaned away continuously before it can dine in peace!  Without its defensive mucus supply the slug is open to attack from a predatory beetle as in these instances, the slugs had expended their mucus reserves. A dying or dead slug is easier to handle and wastes less of the beetles energy and time when having to overcome the slug, kill and consume it.

In essence then killing and injuring more slugs by applying more chemical control, would kill and injure more slugs which would be the preferred specimens for predatory beetles to eat over the uninjured alive slugs leaving them to continue unmolested eating away at the farmers crops!Not a sensible idea at all!

Predatory beetles, infected slugs and nematodes

Today nematodes have been massed bred and used as a biocontrol for slugs. The life cycle of the nematodes, Phasmarhabdatis hermaphrodita,  requires them to feed upon and breed from the slug that they have infected. Whilst at this stage the slug itself and therefore the nematodes are likely to be found by predatory ground beetles and preyed upon. The nematodes themselves would become prey. Researchers experimented with this aspect and found that slugs infected by the nematodes actually deterred the beetles from consuming the infected slug. Even though the nematode cannot infect an insect host. The nematodes and bacteria mixture in effect stops the slug and therefore the nematodes, being preyed upon.



Refs-  besides my own research, experience and observations…..

Edwards C.A , et al (2009), ” The relative toxicity of metaldehyde and iron phosphate-based molluscicides to earthworms” Ohio State University, Columbus, Ohio, USA

A. M. Langan, G. Pilkington & C. P. Wheater, (2001), ” Feeding preferences of a predatory beetle (Pterostichus madidus) for slugs exposed to lethal and sub-lethal dosages of metaldehyde”, Entomologia Experimentalis et Applicata 98: 245–248, 2001.Kluwer Academic Publishers. Netherlands

see paper here…….     langan_et al_2001

Pilkington, G, (2001), “Control of the grey field slug (Deroceras reticulatum): A conflict between the use of metaldehyde and predatory beetles (Pterostichus madidus), unpublished

Photographs with thanks from…… Richard Carter ground beetle used in research devils coach horse songthrush   song thrush with snail







{ 9 comments… read them below or add one }

marie September 22, 2011 at 9:43 pm

Hi George, I found that really interesting, I do try to peruade people not to use the pellets, I tell them that the slugs like the pellets and will travel to find them,,,,,and if they miss them then the first thing they find,,,,your lettuce,,,they will eat that. So if you don’t have a serious problem to start with then you will after using the pellets, and you will also have your neighbours problem too ‘cos his slugs like pellets as well. We don’t sell many,,,one pack this year I think.cheers marie



nurturingnature September 23, 2011 at 6:37 am

Hi Marie, thanks Marie, at least they are not methiocarb as used by farmers which have ben found to be lethal to worms, beetles and soil microbes!


Helen September 23, 2011 at 9:02 am

Hi George,
Great article, good details and fabulous piccies, I love the violet ground beetle. I enjoyed reading it whilst eating my weetabix 🙂 but the big question is, who did the Thrush research?!!!


nurturingnature September 23, 2011 at 9:25 am

Aw, thanks Helen. Sorry to say that question is still vexing researchers 🙁 with the usual suspects..farming practices,land drainage, pesticides…..


wetworld November 20, 2012 at 12:19 pm

What a wonderful article. I was brought here because only this soggy year I had learned that carabid (ground) beetles predated on slugs.

This year was so wet that despite a lifetime of spurning slug pellets, I was forced to allow their experimental use in the greenhouse. The theory given was that since birds could not get into the greenhouse, using metaldehyde slug pellets should be harmless.

Your article has pointed out that the dying slugs become food for the ground beetles, which consequently cannot be bothered to hunt down the living slugs. As ground beetle populations lag behind the population cycle of their food, it seems clear that leaving dying slugs about is a losing game. I used to just go out at night with a light and cut up the slugs with scissors and leave them in situ. Now I shall definitely collect the slugs, and dispose of them (by stirring wood ash into them, which is actually less time-consuming!). And I am re-inspired to put in a pond and stock it with frogs. Thanks again.


nurturingnature November 20, 2012 at 7:16 pm

Hi wetworld, thanks for your kind comments. It makes sense really, why suffer its mandibles becoming slimed up, leaving it without their use and then have to spend time in the open cleaning them up, because the live healthy slug protected itself? It can safely can dine on a slug that is dead or dying and less able to produce slime. Metaldehyde forces slugs to produce its defensive slime in copious amounts,the slug expends it very quickly and there is little or no slime left to produce. Hence the slug dehydrates having used up water to produce the slime. Although depending upon many factors, some slugs can hide away in a dark damp place and may recover by absorbing moister from the surrounding material that is in direct contact with its skin.

Cheers George


Liz August 22, 2016 at 6:39 am

An excellent article – which raises more questions. How common are naturally occurring nematodes? I understand that slugs commonly have nematodes which are harmful to hedgehogs. I read that hedgehogs will only eat slugs when there is no other food available. A lot of gardeners believe that hedgehogs will keep the slug population down, and welcome them in their gardens for this reason. Is it true that frogs are not adversely affected by nematodes? I have a small pond, but it seems to have more lizards than frogs! Will lizards feed on slugs?


nurturingnature August 22, 2016 at 9:45 pm

Thanks. Not sure what you mean by lizards Liz! Slugs are not tope of the list for slugs, beetles are! These nematodes target slugs, don’t think it is the same ones you mention re hedgehogs and frogs. Cheers, george


Liz August 23, 2016 at 9:15 am

Thank you – now I have subscribed I can read all of the other articles to answer my questions! We have newts in our pond of course!


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